Comparison of, and the effects of altered freshwater inflow on, fish assemblages of two contrasting South African estuaries: the cool-temperate Olifants and the warm-temperate Breede

  • SJ Lamberth Marine Biology Research Institute, Department of Zoology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa; Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa
  • L van Niekerk Council for Scientific an Industrial Research (CSIR), Environmentek, PO Box 320, Stellenbosch 7599, South Africa
  • K Hutchings Marine Biology Research Institute, Department of Zoology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa

Abstract

This study compares the fish assemblages of the permanently open cool-temperate Olifants and warmtemperate Breede estuaries on the west and south coasts of South Africa respectively, and their responses to altered freshwater flows. Both estuaries have experienced a >35% reduction in mean annual runoff (MAR) from a historical reference condition to the present day with a >60% reduction possible under future flow scenarios. With the exception of species that have both marine and estuarine breeding populations, the Olifants Estuary fish assemblage has experienced an overall 20% decrease in abundance from reference (pristine state) to the present day and will gradually decline to 55% of reference with a predicted future 60% reduction in MAR. Consequently, future reductions in flow are likely to result in the Olifants Estuary progressing towards a low biomass, low diversity, marine-dominated system. In contrast, reduced freshwater flows in the Breede Estuary are likely to experience an overall reduction in the abundance of species that breed only in estuaries, and in freshwater and catadromous species. Collectively, entirely estuarine-dependent fish will increase in abundance, but considered individually some important exploited species such as Argyrosomus japonicus and Pomadasys commersonnii will collapse to 50% of historical numbers once there has been a 64% reduction in MAR. Overall, fish abundance in the estuary has increased by 6% from reference to the present day and is likely to increase to 115% of reference with future reductions in flow. Some species with a preference for fresh and brackish water will be all but lost from the system, but overall diversity is likely to increase with the range expansion of warm-temperate and subtropical marine species westward. In all, the fish assemblage of the Breede Estuary will experience a gradual change from a relatively high-diversity, low-abundance, freshwaterrich system under historical flow conditions to a highdiversity, high-abundance, marine-dominated system with future reductions in flow.

Keywords: abundance and diversity, ecological flow requirements, estuarine dependency, Freshwater Reserve (Environmental Flow Requirements), salinity preferences

African Journal of Marine Science 2008, 30(2): 311–336
Published
2008-10-20
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X