Ichthyological
research in the Eastern Cape Province has mostly been conducted in larger
permanently open estuaries, including systems such as the Swartkops (Melville-Smith
& Baird, 1980; Beckley, 1983; De Wet & Marais, 1990; Marais, 1990;
Baird et al., 1996), Sundays
(Beckley, 1984; Whitfield & Paterson, 1995; Baird et al., 1996; Whitfield & Harrison, 1996),
Kariega (Ter Morshuizen & Whitfield, 1994; Paterson & Whitfield, 1996;
Paterson, 1998) and Great Fish estuaries (Ter Morshuizen et al., 1996a; Ter Morshuizen et al., 1996b; Whitfield et al., 1994). Some insight has been provided
into the fish assemblages of smaller systems, including the Kabeljous, Seekoei,
Van Stadens (Dundas, 1994) and East Kleinemonde estuaries (Cowley, 1998),
but in general temporarily closed estuaries have been poorly researched.
This lack of baseline information from smaller systems emphasizes the need
to document and interpret their fish community structures. There is a particular
requirement for additional information such as species composition, richness/diversity
indices and the length frequencies and longitudinal distributions of species
found in these and other types of estuaries along the Eastern Cape coast.
There is a similar need for published data on the physical variability within
these systems, e.g. the Great Fish (Ter Morshuizen et al.,
1996a; Ter Morshuizen et al.,
1996b; Whitfield et al., 1994),
Keiskamma (Read, 1983) and East Kleinemonde (Cowley, 1998; Cowley & Whitfield,
2001; Cowley et al., 2001). This study compares the species composition,
relative abundance, richness and diversity, longitudinal distributions and
length frequency data for the fish communities in 10 Eastern Cape estuaries.
A study of the physical variability in these systems was also carried out,
but a detailed analysis of the physical data in combination with fish community
data has been analysed in another manuscript (Vorwerk et al., submitted).
MATERIALS AND
METHODS
PHYSICO-CHEMICAL SAMPLING
Five sites up the length of each estuary were selected, with one site occurring
in each of the following reaches; mouth, lower, middle, upper and head reaches.
Water temperature was measured at the time of fish sampling using an alcohol
thermometer. Water samples were simultaneously collected at the same sites
for laboratory analyses of salinity (using a Reichert optical salinometer)
and turbidity (using a Hach 2100A turbidimeter). Additionally, a winter and
summer set of physico-chemical samples were collected in all the study estuaries
during July 1999 and February 2000 respectively. This involved sampling temperature,
salinity and turbidity in the water column at 1 m intervals (with a minimum
of a surface and bottom sample) at the five sites along each estuary. Sampling
was conducted during early mornings (approximately 06h00 in summer and 07h00
in winter) and mid-afternoons (approximately 14h00-15h00) to obtain measurements
during the coolest and warmest periods of the day. Information on the area
cover of submerged macrophytes in each system were obtained from Colloty (2000).
These data are not considered to offer an insight into the long-term variability
in these systems, but indicate the physico-chemical variability as recorded
during the study period.
During the February 2000 physico-chemical expedition, a sediment sample was
collected from each region (mouth, lower, middle, upper and head) of each
estuary. These samples were then subjected to organic content and particle
size distribution determinations as described in Black (1965). The samples
were classed as gravel (>2 mm), coarse sand (2-0.5 mm), fine sand (0.5-0.063
mm), silt (<0.063 mm) or a combination of these categories (Walsh et al.,
1999).
The width and depth of the estuary was determined at five sites within each
system. In the smaller systems a rope marked at 2.5 m intervals was strung
across the channel. A measuring pole was then used at every marker to determine
the depth at that point. In the larger systems a Lowrance depth sounder was
used to take a reading every 3 sec while crossing the estuary at a steady
speed. The full width of the estuary at the point of the cross-section was
measured using a graduated rope.
ICHTHYOFAUNAL SAMPLING
The fishes in the different estuaries were sampled bi-annually, during June
or July (winter) and January or February (summer), on one or 2-3 consecutive
days depending on the size of the estuary (the dates for sampling in each
estuary are reflected in Table 1). A range of gear types, including a small
and large seine net and a fleet of gill nets were used. Different gear types
targeted specific groups and/or size ranges of fishes (Table 2). Fish were
identified using Smith & Heemstra (1995) and van der Elst & Wallace
(1976). The fish were then assigned to an estuarine association category
according to Whitfield (1998).
The number of large mesh seine net hauls varied in each system (Table 3) depending
on the size of the estuary and a major decline in the rate of collection of
new species from that system. In each estuary all possible littoral habitats
were sampled, with the exception of areas with submerged obstructions. The
net was laid in a semi-circle from the bank by motorised boat and then hauled
in by three or four people. All fish captured were identified and measured
to the nearest millimetre standard length (SL) before being returned to the
water alive.
The small mesh seine net sampling protocol was identical to that used with the
large seine except that due to the large numbers of individuals, the fish
captured in this net were preserved in formalin and returned to the laboratory
where identification and measurements (mm standard length) were conducted.
Monofilament gill nets were used to sample larger individuals of both marine
spawning and freshwater species, as members of both groups are known to actively
avoid seine nets (Cowley, 1998). The nets were 10 m in length and 2 m in
depth consisting of three equal length sections of 45 mm, 75 mm and 100 mm
stretch meshes. Generally two nets were used in each reach (upper, middle
and lower) of an estuary (see Table 3), although this was changed depending
on the size of the system. No gill nets were set in the very small estuaries
as these were adequately swept with the seine net gear. The gill nets were
deployed in the evening (at approximately 18h00) and lifted the following
morning (at approximately 06h00). All fish captured were identified and measured
to the nearest millimetre standard length.
SPECIES COMPOSITION, DIVERSITY
AND ESTUARINE ASSOCIATION
The richness and diversity of the fish communities in each estuary were expressed
using Margalefs richness index and the Shannon-Wiener diversity index respectively.
Margalefs index (Equation 1) calculates the number of species relative to
the number of individuals in the sample, which reduces sample size bias.
d = (S-1)/ log N
(Equation 1)
Where d is Margalefs index, S
is the number of species and N
is the number of individuals (Clarke & Warwick, 1994).
The Shannon-Wiener diversity index (Equation 2) incorporates equitability in
its calculation (Zar, 1996). This gives a better assessment of composition
diversity, as it indicates whether a community is dominated by a few species.
H = - Si pi (log pi)
(Equation 2)
Where H is the resultant diversity,
i is the sample number and pi
is the proportion of the total count represented by the ith species (Clarke & Warwick, 1994).
Correlations between these indices and catchment size, mean annual run-off (MAR),
estuarine area and linear length were tested using Spearmans Rank Correlation
Coefficient (p-values were determined from Zar (1996)).
The species composition of each estuary was presented as a numerical contribution
by each species. This was determined by calculating the percentage each species
represented of the total catch. Species were categorised according to Whitfield
(1998) and the contribution of each category calculated for each estuary,
based on the number of species and relative abundance.
LONGITUDINAL
DISTRIBUTION ANALYSIS
Longitudinal distributions were investigated using non-parametric multivariate
analysis (Clarke & Ainsworth, 1993; Clarke & Warwick, 1994) from the
PRIMER Package (Version 4.0, Plymouth Marine Laboratory). The estuaries were
first analysed on an individual basis by calculating a catch per unit effort
(CPUE) for each species either per seine net haul or gill net in each system.
A combined analysis of all the estuaries was also conducted by calculating
a catch per unit effort (CPUE) for each species by dividing the total number
of individuals of each species caught by the total number of seine or gill
net hauls in each reach. Both CPUE data sets were averaged either per season
or by combining the seasons. These data were standardised and root-root transformed
before producing a Bray-Curtis similarity matrix. The clusters were produced
using a group average hierarchical sorting strategy. The relationships between
the estuarine reaches, based on their fish communities, were examined using
dendrograms and non-metric multidimensional scaling (MDS). Analysis of similarity
(ANOSIM) was carried out to determine if the fish communities from each reach
were distinct. Where significant differences were found the SIMPER routine
(from the PRIMER Package) determined the relative contribution of individual
fish species to differences between reaches. A non-parametric Kruskal-Wallis
one-way ANOVA (Analysis of Variance) was used to test for differences in densities
from both the small and large seines between the different reaches of the
estuaries.
Length data were analysed for eight of the most abundant species only. These
included four estuarine resident species (Atherina
breviceps, Gilchristella aestuaria,
Glossogobius callidus and Psammogobius
knysnaensis) and four marine migrant species (Rhabdosargus
holubi, Liza dumerilii,
Liza richardsonii and Pomadasys commersonnii). Data for each species
were combined within the two estuary groups: large closed and permanently
open. No analysis was conducted on the small closed estuaries due to the
limited data sets from these systems. The data were analysed using a Kolmogorov-Smirnov
test to detect differences in lengths of fishes in the large closed and permanently
open estuaries. Information on age at length was obtained from the following
studies: A. breviceps (Ratte, 1989), G. aestuaria (Talbot, 1982), G. callidus (Boullé, 1989), P. knysnaensis (Bennett, 1989), L. dumerilii (van der Horst & Erasmus,
1981), L. richardsonii (de Villiers,
1987), R. holubi (Blaber, 1974)
and P. commersonnii (Wallace,
1975).
This study incorporated 10 estuaries along a 70
km long stretch of the Eastern Cape coast between the towns of Seafield (33° 32' 42° S, 27°
03' 05° E) in the south-east and Hamburg (33°
16' 45° S, 27° 29' 50° E) in the north-west. This area
was selected due to the diversity of estuary types and sizes occurring in close
proximity. The estuaries investigated include the East Kleinemonde, Klein Palmiet,
Great Fish, Mpekweni, Mtati, Mgwalana, Bira, Gqutywa, Ngculura and the Keiskamma
(Figure 1).
The estuaries were classed into permanently open, small closed and large closed
systems. These distinctions were made based firstly on whether the estuary
was either a permanently open or a temporarily open/closed system (predominantly
closed during this study). The closed esuaries were further subdivided based
on their size, with systems under 5 ha in surface area being referred to as
small closed and those with a larger surface area being termed large closed.
EAST KLEINEMONDE ESTUARY
The East Kleinemonde (Figure 2) is a large temporarily open/closed estuary.
The township of Seafield surrounds most of the lower reaches of this system,
as well as the neighbouring West Kleinemonde Estuary. The coastal road (R72)
between Port Elizabeth and East London crosses the estuary approximately 500
m from the mouth.
The estuary is 2.5 km long with a surface area of 17.5 ha. The catchment area
is 46 km2 and provides a mean annual run-off (MAR) of 2 x 106
m3/yr (NRIO, 1987; Badenhorst, 1988). The width of the estuary
is approximately 100 m in the lower and middle reaches and narrows down to
25 m in the upper reaches. The main channel has a maximum depth of 2.5 m,
with most of the estuary having a littoral zone of less than one metre deep.
The cross-sectional area of the system steadily decreases from the mouth (154
m2) to the head (24.5 m2), with an average of 82.1 m2.
The mouth dynamics of this system are well documented (Cowley, 1998). These
data demonstrate that open mouth conditions were only evident 2.5% of the
time, while overwash conditions appeared to be important in promoting marine
influences on the system, occurring 16.4% of the time. During the period
1993-1998 open mouth conditions occurred during every month except March and
July, demonstrating the variable nature of the mouth phase (Cowley, 1998).
The
winter and summer temperatures recorded during the study period averaged 16.9°C and 26.4°C respectively. The winter
temperatures ranged between 14.5°C and 19°C while the summer temperatures varied from 25.4°C
to 27°C (Figure 3). These temperatures are below the maximum (27°C)
and minimum (14.9°C) values recorded by Cowley
& Whitfield (2001).
The seasonal salinities remained relatively constant during the study period
with the average winter and summer salinities being 12.7 and 14.8 respectively.
The only large variation was a mouth region sample during summer that had
a relatively high salinity of 34. The summer salinities ranged from 13
to the high of 34, while winter salinities ranged between 12 and 14 (Figure
3). These salinities were generally higher than the 0-27 recorded by Cowley
& Whitfield (2001).
There were extremely low seasonal turbidity variations, with winter turbidities
of 3 to 4.5 NTU, and summer values ranging from 3.4 to 11 NTU (Figure 3).
The winter and summer averages were 3.5 and 5.9 NTU respectively, with bottom
turbidities being generally higher than surface turbidities during both seasons.
Throughout the East Kleinemonde sediment samples comprised mostly fine sands,
with a general trend of decreasing larger particles (gravel, coarse and medium
sands) as well as silts, from head to mouth (Figure 4). The sediment organic
content showed a decreasing trend from head to mouth, although all sites had
a relatively low organic content of between 0.8% and 6.5% (Figure 4).
A brief botanical survey was conducted by Adams (1997) and revealed submerged
macrophytes, in a continuous band along both banks of the estuary above the
road bridge. A salt marsh was recorded on the west bank just above the road
bridge and small stands of reeds were noted along the length of the estuary,
particularly in the lower and middle reaches.
KLEIN PALMIET ESTUARY
The
Klein Palmiet (Figure 5) is a small temporarily open/closed estuary that enters
the sea approximately 1 km west of the Great Fish Estuary on the coordinates
33° 30' 00° S and 27°
07' 47° E. A game farm surrounds the entire estuary, with a farm road accessing
the beach on the east bank at the mouth. The recent construction of a dam
in the catchment has severely altered the freshwater supply to this estuary.
Prior to construction of the dam the estuarine surface area was 1.44 ha when
full, but declined to approximately 0.4 ha during droughts. The catchment
is 12.6 km2 in size and provides a MAR of 0.67 x 106
m3/yr (Smakthin, V., pers. comm.). The water level at the time
of sampling was very low, with an average depth of 0.4 m and a maximum of
1.2 m in the centre of the pool near the mouth.
During
the study, a single winter and summer physico-chemical sample was collected
on the 15th June 1999 and the 1st February 2000. These
samples showed very little variation in the salinity and turbidity measurements.
The recorded winter salinity was 23 while during summer it was slightly higher
at 28. Similarly the winter turbidity was 5 NTU increasing during summer
to 8.1 NTU. Water temperatures showed a greater variation, rising from a
winter value of 16°C to a summer recording
of 25.2°C.
The sediment of the Klein Palmiet Estuary was dominated by fine sands, which
constituted 99.8% of the sample, with all the other sediment sizes contributing
less than 0.1% each. The organic content of the sediment was the lowest of
all the systems in the study, comprising 0.5% of the sample.
Submerged macrophytes
were recorded in the deeper pools near the mouth, although not in very high
densities. Stands of reeds were observed along the banks of the estuary although
due to the low water levels these plants were approximately 3 m away from
the waters edge.
GREAT FISH ESTUARY
The
Great Fish Estuary (Figure 6) enters the sea at 33°
29' 28° S and 27°
08' 06° E. It has a road bridge crossing the estuary approximately 400 m
from the mouth. Due to freshwater input from the Orange River inter-basin
transfer scheme the estuary has a perennial river inflow.
This large permanently open estuary has a catchment area of 30366 km2
and a MAR of 525 x 106 m3 yr-1 (NRIO, 1987).
The longitudinal length of the system is approximately 15 km, encompassing
a total water area of 192.7 ha. The estuary depth and width data were recorded
on a neap low tide. There was a large shallow bay near the mouth with two
channels flowing through it that were 1.8 m deep. The main estuary channel
had a maximum depth of 6.4 m, with shallow intertidal mudbanks on either side,
resulting in an average depth of 1.37 m. Even at low tide the estuary was
relatively broad, with the narrowest area located in the head-waters (50 m)
and the widest area (180 m) being recorded near the mouth (Figure 6). The
average cross-sectional area was 106.9 m2.
The
seasonal variability in water temperatures during this study was most noticeable
in the upper reaches, with the sea having a moderating influence on the mouth
region (Figure 7). The average winter temperature was 16.1°C (range = 12-19°C),
while the summer average was 24.2°C (range = 18.6-25.6°C). The temperatures
presented by Ter Morshuizen et al.
(1996a, 1996b) were within this range, except for an elevated summer maximum
(28.5°C).
Salinities recorded during this study reflected the elevated fresh water inputs
on this system, with the upper estuary having oligohaline salinities, 0-3
(Figure 7). Salinities in the mouth region were generally higher (5-34),
indicating a strong marine influence at times due to the open mouth (Figure
7). Salinity intruded further along the bottom than in the surface layers,
with the surface waters generally having slightly lower salinities (1-2 lower
in the upper reaches and 5-10 in the lower reaches). The average monthly
salinity recorded in the middle and upper reaches between November 1992 and
January 1995 varied between 0 and 15 (Ter Morshuizen et al.,
1996a; 1996b).
Turbidity also reflected the dual nature of the Great Fish Estuary, with the
upper reaches having higher turbidities from the elevated fresh water input
and the lower reaches having clearer water as a result of the marine influence
(Figure 7). There were no large differences between surface and bottom turbidities,
except in the mouth region where surface and bottom readings sometimes differed
by up to 100 NTU. The turbidity in this estuary is generally higher than
in the surrounding systems, with a winter and summer average of 124.9 NTU
and 176.7 NTU respectively. Ter Morshuizen et al. (1996a; 1996b) recorded
a mean of approximately 200 NTU in the middle and upper reaches of the estuary.
The particle size distribution of sediments in the Great Fish increased from
the head to the lower reaches and decreased again to almost completely fine
sand at the mouth (Figure 8). The organic content in the sediment ranged
between 1% and 2% at all sites, excluding the middle reach site (Figure 8),
which contained over twice the organic content when compared with the other
sites.
This estuary has no submerged macrophytes, probably due to the high turbidity
(Colloty, 2000). There is a relatively large (199 ha) salt marsh area on
the southwest bank in the mouth region. Reeds and sedges do occur intermittently
along the banks, covering a total of 16.6 ha (Colloty, 2000).
MPEKWENI ESTUARY
The
Mpekweni (Figure 9) is a temporarily open/closed estuary that reaches the
sea at 33° 26' 13° S and 27°
13' 57° E. The coastal road between East London and Port Elizabeth (R72)
crosses the estuary 300 m upstream from the mouth. Access to the mouth is
limited to a private road through the Mpekweni Sun resort on the northeast
bank of the estuary.
This system has a catchment of 65 km2 and a MAR of 4.0 x 106
m3/yr (NRIO, 1987). The longitudinal length of the estuary is
6 km, encompassing a total water area of 57.6 ha and an average depth of 1.08
m (maximum = 2.6 m). The mouth area is extremely shallow with only one section
reaching 1 m in depth. The cross-sectional area decreases from the lower
reaches to the head of the system (Figure 9), with an average of 92.3 m2.
The mouth (110 m), lower (140 m) and middle (140 m) reaches are relatively
wide when compared with the upper reaches (40 m) and head (40 m).
Water
temperatures in the Mpekweni during this study ranged from 15.3°C to 18.3°C during winter and 27.6°C to 32.2°C
during summer (Figure 10). The mean temperatures for winter and summer were
16.3°C and 29.4°C respectively. Surface and
bottom temperatures showed very little variability, possibly due to the shallow
nature of this system.
Salinities recorded during this study did not reveal any stratification, with
surface and bottom salinities varying by approximately 0.2. Winter salinities
ranged from 20 to 26, while during summer the variation was between 32
and 35 (Figure 10). The summer increase in salinity may be indicative of
the shallow nature of the system and high evaporative potential.
Turbidities in the system showed no depth, longitudinal or seasonal trend (Figure
10), with the winter and summer averages being 7.1 NTU and 6.8 NTU respectively.
The winter and summer ranges overlapped considerably, being 4-11 NTU and 3-13
NTU respectively.
Sediment particle sizes in the Mpekweni Estuary generally decreased from head
to mouth. The upper reach was the only site that didnt conform to this,
consisting almost exclusively of silts and fine sands. The organic content
of the system increased from the head to the middle reaches and then decreased
towards the mouth (Figure 11).
The Mpekweni has a limited variety of estuarine plants. Colloty (2000) identified
1.59 ha of submerged macrophytes in the estuary and 27.2 ha of supratidal
salt marsh on the northeast bank above the road bridge.
MTATI ESTUARY
The
Mtati (Figure 12) is a temporarily open/closed estuary that reaches the sea
at 33° 25' 24° S and 27°
15' 34° E. Anthropogenic impacts are restricted to the road bridge, which
crosses the estuary approximately 300 m from the mouth and a small residential
development on the northeast bank.
The catchment size of the Mtati is 130 km2 with a MAR of 8.0 x 106
m3/yr (NRIO, 1987). The estuary has a surface area of 37.9 ha
and a length of approximately 4 km. The Mtati is generally a narrow system,
with an average width of 57.5 m, except for a bay area above the road bridge
where it widens to approximately 110 m. The average recorded depth of the
system was 1.6 m, although the main channel was generally deeper than 2 m,
reaching a maximum of 3.9 m in the lower reaches. The average cross-sectional
area was 57 m2 although this was mostly due to a relatively high
value of 121 m2 in the lower reaches.
Based
on measurements taken on the 31st May 1999 and the 9th
February 2000, temperatures in the Mtati Estuary showed a large seasonal variation,
with a winter average of 13°C
and a summer average of 28.8°C. There was a small range during both seasons, 11-15°C
during winter and 27-31°C during summer (Figure
13). There was no evidence of temperature stratification, possibly due to
the shallowness of the estuary and the effects of wind mixing.
The salinity throughout the system was fairly uniform with only a small reduction
towards the head during winter (Figure 13). The mean summer salinity (20.1)
was higher than the mean winter salinity (16.5) possibly due to the shallow
warm waters promoting evaporation during the summer. The overall turbidity
in the Mtati was low with an average of 9.2 NTU. Winter turbidities were
slightly lower with a mean of 5.1 NTU while the summer mean was 13.3 NTU (Figure
13).
Sediment particle sizes in the Mtati Estuary decreased from head to mouth, with
the mouth sample comprising mostly fine sands (Figure 14). The percentage
organic content of the sediments also decreased from the head to the mouth
(Figure 14). The only site not following this trend was the lower reach site,
which had a higher organic content than the middle reaches.
The botanical importance of submerged macrophytes to this system is relatively
low, with only 3.2 ha being identified (Colloty, 2000). The main contributors
to productivity were the supratidal salt marsh area (54.3 ha) and reed stands
along the banks (26.2 ha).
MGWALANA ESTUARY
The
Mgwalana (Figure 15) is a temporarily open/closed estuary reaching the sea
at 33° 24' 52° S and 27°
16' 32° E. Access to the mouth area is limited to a private road through
a small holiday resort on the northeast bank. The main anthropogenic influence
on this system is the coastal road, which crosses the main channel approximately
400 m upstream of the mouth. Some small walls have been built on the northeastern
side of the mouth region in an attempt to stop erosion of the beach access
road.
This system has a relatively large catchment area of 200 km2 and
a MAR of 12 x 106 m3/yr (NRIO, 1987). The length of
the estuary is 6.5 km and encompasses a total water surface area of 62.9 ha.
The mouth area of the Mgwalana is very shallow (less than 0.25 m in depth),
with the rest of the system being slightly deeper (average depth = 0.6 m),
and having a maximum depth of 1.3 m. The estuary is also narrower relative
to the other study systems, having a maximum width of 180 m and an average
of 54.4 m. The system has a small average cross-sectional area of 29.7 m2
with only the lower reaches being higher at 60 m2.
The
Mgwalana revealed little temperature variation along the length of the estuary.
Winter temperatures ranged from 14.3°C
to 18°C (average = 16.1°C),
while the summer range was between 25°C and 32°C
(average = 28.7°C) (Figure 16). Similarly, the
biannual samples yielded very little variability in salinity, particularly
during winter when all salinities were 25, while during summer they ranged
from 28 to 31. During summer there was a slightly reversed salinity gradient,
with the mouth having a mean of 28 and the head having a higher mean of 31
(Figure 16).
A turbidity gradient was evident in this estuary, with values at the head (>30
and 60 NTU) being at least triple those at the mouth (<10 NTU) during both
winter and summer (Figure 16). This may be due to wind driven mixing of the
water column stirring up some of the finer sediments in the upper reaches.
The sediment composition of the Mgwalana Estuary was similar from head to mouth,
consisting primarily of fine sands (Figure 17). The only site that did not
follow this trend was the lower reaches where there was a higher percentage
of medium (18.9%) and coarse (6.9%) sands. The percentage organic content
in the estuary decreased from the head (25.6%) to the mouth (1.5%) (Figure
17). The overall organic content of this system and the Keiskamma was relatively
high when compared with the other study estuaries.
Colloty (2000) recorded very low densities of submerged macrophytes in the Mgwalana,
with a total area cover of only 1.1 ha. There is a small supratidal salt
marsh area (7.6 ha) on the northeast bank above the coastal road bridge, but
the majority of the primary productivity arises from patches of reeds and
sedges along the banks of the estuary (total area cover = 48.8 ha).
BIRA ESTUARY
The
Bira (Figure 18) was the largest of the temporarily open/closed estuaries
in this study, with a tidal influence extending 9 km up the estuary and encompassing
an estuarine surface area of 122.3 ha. The system has a 255 km2
catchment area and a MAR of 13 x 106 m3/yr (NRIO, 1987).
The estuary enters the sea at 33° 22' 58°
S and 27° 19' 47° E. The main coastal road (R72) bridge crosses the system approximately
600 m from the mouth, with access to the mouth area via a road on the northeastern
bank.
The Bira is a relatively shallow system for its size, with an average depth
of 1.2 m, and a maximum of only 2 m. The mouth area has a fairly shallow
bay with a short, deep channel (1.8 m) along the east bank, adjacent to the
retaining wall. This bay widens above the road bridge, before the system
narrows again towards the head (40 m). The average cross-sectional area is
also relatively low at 80.6 m2, with only the lower reaches exceeding
this average at 188 m2.
The
Bira demonstrated very little longitudinal variability in winter and summer
temperatures (Figure 19). There was also very little temperature variability
with depth, but a large seasonal difference was evident with a winter mean
of 13.8°C and a summer mean of 28.3°C.
Salinity variation within the system was very low, with a winter mean of 25.6
and a summer mean of 21.9. A small longitudinal change in salinity was recorded
between the mouth and head of the estuary (Figure 19). There was greater
variability in turbidity, with a peak in the middle reaches (Figure 19) that
may be due to outflow from a small tributary entering the estuary in this
region. The summer turbidity was higher than winter throughout the system,
although during both seasons the longitudinal trends were similar.
The sediments of the Bira Estuary indicate a reduction in large particles and
an increase in fine sands from the head to the mouth (Figure 20). The percentage
organic content of the sediments was relatively low (<10%) throughout the
estuary, with the middle, upper and head reaches having a higher organic content
than the lower reaches and mouth region (Figure 20).
According
to Colloty (2000) the Bira has limited macrophyte primary producers, with
a very small area (2.6 ha) of supratidal salt marsh on the northeast bank
above the road bridge and a total of only 5.3 ha of submerged macrophytes.
There are several patches of reeds and sedges (total area = 15.2 ha), although
most of these plants occur on the banks in the lower reaches.
GQUTYWA
ESTUARY
The
Gqutywa (Figure 21) was one of the most pristine systems in this study, with
no direct anthropogenic impacts. The coastal road does not cross this system
and the only access to the mouth area is via the beach. This temporarily
open/closed system enters the sea at 33° 21' 59° S and 27°
21' 34° E. The MAR of 6.0 x 106 m3/yr arises from a
catchment area of 85 km2 (NRIO, 1987). The linear length of the
estuary is 3 km encompassing a total estuarine surface area of 39.9 ha.
The Gqutywa has a shallow embayment (average depth = 0.9 m) in the mouth region
extending up into the middle reaches with one deep channel (1.9 m) along the
east bank. The head and upper regions are slightly deeper on average (average
depth = 1.0 m), although the channel is slightly shallower at 1.4 m. The
system is fairly narrow in the head and upper reaches (27.5 m and 45 m respectively)
but widens in the middle and lower reaches (98 m and 137.5 m) before narrowing
near the mouth. The average cross-sectional area is relatively low (62.2
m2), although the lower reaches has a substantially higher cross-sectional
area of 165 m2.
Sampling
in the Gqutywa demonstrated very little variability in temperature with depth
and distance up the estuary. There was a slight decrease in temperature up
the system during winter and a slight increase during summer (Figure 22).
The mean winter temperature was 17.5°C
(range = 15-19.3°C) while the summer mean was 28.6°C (range
= 27.4-30.2°C).
Salinity also revealed very little variability, with only a small seasonal variation
(Figure 22). The winter salinities were 25 with only two values lower than
this in the head region, while the summer salinities ranged between 27 and
29 (mean = 28.1). The summer and winter turbidities showed similar trends,
peaking in the upper reaches and decreasing towards the head region (Figure
22). Summer turbidities were generally higher than winter, with means of
13.3 NTU and 8.6 NTU respectively.
Sediment particle sizes in the Gqutywa decreased from the head to the lower
reaches, with the mouth having slightly larger particles than the lower and
middle reaches (Figure 23). The organic content of the Gqutywa increased from
the head of the system towards the middle reaches and then decreased towards
the mouth (Figure 23).
Submerged
macrophyte densities in the Gqutywa during this study were very low, with
a total area cover of 2.5 ha. In addition there was only a small salt marsh
area on the northeast bank in the middle reaches covering an area of 1.2 ha.
There were reed and sedge patches on both banks (total area = 3.8 ha).
NGCULURA
ESTUARY
The
Ngculura (Figure 24) is a small temporarily open/closed estuary with two holiday
homes situated 500 m away from the system on the northeast bank. It is not
used for any form of boating due to its small size and no roads have encroached
on the channel. The system enters the sea at 33°
21' 21° S and 27° 22' 23° E approximately 700 m east of
the Gqutywa.
The estuary has a small catchment of 15 km2 that provides a MAR of
1.17 x 106 m3/yr (NRIO, 1987). Tidal influence, when
linked to the sea, is only evident approximately 600 m up the system resulting
in a total estuarine surface area of only 1.6 ha. The estuary is very narrow
(average width = 14 m), the widest point being the mouth area at 20 m, and
the narrowest being the head region at 10 m. The estuary is generally shallow
(average depth = 0.8 m) with only the upper reaches and head region being
deeper at 1.5m and 1.8 m respectively. The average cross-sectional area was
10.9 m2.
Water
temperatures in the Ngculura showed summer/winter differences but no longitudinal
or depth trends (Figure 25). The mean winter temperatures were 16.7°C
while the mean summer temperatures were 27.9°C. The low variability may be due to the shallow nature of the system
and limited water volume. The salinity was very low throughout the estuary
during both seasons, with winter having an average 2 and summer 3.2 (Figure
25). There was almost no longitudinal or depth variability within the system.
The estuary also had very low turbidities during both seasons, with the winter
turbidity being lower than summer (means of 3.9 NTU and 12.8 NTU respectively)
(Figure 25). A slight increase in turbidity gradient from mouth to head was
evident during both seasons, although barely detectable during winter.
Fine sands dominated the sediments in all reaches (Figure 26). This arises
through most of the estuary being situated in a wind blown dune area of the
beach, with only the head and upper reaches extending beyond that into a vegetated
dune ridge. The percentage organic content was also extremely low in this
system, peaking with a value of 6.9% in the upper reaches (Figure 26) where
dune vegetation was present.
Colloty
(2000) recorded very low macrophyte densities, with no submerged macrophyte
beds or salt marsh areas. The only plants present in the system are reeds
and sedges, with a relatively large patch near the mouth and a few very small
areas further upstream (total area cover = 0.7 ha).
KEISKAMMA
ESTUARY
The
Keiskamma (Figure 27) is a large, permanently open estuary, with the mouth
situated at 33° 16' 45° S and 27° 29' 50° E. This system represented the eastern boundary of the study area
with the small town of Hamburg situated on the southwestern bank.
The Keiskamma has a large catchment (2745 km2) with a MAR of 170.48
x 106 m3/yr (NRIO, 1987). The estuary is approximately
12 km long, encompassing a total estuarine area of 197 ha. The estuary has
an embayment near the mouth with depths ranging from 0.5 m to 2 m. The main
estuary channel has a minimum 1.6 m depth, attaining a maximum of 2.7 m in
the upper reaches. The average depth is 1.35 m with the middle and lower
reaches having relatively shallow banks on either side of the main channel,
while the upper reaches and head of the system have steep slopes from the
bank into the main channel. The average cross-sectional area is 116 m2
and the average width is 86.5 m with a minimum of 30 m in the upper reaches
and a maximum of 172.5 m near the mouth.
There
was little longitudinal variability in winter water temperatures (Figure 28),
with a mean of 17.6°C
and a recorded range of 15.5°C to 19°C. Summer temperatures increased with distance up the estuary from
a mean of 21.3°C at the mouth to 27°C at the head of the system (Figure 28). Summer temperatures ranged
from 16°C
to 28°C. Read (1983) recorded an identical maximum
but lower minimum (12°C)
temperature.
Longitudinal salinity trends during both winter and summer decreased from the
mouth (31 and 31.8 respectively) to the head (6.3 and 0 respectively)
thus indicating a perennial fresh water input together with open mouth conditions
(Figure 28). The average salinities during winter and summer were 19.5 and
11.8 respectively.
No longitudinal turbidity trends were evident during winter, although during
summer the turbidity increased with distance up the estuary, peaking in the
upper reaches (Figure 28). The summer turbidities were generally higher than
the winter turbidities with a mean of 74.8 NTU during summer and 15.5 NTU
during winter.
Sediment composition in the Keiskamma Estuary did not reveal any distinct trends
(Figure 29). The middle reach samples were dominated by silt, and the mouth
region by fine-grained sands. Similarly the percentage organic content of
the sediments did not show any general trends, with a peak of 27.4% in the
upper reaches, a minimum of 1.7% at the mouth; the remaining samples contained
between 10% and 15% organics (Figure 29).
Colloty (2000) found that the macrophyte vegetation of the estuary comprised
mainly salt marsh, reed and sedge species, with a collective cover of approximately
112 hectares. In addition there are relatively small stands of submerged
macrophytes (total area = 11 ha).
SPECIES COMPOSITION
AND DIVERSITY
Species composition
A total of 75533 fish representing 57 species were captured using three gear
types in all the estuaries during the study. Seasonal catches combining all
the gear types for all the estuaries were similar, with summer and winter
totals of 38211 individuals comprising 48 species and 37322 individuals of
47 species respectively (Table 4).
The
number of species in each system ranged from eight in the Ngculura to 30 in
the Keiskamma and Great Fish estuaries (Table 5). A total of 39 species were
recorded during both summer and winter, with only 15 species being restricted
to either season, most of which were rare taxa. Gilchristella aestuaria (average contribution
= 37.8%) and Atherina breviceps
(average contribution = 31.1%) numerically dominated the catches in all the
estuaries (Table 5). Other important species included Rhabdosargus
holubi (average contribution = 8.8%), Liza
richardsonii (average contribution = 5.5%), Glossogobius callidus (average contribution
= 5.1%), Myxus capensis (average
contribution = 2.7%) and Mugil cephalus
(average contribution = 1.3%).
Richness and diversity indices
The permanently open systems (Keiskamma and Great Fish) had the greatest richness
index values in terms of seine netting (11.9 and 15.8 respectively) and a
combination of seine and gill netting (14.3 and 16.4) (Table 6). In terms
of gill netting, the Great Fish had the lowest richness (5.45), while the
Keiskamma maintained a high richness (14.3). The Margalef index calculated
for the communities correlated significantly with the linear length of each
estuary when the gears were combined (rs=0.91; p<0.001) and when
analysing seine net results independently (rs=0.91;
p<0.001) (Figure 32). Other physical characteristics of each estuary that
increased proportionally with richness (seine and gill net data combined)
were the catchment size (rs=0.71; p<0.02), mean annual
run-off (rs=0.79;
p<0.02) and estuarine area (rs=0.94;
p<0.001). Similarly, the trends in richness from the seine net data correlated
with these physical characteristics, except for a slightly higher correlation
for the mean annual run-off (rs=0.84;
p<0.005).
The Shannon-Wiener index did not show any trends relative to the physical characteristics
of the estuaries. The two open estuaries had very different diversities,
with the Keiskamma having a much lower diversity (0.51). Nine species comprised
90% of the catch, of which Gilchristella
aestuaria contributed 65% (Table 7). The Great Fish had the greatest
diversity for the seine net data (0.80), with the smallest temporarily open/closed
estuary, the Ngculura, having a similarly high diversity of 0.79. The remaining
closed estuaries had relatively low diversity indices in the seine and combined
gear results (Table 7). The gill net diversity was relatively high in all
systems with only the Great Fish having an index value less than 0.75.
ESTUARINE ASSOCIATION
The permanently open estuaries had more marine and freshwater species than the
temporarily closed systems (Figure 30), with the latter recording a higher
number of marine species dependent on estuaries (category IIa). Neither the
permanently open or temporarily open/closed estuaries conformed to the general
trends in overall numbers of species from southern African systems. The estuaries
in this study revealed a decrease in number of species from category IIa to
III, compared with an increase across these categories in the southern African
data (Figure 30).
The
abundance of individuals in each estuarine dependence category illustrated
different results from the number of species in each category. The estuarine
resident species were dominant in all the systems except the small Ngculura
Estuary (Figure 31) where the marine migrants Rhabdosargus
holubi, Liza richardsonii and
Myxus capensis comprised a major
proportion of the total catch (Table 5). The permanently open estuaries
had a greater abundance of marine migrants compared with the closed systems,
although the large Bira and Mgwalana estuaries had equivalent proportions
of marine migrants (Figure 31).
LONGITUDINAL DISTRIBUTION
Analysis on an individual estuary basis provided no strong evidence of longitudinal
distributions using either gear type. However, analysis of the seine net
composition data when all the estuaries were combined provided some indication
of longitudinal trends in fish distribution within the estuaries (Figure 33
and 35). When using multidimensional scaling, a general gradient was apparent
in the seine net data (Figure 34) but no trends were found in the gill net
data set (Figure 36). The results from a Kruskal-Wallis ANOVA run on the
fish densities in the different reaches similarly produced no significant
difference from either the large (p=0.19) or small seine data set (p=0.92).
When the seine net fish composition data from the different reaches were tested
using ANOSIM, the upper and lower reaches were found to differ significantly
(p=0.01), while neither the upper nor lower were significantly different from
the middle reaches. The SIMPER routine showed that 50% of the dissimilarity
between the upper and lower reaches was accounted for by seven species. These
included three estuarine spawning species; Atherina
breviceps (10.3%), G. aestuaria (8.8%), Glossogobius callidus (6.6%); and four marine
species; R. holubi (6.8%), L. richardsonii (6.4%), Liza
dumerilii (5.6%) and M.
capensis (5.4%).
Although
on a community basis the three reaches did not separate out, some species
(when analysed individually) revealed specific range preferences. A. breviceps and G. aestuaria demonstrated opposite habitat
preferences (Figure 37), with A. breviceps
dominating the lower reaches and G. aestuaria
becoming more abundant further upstream. The freshwater Oreochromis mossambicus exhibited a preference for the upper
reaches with the majority (43%) of the individuals being captured in this
region (Figure 38). The catch of M. capensis,
a catadromous species, was also highest in the upper reaches (Figure 38).
In contrast, the dominant marine migrant R.
holubi showed a relatively uniform distribution throughout the
three estuarine reaches (Figure 38).
Estuarine
resident species
G. aestuaria comprised individuals from a range of size classes in both the permanently
open and large closed estuaries (Figure 39). The modal size class in the
large closed systems was 25-30 mm compared with 40-45 mm in the permanently
open estuaries. The mean size range of 40 mm SL (±8.7 SD) in the open systems
was significantly larger (p<0.001) than the mean size of 31 mm SL (±8.2
SD) in the closed estuaries. Conversely, the mean size class in each individual
system within each estuary type (Figure 40) was similar, with the majority
of individuals being approximately 1 year old in the large closed estuaries
and 1 year or older in the permanently open estuaries (Figure 39).
The other pelagic estuarine resident, A. breviceps, produced a left skewed size
class distribution in the large closed estuaries and a bimodal distribution
in the permanently open estuaries (Figure 41). The modal size class in the
large closed systems was 25-30 mm and in the permanently open estuaries the
two peaks included the 30-35 mm and 55-60 mm size classes. There was very
little mean size class variation between individual systems within each estuary
type (Figure 42). When comparing estuary types, however, there were significant
differences (p<0.001) with the permanently open estuaries having a mean
size of 45 mm SL (±12.2 SD) and the large closed systems having a mean of
34 mm SL (±8.6 SD). There is consequently a large difference in the average
age of this species within the different estuary types with the majority of
captured individuals in the closed estuaries being less than 1 year and the
majority of individuals sampled in the open systems being approximately 1
year or older (Figure 41).
G. callidus length frequency distribution revealed a left skewed distribution
in the temporarily open/closed systems, with only six individuals from four
size classes recorded in the permanently open estuaries (Figure 43). The
modal size class of 35-40 mm in the large closed estuaries coincided with
an age of 1 year with very few specimens being above two years of age (Figure
43). There was little variation in the mean length of fish within or between
estuary types (Figure 44), with the mean length in the permanently open estuaries
being 44 mm SL (±7.8 SD) and the mean for the temporarily open/closed systems
being 41 mm SL (±13.8 SD).
A second gobiid species, P. knysnaensis, had a similar length frequency distribution
in all the estuaries, with a slightly larger modal length in the permanently
open systems (Figure 45). The majority of fish in both estuary types were
approximately 1 year old. Figure 46 shows the limited differences in mean
fish length between the individual systems, which is mirrored by the estuary
types with an identical mean of 34 mm SL.
Marine migrant species
The length frequency distribution of P. commersonnii was similar in all the estuaries,
with smaller size classes predominating (Figure 47). The majority of individuals
were between 70 mm and 120 mm in both estuary types. There was a lack of
variation in fish size in the individual systems (Figure 48) and between estuary
types, with the permanently open systems having a mean fish length of 139
mm SL (±67.3 SD) and the large closed estuaries producing a mean of 145 mm
SL (±85.1 SD). However the largest individuals were captured in the closed
systems, with a maximum size of 562 mm, compared with 421 mm in the open estuaries.
R. holubi produced a left-skewed distribution in the large temporarily open/closed
estuaries with the majority of individuals being under 1 year old (Figure
49). Similarly, in the open systems most fish were less than 1 year of age,
with a slightly more condensed length frequency distribution of individuals
recorded. Very little variation was evident in the mean R.
holubi lengths between individual estuaries of the same type (Figure
50), yet a significant difference (p<0.005) between estuary types was calculated
using the Kolmogorov-Smirnov test. The mean length in the open systems was
75 mm SL (±17.3 SD) and in the closed estuaries was 78 mm SL (±19.4 SD).
The maximum sized individuals in the closed systems (222 mm SL) were considerably
larger than those in the open estuaries (165 mm SL).
For L. dumerilii a slightly left-skewed
distribution was evident in the large closed estuaries compared with a much
stronger left-skewed plot in the permanently open systems (Figure 51). Thirty-three
percent of the individuals in the closed estuaries were below 1 year of age
compared with 70% of individuals in the permanently open systems. Minimal
differences were evident when comparing individual systems within each estuary
type (Figure 52). However, the length frequency of L.
dumerilii in the permanently open estuaries was calculated to be
significantly smaller (p<0.001) than those in the large closed systems.
This was evident in the mean fish length of 105 mm SL (±44.9 SD) for open
estuaries and 137 mm SL (±36.3 SD) in closed estuaries.
Similar
length frequency distributions were produced for the other important mugilid
species, L. richardsonii (Figure
53). In the large temporarily open/closed estuaries, most individuals were
1-2 years old, whereas in the permanently open systems most individuals were
less than 1 year of age. The open systems contained significantly smaller
individuals (p<0.001) with a mean size of 118 mm SL (±54.6 SD) compared
with L. richardsonii in the
closed estuaries with a mean of 142 mm SL (±42.0 SD). The variation between
different systems within the large closed estuary type was relatively small,
with the two open systems showing increased differences in both mean size
and ranges (Figure 54).
Seasonal
differentiation
The lack of seasonal differentiation in the number of individuals caught during
this study is surprising, as several authors (e.g. Bennett, 1989; Harrison
& Whitfield, 1995) have described large seasonal variations in estuarine
ichthyofaunal densities. Similarly, the number of species in estuaries reportedly
varies seasonally (Harrison & Whitfield, 1995), but this was not evident
during this study. The lack of seasonal variation in temporarily closed estuaries
may be due to the mouth status of these systems (predominantly closed during
the study period), preventing large immigrations or emigrations of species.
Conversely, the permanently open systems retained a connection to the sea
for the duration of the study, allowing species to move freely.
Two species, G. aestuaria and A. breviceps,
numerically dominated the catches (more than 60% of the catch) in every estuary
except the Ngculura and the Great Fish (Table 5). These planktivorous fishes
are both estuarine residents, completing their entire lifecycles within estuaries
(Whitfield, 1996). To avoid competition these species undergo spatial segregation
(Harrison & Whitfield, 1995; Cowley & Whitfield, 2001), with A.
breviceps densities dropping fourfold from the lower to the upper
reaches and G. aestuaria densities increasing twofold
in the same direction (Figure 36). Cowley & Whitfield (2001) found an
identical trend in the East Kleinemonde Estuary, with other authors identifying
similar distribution patterns for G. aestuaria
(Harrison & Whitfield, 1995). Whitfield (1980a) identified the converse
distribution of G. aestuaria in the Mhlanga Estuary in Kwazulu-Natal,
with the highest densities occurring in the lower reaches and the numbers
decreasing further upstream. This may be due to the lack of A.
breviceps in the Mhlanga Estuary, thus opening up the lower reaches to colonisation by G. aestuaria.
The
third most dominant species was R. holubi,
representing between 5% and 25% of the overall catch in all the estuaries
except the permanently open systems and the Klein Palmiet (Table 5). The
large R. holubi populations may be attributed to
this species being able to recruit during marine overwash conditions (Cowley
et al., 2001) and hence may
have growing populations in closed estuaries. This species was unique in
its universal distribution through the different reaches of the estuaries,
with approximately 33% in each reach (Figure 38). These distributions are
similar to those recorded by several authors for this species (Hanekom &
Baird, 1984; Whitfield et al., 1989).
R.
holubi has also been reported to associate with submerged macrophyte beds
(Hanekom and Baird, 1984; Whitfield et al., 1989), yet in the three systems
in this study where R. holubi predominated, the Mgwalana (14.3%), the Bira (17.2%)
and the Ngculura (24.4%), very low macrophyte densities were reported (Colloty,
2000). Similar results have been recorded in the mouth area of the East Kleinemonde,
where there were very low macrophyte densities but high R. holubi abundance (Cowley, 1998). However,
where large catches of R. holubi were made away from macrophyte
beds in the East Kleinemonde, Cowley & Whitfield (2001) noted the presence
of thick filamentous algal mats.
Blaber (1985) commented that the mugilids are probably the most abundant family
of marine fishes in south-east African estuaries. The mugilids were found
to represent a minimum of 5% of the catch in all the systems except the Gqutywa,
Klein Palmiet, Mgwalana and Mpekweni (Table 5). Of the eight mugilid species
captured during this study L. richardsonii
and M. capensis were the most common (Table 4).
M. capensis is a catadromous
fish whose overall distribution was highest in the upper reaches, with a decline
in density in the middle reaches, and an increase again near the mouth (Figure
38). Most authors have reported a linear trend for this species, increasing
from lower to upper reaches (Whitfield et al., 1994; Cowley & Whitfield, 2001).
The relatively high numbers in the lower reaches of closed estuaries during
this study may be due to some individuals awaiting an opportunity to migrate
out to sea to breed.
The dominant freshwater species in the study area, O. mossambicus,
demonstrated an increasing linear trend from the lower reaches to the upper
reaches (Figure 37). Whitfield & Blaber (1979) related the distribution
of this species within estuaries to several factors including salinity stability,
slow water currents, suitable breeding areas, marginal vegetation and the
absence of marine competitors and piscivores. These authors remarked that
this species would occur where four of these factors were favourable, and
be abundant if more than four factors were optimal. The increasing abundance
of this species further upstream during this study, may be due to the increasing
number of favourable factors, e.g. fewer piscivorous predators and marine
competitors, slower water currents, more stable salinities and an increase
in the marginal vegetation.
Richness
and diversity
The dominance by a few species (e.g. A. breviceps and G. aestuaria) in the overall catch (all gear types), resulted in relatively
low Shannon-Wiener diversity indices for the majority of estuaries (Table
7). Two of the closed systems, the Ngculura and East Kleinemonde, had higher
diversities (0.83 and 0.72 respectively) as greater proportions of species
contributed >5% of the catch. Although G. aestuaria accounted for nearly 50% of
the catch in the Great Fish, eight other species contributed between 2% and
19%, thus accounting for a relatively high richness (0.75).
Similar trends were evident in the seine net fish diversity, with the Ngculura
(0.79), East Kleinemonde (0.76) and Great Fish (0.80) having the highest overall
diversities (Table 7). The fish diversity in the gill nets was high in most
systems due to the relatively low densities and high species numbers normally
caught by this gear. The low diversity in the Great Fish was possibly due
to this estuary producing very poor gill net catches in terms of both fish
densities and species numbers. These low species numbers in the Great Fish
gill nets were reflected by the low species richness (5.45) for gill netting
in this system (Table 6). The gill net catches in the remaining estuaries,
except for the Mgwalana and East Kleinemonde, all produced Margalefs richness
values >10. Once again, the seine net results controlled the resultant
richness when both gear types were combined (Table 6).
The strong correlation between estuary size and fish species richness values
(Table 6), is similar to trends highlighted by other authors. Whitfield (1980b)
considered estuary size to be one of the major controlling factors of species
richness in Maputaland estuaries. Similarly, Marais (1988) found that fish
abundance and biomass in Eastern Cape estuaries could be correlated to catchment
size (r=0.46, p<0.001; r=0.59, p<0.001 respectively). Suggestions have
been made that it is not estuarine or catchment proportions that influence
these trends, but more likely the hydrological consequences of the dimensions
(Marais, 1988; Whitfield, 1996). Hydrological factors include increased nutrient
input into systems with perennial freshwater inputs (Whitfield, 1996), positive
salinity gradients and increased turbidity associated with larger systems
(Marais, 1988). An important consideration is the effect of river flow and
tidal prism on mouth status, with the smaller estuaries tending to close for
longer periods. A prolonged closed phase reduces the recruitment potential
of juvenile marine fish and prevents adult emigration back to the sea. Additionally,
during the closed phase estuarine salinities may increase due to evaporation
or decrease due to dilution with freshwater, resulting in only strongly euryhaline
species surviving these conditions (Whitfield, 1983).
The relatively large proportion of category IIa and IIb species occurring in
the temporarily open/closed systems may be explained by their strong attraction
to estuaries. Cowley & Whitfield (2001) recorded that some of these species,
R. holubi in particular, can recruit during
overwash conditions. However, once they have entered these estuaries, there
is no means of leaving until the following mouth opening event. The low number
of marine straggler species (category III) in the temporarily open/closed
estuaries may be related to their non-dependence on estuaries when compared
with category II taxa.
The low proportion of category III species recorded in the permanently open
estuaries during this study was surprising due to the accessibility of these
systems to all marine species. An earlier study on the Great Fish reported
four additional category III species (Whitfield et al.,
1994), but this coincided with a greater marine influence in the middle reaches
relative to this study (5-18 vs 0-3 ). The high proportion of
species more dependent on estuaries (category II) in the two open estuaries
is probably due to the increased opportunity for recruitment into these estuaries.
The high representation of freshwater species in the Great Fish and Keiskamma
estuaries can be attributed to the strong perennial river flow, thus allowing
these species to enter and retreat from estuarine waters as conditions changed.
Additionally, two of the category IV species in the Great Fish were alien
species (Clarias gariepinus
and Cyprinus carpio) which added
to the representation of this group (Laurenson & Hocutt, 1984; Laurenson
et al., 1989).
The estuarine resident component of the communities dominated (>70%) in all
the estuaries except the Ngculura (34.7%) and the Great Fish (51.5%) (Figure
31). The dominance by number of this group is not surprising as the species
which contribute to the estuarine resident component are small and complete
all or most of their life-cycles within estuaries (Whitfield, 1990). In contrast,
the marine migrants spend only short periods in estuaries, either utilising
these systems as a nursery area for juveniles or as a feeding ground when
adult.
In a similar study of a temporarily open/closed estuary in Australia, the marine
straggler and estuarine resident components of the Wilson Inlet community
contributed a higher proportion of species (10% and 55% respectively) (Potter
et al., 1993) compared to temporarily open/closed systems in this Eastern
Cape study (2% and 23% respectively). The higher marine straggler representation
in the Wilson Inlet may be due to the higher frequency of mouth opening during
the Potter et al. (1993) study when compared with the temporarily open/closed
systems in the Eastern Cape. In the permanently open Nornalup-Walpole Estuary,
Potter & Hyndes (1994) identified a similar percentage (57%) of marine
migrant species to that identified in the permanently open estuaries in the
Eastern Cape (49%). The estuarine resident component in the Nornalupe-Walpole
contributed a greater percentage of species (43%) relative to this Eastern
Cape study (23%). The higher contribution of estuarine resident species in
the Nornalup-Walpole estuary compared with permanently open Eastern Cape estuaries
may be as a result of extensive lacustrine conditions in the former system.
The lack of distinguishable fish assemblages in the different estuarine reaches
is surprising, given reported (Cowley, 1998; Whitfield, 1980a) and observed
(Figure 37; Figure 38) longitudinal density differences by the dominant species.
The observed assemblage differences in the seine net results between the upper
and lower reaches were due to some species (e.g. O. mossambicus, M. capensis, G. aestuaria
and A. breviceps) being found at the longitudinal extremes.
The species that accounted for the community differences were dominated by
estuarine residents (category I), followed by marine species dependent on
estuaries for their juvenile stages (category II).
LENGTH
FREQUENCIES
The small sizes of estuarine resident species relative to marine migrant taxa
is partly due to the estuarine residents having predominantly stenotopic traits
and the marine migrants having mainly eurytopic traits (sensu Ribbink, 1994).
Whitfield (1990) also suggests that a small body size is well suited to an
estuarine life-history style, with most South African estuaries providing
extensive littoral areas that small species can utilise very effectively.
The larger modal size of estuarine resident species in permanently open compared
to closed systems may be indicative of a higher survival rate of smaller individuals
in closed estuaries. The loss of larvae and early juveniles from open estuaries
due to ebb tidal flushing is a distinct possibility. In contrast, all these
size classes would be retained in closed estuaries, thus elevating their relative
contribution to the overall population. Cyrus & Blaber (1987a; 1987b)
have suggested that small fish undergo lower predation rates in turbid water
environments. In this study, the open estuaries had higher turbidity levels
than the closed systems, thus providing better protection to the larger (more
visible) size cohorts of the small pelagic estuarine species.
In the permanently open estuaries the maximum size of the two goby species was
smaller than in the closed systems, whereas the two planktivorous species
(A. breviceps and G.
aestuaria) revealed the opposite trend. These results may be due
to the availability of food resources. Feeding studies on G.
aestuaria and A. breviceps
have shown that these species feed mainly on a variety of small crustaceans
and insect larvae (Coetzee, 1982; White & Bruton, 1983; Cyrus et al.,
1993). Blaber (1979) found that in turbid estuaries, G.
aestuaria was a planktonic filter feeder, whereas in clear water
systems this species was a visual predator (Blaber et al., 1981). The two
goby species, G. callidus and
P. knysnaensis, also feed on
small crustaceans and insect larvae but are not planktivorous (Whitfield,
1988; Bennett & Branch, 1990). The trends of larger gobiid individuals
in the closed estuaries and larger clupeid and atherinid individuals in the
open systems may be due to differences in food resources in the different
systems. The open estuaries are often plankton rich due to riverine and marine
nutrient inputs (Froneman, 2000; Grange et al., 2000), while the closed systems
have numerous small crustaceans associated with the extensive submerged and
emergent macrophyte beds (Reavell & Cyrus, 1989; Whitfield, 1980c).
The length-frequency distributions of G. aestuaria in an earlier study of the closed
Mhlanga Estuary (Harrison & Whitfield, 1995) showed similar trends and
an identical modal size class of 30 mm when compared with the temporarily
open/closed systems in this study. Kok & Whitfield (1986) reported almost
identical size distributions for A. breviceps in the Swartvlei Estuary during
the open and closed mouth stages. The current study identified significantly
different A. breviceps length-frequency distributions,
with the temporarily open/closed systems having a modal size class half that
recorded in the Swartvlei system during closed mouth phases.
The gobiid species, G. callidus
and P. knysnaensis, had similar length frequencies in temporarily
open/closed estuaries during this study when compared with an earlier investigation
of the East Kleinemonde Estuary (Cowley & Whitfield, 2001). The modal
size class of G. callidus in this study was higher than
that recorded by Cowley & Whitfield (2001), while P. knysnaensis had an identical modal size
class during both studies. The mean size of G.
callidus during this study was the same as the earlier East Kleinemonde
study, with means of 41.0±13.8 mm and 41.6±12.8 mm respectively. Also, the
modal size class and length-frequency distributions of G. callidus in the closed estuaries during
this study and in the closed Damba and Zotsha estuaries were similar (Harrison
& Whitfield, 1995).
The age of estuarine species peaks at less than 1 year in the temporarily open/closed
estuaries compared with 1 year or older in the permanently open systems.
All these species commence breeding at approximately 7-9 months of age (Bennett,
1989; Boullé, 1989; Ratte, 1989; Talbot, 1982). As discussed earlier, the
difference in dominant age classes is probably a feature of differential mortality
or resource availability.
Three of the four marine migrant species, namely P. commersonnii, L. dumerilii and L. richardsonii, had different length-frequency
distributions in the two estuary types. These species had a greater proportion
of smaller individuals in the permanently open systems and fewer middle to
large size class individuals compared with the temporarily open/closed estuaries.
The modal size class for all four species was however similar in both estuary
types. For example, P. commersonnii and R. holubi in the permanently open estuaries
was one size class greater than in the closed systems, while L. dumerilii was one size class smaller in
the open estuaries. L. richardsonii had an identical modal size
class in both estuary types. The relatively similar modal distributions between
these two estuary types may be linked to the opening of temporarily closed
estuaries coinciding with peak recruitment of the postflexion larvae and 0+
juveniles of these species.
The larger maximum sized marine migrant individuals occurring in temporarily
open/closed estuaries may be as a result of these species being trapped in
closed estuaries for extended periods. In addition, the inability to reproduce
in the estuarine environment means that surplus energy obtained from feeding
is used for growth and not channelled into gonad development. The fish occupying
permanently open systems are often 0+ juveniles that reside in these systems
for 1-3 years before departing in time for the breeding season (Bennett, 1989;
Whitfield, 1990). Adults of certain marine species are known to enter open
systems to feed, but this is generally for short periods, hence the lower
catches of these large individuals during this study.
This
study describes the physical variability that occurred in 10 Eastern Cape
estuaries, as well as discussing the fish community structure of these systems.
The calculated Margalefs species richness index in the permanently open systems
was higher than that calculated for the temporarily open/closed estuaries,
although there were no trends identified when using the Shannon-Wiener diversity
index. There were very minor longitudinal distribution trends in fish assemblages
when all the estuaries were combined for analysis, with only the lower and
upper reaches showing significant differences. When examined on an individual
estuary basis no overall longitudinal distribution trends were evident although
some fish species did reveal zonation patterns. The length frequency histograms
for the different estuary types were varied for the estuarine resident species
but similar for the marine migrant species. The length frequency differences
in fish species between the permanently open and temporarily open/closed estuaries
are probably be related to the variable access these species have to the marine
environment, predation effects and the differences in foraging strategies
by the various taxa.
REFERENCES
-
ADAMS,
J.B. 1997. Botanical survey of the Mpekweni and East Kleinemonde estuaries.
University of Port Elizabeth, Unpublished Report.
-
BADENHORST,
P. 1988. Report on the dynamics of the Kleinemonde West and East estuaries
(CSE 13 & 14). CSIR Report EMA/T 8805. 19 pp.
-
BAIRD,
D., MARAIS, J.F.K. & DANIEL, C. 1996. Exploitation and conservation
of angling fish in two South African estuaries. Aquatic Conservation: Marine and Freshwater Ecosystems
6: 319-330.
-
BECKLEY,
L.E. 1983. The ichthyofauna associated with Zostera capensis Setchell in the Swartkops
estuary, South Africa. South African
Journal of Zoology 18:
15-24.
-
BECKLEY,
L.E. 1984. The ichthyofauna of the Sundays Estuary, South Africa, with particular
reference to the juvenile marine component. Estuaries
7: 248-258.
-
BENNETT,
B.A. 1989. A comparison of the fish communities in nearby permanently open,
seasonally open and normally closed estuaries in the south-western cape,
South Africa. South African Journal of Marine Science 8: 43-55.
-
BENNETT,
B.A & BRANCH, G.M. 1990. Relationships between production and consumption
of prey species by resident fish in the Bot, a cool temperate South African
estuary. Estuarine, Coastal and Shelf
Science 31: 139-155.
-
BLABER,
S.J.M. 1974. The population structure and growth of juvenile Rhabdosargus
holubi (Steindachner) (Teleostei: Sparidae) in a closed estuary.
Journal of Fish Biology 6: 455-460.
-
BLABER,
S.J.M. 1979. The biology of filter feeding teleosts in Lake St Lucia, Zululand.
Journal of Fish Biology 15:
37-59.
-
BLABER,
S.J.M. 1985. The ecology of fishes of estuaries and lagoons of the Indo-Pacific
with particular reference to Southeast Africa. In: Fish
Community Ecology in Estuaries and Coastal Lagoons: Towards an Ecosystem
Integration. Yanez-Arancibia, A. (ed.). UNAM Press, Mexico. pp.
274-266.
-
BLABER,
S.J.M., CYRUS, D.P. & WHITFIELD, A.K. 1981. The influence of zooplankton
food resources on the morphology of the estuarine clupeid Gilchristella
aestuarius (Gilchrist, 1914). Environmental
Biology of Fishes 6:
351-355.
-
BLACK,
C.A. 1965. Methods of Soil Analysis,
Part 1, Physical and Mineralogical Properties, including Statistics of Measurement
and Sampling. American Society of Agronomy, Wisconsin.
-
BOULLÉ,
D.P. 1989. Aspects of biology and ecology
of a small stream fish community. B.Sc. (Hons) Project Report,
Rhodes University, Grahamstown. 45pp.
-
CLARKE,
K.R. & AINSWORTH, M. 1993. A method of linking multivariate community
structure to environmental variables. Marine Ecology Progress Series 92: 205-219.
-
CLARKE,
K.R. & WARWICK, R.M. 1994. Change in Marine Communities: An approach to Statistical
Analysis and Interpretation. Plymouth Marine Laboratories, Plymouth.
144 pp.
-
COETZEE,
D.J. 1982. Stomach content analyses of Gilchristella aestuarius and Hepsetia breviceps from the Swartvlei system
and Groenvlei, southern Cape. South African Journal of Zoology 17: 59-66.
-
COLLOTY,
B.M. 2000. The botanical importance rating
of the estuaries of Ciskei / Transkei. Ph.D. thesis, University
of Port Elizabeth, Port Elizabeth. 230pp.
-
COWLEY,
P.D. 1998. Fish population dynamics in
a temporarily open/closed South African estuary. Ph.D. thesis,
Rhodes University, Grahamstown. 145pp.
-
COWLEY,
P.D. & WHITFIELD, A.K. 2001. Ichthyofaunal characteristics of a typical
temporarily open/closed estuary on the south east coast of South Africa.
Ichthyological Bulletin of the J.L.B.
Smith Institute of Ichthyology 71: 1-19.
-
-
CYRUS,
D.P. & BLABER, S.J.M. 1987a. The influence of turbidity on juvenile
marine fish in the estuaries of Natal, South Africa. Continental Shelf Research 7: 1411-1416.
-
CYRUS,
D.P. & BLABER, S.J.M. 1987b. The influence of turbidity on juvenile
marine fishes in estuaries. Part 1. Field studies at Lake St Lucia on the
southeastern coast of Africa. Journal of Experimental Marine Biology and Ecology 109: 53-70.
-
CYRUS,
D.P., WELLMAN, E.C. & MARTIN, T.J. 1993. Diet and reproductive activity
of the estuarine roundherring Gilchristella aestuaria in Cubhu, a freshwater
coastal lake in northern Natal, South Africa. Southern African Journal of Aquatic Sciences 19: 3-13.
-
DE
VILLIERS, G. 1987. Harvesting harders Liza richardsonii in the Benguela upwelling
region. South African Journal of Marine
Science 5: 851-862.
-
DE
WET, P.S. & MARAIS, J.F.K. 1990. Stomach content analysis of juvenile
Cape stumpnose Rhabdosargus holubi
in the Swartkops estuary, South Africa. South African Journal of Marine Science 9: 127-133.
-
DUNDAS,
A. 1994. A comparative analysis of fish
abundance and diversity in three semi-closed estuaries in the Eastern Cape.
M.Sc. thesis, University of Port Elizabeth, Port Elizabeth. 81pp.
-
FRONEMAN,
P.W. 2000. Preliminary investigation of food web structure in contrasting
estuaries. African Journal of Aquatic Science 25: 13-22.
-
GRANGE,
N., WHITFIELD, A.K., DE VILLIERS, C.J. & ALLANSON, B.R. 2000. The response
of two South African east coast estuaries to altered river flow regimes.
Aquatic Conservation: Marine and Freshwater
Ecosystems 10: 155-177.
-
HANEKOM,
N. & BAIRD, D. 1984. Fish community structure in Zostera
and non-Zostera regions of the Kromme estuary, St
Francis Bay. South African Journal of
Zoology 19: 295-301.
-
HARRISON,
T.D. & WHITFIELD, A.K. 1995. Fish community structure in three temporarily
open/closed estuaries on the Natal coast. Ichthyological
Bulletin of the J.L.B. Smith Institute of Ichthyology 64: 1-80.
-
KOK,
H.M. & WHITFIELD, A.K. 1986. The influence of open and closed mouth
phases on the marine fish fauna of the Swartvlei estuary. South
African Journal of Zoology 21: 309-315.
-
LAURENSON,
L.J.B., HOCUTT, C.H. & HECHT, T. 1989. An evaluation of the success
of invasive fish species of the Great Fish River. Journal of Applied Ichthyology 5: 28-34.
-
MARAIS,
J.F.K. 1988. Some factors that influence fish abundance in South African
estuaries. South African Journal of Marine
Science 6: 67-77.
-
MARAIS,
J.F.K. 1990. Body composition of ten marine migrants and one freshwater
fish species caught in estuaries of the Eastern Cape, South Africa. South
African Journal of Marine Science 9:
135-140.
-
MELVILLE-SMITH,
R & BAIRD, D. 1980. Abundance, distribution and species composition
of fish larvae in the Swartkops estuary. South African Journal of Zoology 15: 72-78.
-
NRIO. 1987. Basic physical geography/hydro data for estuaries of the south-eastern
Cape (CSE 1-59). CSIR, Stellenbosch. NRIO Data Report D8703.
-
PATERSON,
A.W. 1998. Aspects of the ecology of
fishes associated with salt marshes and adjacent habitats in a temperate
South African estuary. Ph.D. thesis, Rhodes University, Grahamstown.
200pp.
-
PATERSON,
A.W. & WHITFIELD, A.K. 1996. The fishes associated with an intertidal
salt marsh creek in the Kariega estuary, South Africa. Transactions
of the Royal Society of South Africa 51:
195-218.
-
POTTER,
I.C. & HYNDES, G.A. 1994. Composition of the fish fauna of a permanently
open estuary on the southern coast of Australia, and comparisons with a
nearby seasonally closed estuary. Marine
Biology 121: 199-209.
-
POTTER,
I.C., HYNDES, G.A. & BARONIE, F.M. 1993. The fish fauna of a seasonally
closed Autralian estuary. Is the prevalence of estuarine spawning species
high? Marine Biology 116:
19-30.
-
RATTE,
T.W. 1989. Population structure, production,
growth, reproduction and the ecology of Atherina breviceps
Valenciennes, 1935 (Pisces, Atherinidae) and Gilchristella aestuaria (Gilchrist, 1914) (Pisces, Clupeidae), from two southern Cape coastal
lakes. Ph.D. thesis, University of Port Elizabeth, Port Elizabeth.
319pp.
-
READ,
G.H.L. 1983. The effect of a dry and a wet summer on the thermal and salinity
structure of the middle and upper reaches of the Keiskamma Estuary, Ciskei.
Transactions of the Royal Society of
South Africa 45: 45-62.
-
REAVELL,
P.E. & CYRUS, D.P. 1989. Preliminary observations on the macrocrustacea
of coastal lakes in the vicinity of Richards Bay, Zululand, South Africa.
Southern African Journal of Aquatic Sciences
15: 103-128.
-
RIBBINK,
A.J. 1994. Biodiversity and speciation of freshwater fishes with particular
reference to African cichlids. In: Aquatic Ecology: Scale, Pattern and Process.
Giller, P., Hildrew, A. & Raffaelli, D. (eds). Blackwell Scientific Publications, Oxford. pp. 261-288.
-
SMITH,
M.M. & HEEMSTRA, P.C. 1995. Smiths Sea Fishes. Southern Book Publishers,
Johannesburg.
-
TALBOT,
M.M.J. 1982. Aspects of the ecology and
biology of Gilchristella aestuarius (G & T) (Pisces: Clupeidae) in the Swartkops estuary,
Port Elizabeth. M.Sc. thesis, University of Port Elizabeth,
Port Elizabeth. 128 pp.
-
TER
MORSHUIZEN, L.D. & WHITFIELD, A.K. 1994. The distribution of littoral
fish associated with eelgrass Zostera capensis beds in the Kariega estuary,
a southern African system with a reversed salinity gradient. South African Journal of Marine Science 14: 95-105.
-
TER
MORSHUIZEN, L.D., WHITFIELD, A.K. & PATERSON, A.W. 1996a. Distribution
patterns of fishes in an Eastern Cape estuary and river with particular
emphasis on the ebb and flow region. Transactions of the Royal Society of South Africa 51: 257-280.
-
TER
MORSHUIZEN, L.D., WHITFIELD, A.K. & PATERSON, A.W. 1996b. Influence
of freshwater flow regime on fish assemblages in the Great Fish River and
estuary. Southern African Journal of
Aquatic Sciences 22: 52-61.
-
VAN
DER ELST, R.P. & WALLACE, J.H. 1976. Identification of the juvenile
mullet of the east coast of South Africa. Journal of Fish Biology 9: 371-374.
-
VAN
DER HORST, G. & ERASMUS, T. 1981. Spawning time and spawning grounds
of mullet with special reference to Liza dumerilii (Steindachner, 1869). South African Journal of Science 77: 73-78.
-
VORWERK, P.D., WHITFIELD, A.K., COWLEY, P.D. & PATERSON, A.W. Submitted.
The influence of selected environmental variables on fish community structure
in a range of Eastern Cape estuaries. Environmental Biology of Fishes.
-
WALLACE,
J.H. 1975. The estuarine fishes of the east coast of South Africa. Part
I. Species composition and length distribution in the estuarine and marine
environments. Part II. Seasonal abundance and migrations. Investigational
Report of the Oceanographic Research Institute 40:
1-72.
-
WALSH,
J.H., PETERS, D.S. & CYRUS, D.P. 1999. Habitat utilization by small
flatfishes in a North Carolina estuary. Estuaries 22: 803-813.
-
WHITE,
P.N. & BRUTON, M.N. 1983. Food and feeding mechanisms of Gilchristella
aestuarius (Pisces: Clupeidae). South
African Journal of Zoology 18:
31-36.
-
WHITFIELD,
A.K. 1980a. Factors influencing the recruitment of juvenile fishes into
the Mhlanga estuary. South African Journal of Zoology 15: 166-169.
-
WHITFIELD,
A.K. 1980b. A checklist of fish species recorded from Maputaland estuarine
systems. In: Studies on the Ecology of Maputaland. Bruton, M. and Cooper, K. (eds). Rhodes
University, Grahamstown. pp. 204-209.
-
WHITFIELD,
A.K. 1980c. A quantitative study of the trophic relationships within the
fish community of the Mhlanga estuary, South Africa. Estuarine,
Coastal and Shelf Science. 10:
417-435.
-
WHITFIELD,
A.K. 1983. Factors influencing the utilization of Southern African estuaries
by fishes. South African Journal of Science 79: 362-365.
-
WHITFIELD,
A.K. 1988. The fish community of the Swartvlei estuary and the influence
of food availability on resource utilization. Estuaries
11: 160-170.
-
WHITFIELD,
A.K. 1990. Life-history styles of fishes in South African estuaries. Environmental
Biology of Fishes 28:
295-308.
-
WHITFIELD,
A.K. 1996. A review of factors influencing fish utilization of South African
estuaries. Transactions of the Royal
Society of South Africa 51: 115-137.
-
WHITFIELD,
A.K. 1998. Biology and ecology of fishes in southern African estuaries.
Ichthyological Monographs of the J.L.B.
Smith Institute of Ichthyology 2: 223 pp.
-
WHITFIELD,
A.K. & BLABER, S.J.M. 1979. The distribution of the freshwater cichlid
Sarotherodon mossambicus in
estuarine systems. Environmental Biology
of Fishes 4: 77-81.
-
WHITFIELD,
A.K. & HARRISON, T.D. 1996. Gilchristella aestuaria (Pisces: Clupeidae)
biomass and consumption of zooplankton in the Sundays estuary. South African Journal of Marine Science 17: 49-53.
-
WHITFIELD,
A.K. & PATERSON, A.W. 1995. Flood-associated mass mortality of fishes
in the Sundays Estuary. Water SA 21:
385-389.
-
WHITFIELD,
A.K., BECKLEY, L.E., BENNETT, B.A., BRANCH, G.M., KOK, H.M., POTTER, I.C.
& VAN DER ELST, R.P. 1989. Composition, species richness and similarity
of ichthyofaunas in eelgrass Zostera capensis beds of southern Africa.
South African Journal of Marine Science 8: 251-259.
-
WHITFIELD,
A.K., PATERSON, A.W., BOK, A.H., & KOK, H.M. 1994. A comparison of the
ichthyofaunas in two permanently open eastern Cape estuaries. South
African Journal of Zoology 29:
175-185.
-
ZAR,
J.H. 1996. Biostatistical Analysis. Prentice-Hall
International, London.