The ichthyofaunal composition of the Mngazi and Mngazana estuaries: a comparative study

The fish community structures of two contrasting estuaries, one with a well developed mangrove forest (Mngazana) and the other without mangroves (Mngazi) were compared. Members of the Mugilidae and Gobiidae families dominated the catch composition in both estuaries. In terms of estuary-dependence categories, euryhaline marine-spawning taxa dominated in both estuaries, emphasising the importance of estuarine habitats as nursery areas. The Mngazi Estuary contained 18% more estuarine-spawning fishes in terms of abundance than the Mngazana Estuary, probably due to the reduced tidal influence caused by a narrow mouth opening. Conversely, the higher diversity of species in the Mngazana Estuary (66 versus 49) was attributed to the greater influence of the marine environment due to the wide permanently open mouth as well as the presence of a wider variety of habitats in this system. Similarity analysis revealed no significant correlations between the fish community structure and the physical properties (salinity, temperature and turbidity) that were investigated in both estuaries. A geographic division of the estuaries into lower, middle and upper reaches revealed greatest abundance (CPUE) in the middle reaches and highest diversity in the lower and middle reaches of both estuaries. Tropical and temperate species were recorded in both estuaries, thus confirming the biogeographical transitional nature of these systems (i.e. situated close to the boundary between the subtropical and warm temperate regions of the southern African coastline). However, contrary to previous studies, which recorded seasonal changes in the proportions of tropical and temperate species, the proportions of tropical/temperate species remained unchanged at 71% during the January and June sampling occasions. Global warming as a possible reason for the increased dominance of tropical species irrespective of season is discussed.