Long-term variations in composition and abundance of fish in the Berg Estuary, South Africa

Abstract

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 South Africa (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. A number of water bodies of varying size on the floodplain adjacent to the main channel that were flooded only during the high-flow season were sampled but these proved to be of little importance to the fish fauna of the estuary, yielding mostly exotic freshwater species.