Ichthyofaunal assemblages in estuaries: A South African case study

This review places the life-history styles of fishes associated with South African estuaries in a global context and presents a classification system incorporating all the major life-history categories for estuary-associated fish species around the world. In addition, it documents the early life his tories of the major fish groups in South African estuaries, with particular emphasis on the differing modes of estuarine utilization by marine, estuar ine and freshwater taxa. This review details factors influencing the ichthyofaunal community structu re in South African estuaries. The availability of fish for recruitment int o an estuary depends primarily upon the distributional range of euryhaline marine and estuarine species, with tropical and temperate taxa showing mark ed abundance trends. Within a particular biogeographic region, however, est uarine type and prevailing salinity regime have a major influence on the de tailed ichthyofaunal structure that develops. There is an increasing prepon derance of marine fish taxa when moving from a freshwater-dominated towards a seawater-dominated type of system, and a decline in species diversity be tween subtropical estuaries in the north-east and cool temperate systems in the south-west. Similar declines in fish species diversity between tropica l and temperate estuaries in other parts of the world are highlighted. Fish assemblages in estuaries adjust constantly in response to changing sea sons, salinities, turbidities, etc. Despite persistent fluctuations in both the biotic and abiotic environment, the basic ichthyofaunal structure appe ars to have an underlying stability and to be predictable in terms of the r esponse of individual species to specific conditions. This stability seems to be governed by factors such as the dominance of eurytopic taxa within es tuarine assemblages and the robust nature of food webs within these systems . The predictability arises from factors such as the seasonality associated with estuarine spawning cycles and juvenile fish recruitment patterns. The se patterns, together with a well-documented resilience to a wide range of physico-chemical and biotic perturbations, appear to be an underlying featu re of fish assemblages in estuaries around the world. In contrast to marine fish species, estuary-associated taxa have received l ittle conservation attention. Apart from the designation of protected areas , the main direct means of conserving estuary-associated fish stocks includ e habitat conservation and controls over fishing methods, effort, efficienc y and seasonality. Of these, the conservation of fish habitats, the most im portant, because healthy aquatic environments invariably support healthy fi sh populations. The use of estuarine sanctuaries for fish conservation is b riefly reviewed, as well as the legislation governing the USA National Estu arine Research Reserve System (NERRS) and the Australian Marine and Estuari ne Protected Area (MEPA) system. It is concluded that South Africa requires an expansion of the existing Estuarine Protected Area (EPA) network, as we ll as the upgrading of selected `estuarine reserves' where fishing is permi tted, into `estuarine sanctuaries' where no exploitation of biological reso urces is allowed.