Search

Fish populations in the Berg Estuary on the west coast of South Africa (32°46'S, 18°09'E) were sampled intensively over a four-year period, 2003-2006, in conjunction with a range of physico-chemical variables as part of the Berg River Baseline Monitoring Programme. A considerable body of historic data also exists for the system, dating back a further 10 years, making the Berg Estuary one of the most intensively sampled estuaries in South Africa. Using these data, this paper presents a detailed analysis of the variations in abundance, community structure and size composition of the fish fauna of the Berg Estuary in relation to intra- and inter-annual changes in freshwater flow and other physico-chemical parameters. It also serves to highlight the importance of the Berg Estuary as a nursery area and feeding ground for fish on the South African west coast and provides some insights into the likely impacts the Berg River dam (recently installed in the upper Berg catchment) on the fish fauna of the estuary. It should be noted that the Berg Estuary is one of the longest and largest in SouthAfrica (length=65 km, area=61 km²), and is rated as one of the most important systems in the country from a conservation perspective. Data analysed for the study includes results of ninepling excursions undertaken between 1992 and 2006 during which 42-46 sites were sampled with a 30 m long beach seine net and in some cases with a fleet of gill nets with mesh sizes from 44-145 mm stretched mesh. Total number of fish collected in the seine net hauls per sampling excursion ranged from 20 402-243 226 and biomass from 80-490 kg. Number of species present in the catches ranged from 14-23 species. Fewer species and lower biomass was generally recorded in samples taken during the winter and autumn (high-flow period) relative to summer and spring (low-flow) months. Overall fish abundance (measured as number of individuals per square meter) increased over the study period with an initial modest increase from 2004 onwards followed by a more marked increased from winter 2005 onwards. Species richness (measured as number of species recorded) also exhibited a modest increase over the entire study period, from around 14-18 species between 1992 and 2003, rising to between 17 and 23 species from 2004 onwards. Grouping fish species in accordance with their degree of association with estuaries revealed that marine migrant species accounted for most of the observed variation in abundance and biomass, with estuarine resident species following a more stable trajectory over time. The relative contribution by different groups of fish exhibited no clearly discernible pattern from year to year in relation to measured physico-chemical parameters including freshwater runoff to the estuary, or between seasons. In terms of species composition, Liza richardsonii was the most abundant species overall (average contribution = 59%), followed by Gilchristella aestuaria (22%), Atherina breviceps (8%), Psammogobius knysnaensis (3%), Caffrogobius nudiceps (2%) and Oreochromis mossambicus (2%). Together these six species accounted for 93-99% of the catches in each year (overall average: 97%). Increases in the abundance of L. richardsonii accounted for the major portion of the increase in fish abundance over time, and is thought to be related to the closure of a historic gill net fishery on the estuary in 2003, the primary target for this fishery. Some (but not all) fish species in the estuary displayed clear and consistent distribution patterns up the length of the estuary with freshwater species typically being most abundant in the upper reaches of the estuary and estuarine resident and marine migrant species being more abundant lower down.… [ABSTRACT FROM AUTHOR]

A total of 248 roving creel surveys along the length of the Berg River Estuary, in the Western Cape, South Africa, recorded 626 shore-angler and 88 boat-angler outings over the period December 2002-November 2005. Catch-and-effort information was obtained from catch inspections with 360 handline and 246 rod-anglers. Average total annual linefishing effort was estimated at 449±29 (mean±SE) boat-angler days, 1 299±118 recreational shore-angler days and 1 394±57 subsistence (handline) shore-angler days. The estimated total shore-based linefish catch (excluding boat-based catches) from the estuary for the years 2004 and 2005 was 37 231±1 326 fish and 26 938±706 fish (approximately 8 t and 7 t) respectively. Compared with other estuaries along the South African east coast where angler catches have been surveyed, species diversity in catches from the cool-temperate Berg River Estuary was low, with only 15 species caught, of which three, elf Pomatomus saltatrix (56%), harder Liza richardsonii (31%) and carp Cyprinius carpio (11%), dominated the catch. Average linefish catch per unit effort of most species increased significantly in the two years subsequent to the closure of the long-existing commercial gillnet fishery in March 2003. Length frequency distributions revealed significant increases in the average size and an increased contribution of larger size class elf and harder to the linefish catch over the monitoring period, suggesting a degree of recovery of the estuarine icthyofauna after more than a century of intensive gillnet fishing. [ABSTRACT FROM AUTHOR]

This study compares the fish assemblages of the permanently open cool-temperate Olifants and warm-temperate 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, freshwater-rich system under historical flow conditions to a high-diversity, high-abundance, marine-dominated system with future reductions in flow. [ABSTRACT FROM AUTHOR]