The status of fish conservation in South African estuaries

Abstract

Estuary-dependent fish species are defined as those taxa whose populations would be adversely affected by the loss of estuarine habitats. Of the 155 species regularly recorded in South African estuaries, only 32 (21%) are completely dependent on these systems, but this figure increases to 103 species (66%) if partially dependent taxa are included in the analysis. The conservation of fishes in estuaries on the subcontinent is threatened by a number of factors, including habitat degradation, disruption of essential ecological processes, hydrological manipulations, environmental pollution, overexploitation due to fishing activities and, more recently, climate change and the effects of introduced aquatic animals. Although major threats to fishes are usually linked to environmental degradation, there is increasing evidence that the stocks of certain fish species are overexploited or collapsed. Fish conservation and fisheries management does not depend on the implementation of a single action, but rather the co-ordination of a detailed plan, often in a multidisciplinary context. Some examples of innovative means of contributing to estuarine fish conservation in a South African context include the determination and implementation of the ecological freshwater requirements for estuaries, the zoning of estuaries for different uses and the recognition that the maintenance of ecological processes are vital to aquatic ecosystem health. Apart from the designation of protected areas, the main direct means of conserving fish species and stocks include habitat conservation, controls over fishing methods, effort, efficiency and seasonality, pollution control and the prevention of artificial manipulation of estuary mouths. Since becoming a democracy in 1994, environmental legislation, policy and institutional arrangements in South Africa have undergone some major changes, which, if fully implemented, will be very positive for fish conservation in estuaries on the subcontinent.

Movements and residency of juvenile white steenbras Lithognathus lithognathus in a range of contrasting estuaries

“…However, unlike white steenbras, these fishes undertook frequent and extensive sea trips. Consequently, this resolute dependence on estuarine environments by juvenile white steenbras represents a bottleneck in their life history, making them extremely vulnerable to threats associated with estuaries, in particular fishing pressure (Whitfield and Cowley, 2010).…”

“…The complex synergistic manner in which environmental factors influence the movement patterns of fishes in estuaries (Marshall and Elliott, 1998) suggests that anthropogenic disturbances on estuaries and environmental processes may impact negatively on species using these habitats. Consequently, management and conservation of estuaries should focus not only on the species, but also on protection of critical habitats and the maintenance of ecosystem functioning (Beck et al, 2001;Vasconcelos et al, 2007;Whitfield and Cowley, 2010).…”

“…Estuaries are subject to severe threats, such as habitat degradation, water abstraction, resource exploitation and infrastructural development (Whitfield and Cowley, 2010). Hypersalinity in the upper reaches, caused by a small catchment, low MAR and excessive water abstraction (Grange et al, 2000), limited the upper distribution of tagged white steenbras in the Kariega Estuary.…”

“…Understanding fish dependence on estuaries, their movement patterns therein, and the factors affecting these movements, is essential for effective conservation and sustainable exploitation of estuarine-associated fishery species (Whitfield and Cowley, 2010). Estuaries differ in physiological, hydrodynamic, climatic and biotic characteristics (Whitfield, 1992).…”

Riding the tide: estuarine movements of a sciaenid fish, Argyrosomus japonicus

“…Over-fishing poses the single biggest threat to estuarine-associated fishes in South Africa (Whitfield & Cowley 2010), and the juvenile estuarinedependent phase has been viewed as a life history bottleneck (Lamberth & Turpie 2003). The spawner biomass per recruit ratio of Argyrosomus japonicus has been re duced to between 1.0 and 4.5% of its unfished condition, probably caused by over-fishing during the juvenile estuarine-dependent phase (Griffiths 1997a).…”