The fish community of the berg river estuary and an assessment of the likely effects of reduced freshwater inflows

Abstract

Data concerning the species composition, abundance and distribution of fishes inhab~ing the Berg River estuary are presented and used to assess the value of the estuary to fish, and the likely effects of reduced freshwater inflows. A total of 31 species was recorded, eight of which were classified as estuarine residents, 11 as marine estuarine-opportunists, six as marine stragglers and six were freshwater species. Liza richardsonii, an opportunist, was the most abundant (54% N) and widely distributed species encountered during the winter and summer seine-net surveys. Three other species (all residents) contributed more than 10% of the total catch; Atherina breviceps and Caffrogobius nudiceps occurring most abundantly in the lower estuary, and Gilchriste/la aestuaria in the middle estuary. The low number of species compared with estuaries in other regions of the South African coast reflect a well-established marine biogeographical trend. The higher proportions of resident species, entirely dependent species and partially dependent species suggest, however, that west coast estuaries may be more important to the fish that inhabit them than are estuaries in other regions. It is concluded that the fish community of the estuary is already suffering the effects of habitat degradation and that further reductions in freshwater inflows are not desirable. Data betreffende die spesiessamestelling, talrykheid en verspreiding van visse wat die Bergriviermond bewoon, word aangebied en gebruik in 'n raming van die waarde van die mend vir die bewonende visse, en die moontlike gevolge van verminderde varswaterinvloei. 'n Totaal van 31 spesies is aangeteken, agt waar• van geklassifiseer is as mondinwoners. 11 as mariene trekvisse, ses as mariene swerwers en ses as vars• watersoorte. Liza richardsonii, 'n trek vis, was die volopste (54% N) en mees wydverspreide spesie wat te9ge• kom is gedurende die winter• en someropnames. Die ander spesies (almal inwoners) het meer as 10% byge• dra tot die totale vangs, met Atherina breviceps en Caffrogobius nudiceps wat volopste in die laer-, en Gilchristella aestuaria in die middelgedeeltes van die strandmeer, voorkom. Die klein aantal spesies weer-spie91 'n geed vasgelegde mariene biogeografiese neiging. Die grater proporsie van inwonerspesies, totaal afhanklike en gedeeltel1k afhanklike spesies suggereer egter dat Weskusriviermonde belangriker mag wees vir die vi sse wat hulle bewoon as riviermonde in ander streke. Die gevolgtrekking word gemaak dat die visgemeenskap van die riviermond reeds onder die gevolge van habitatvernieling ly en dat verdere verminderings in varswaterinv1oei onwenslik is.

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

“…These provided data comparable with that collected during the baseline study and were analysed in conjunction with the baseline data. For simplicity and clarity in this paper the four historical studies are referred to as Winter 1992 and Summer 1993 (from Bennett, 1993) and Summer 1994 andSummer 1996 (S. Lamberth &B. Clark, unpubl.…”

“…Compared to marine coastal waters, estuaries are considered to be highly productive, with calm, shallow, warm waters, and lowered salinities (Bennett, 1993). These attributes promote rapid growth and/or reduced mortality for fish species, hence making them ideal environments for colonisation by juveniles of marine species.…”

“…The Berg River is the largest river in the Western Cape, a region that experiences long, dry summer seasons followed by elevated river flows driven by winter rainfall (Bennett, 1993). The effects of the ocean tidal cycle can be measured up to 65 km from the mouth (Day, 1981), making it one of the longest and largest estuaries in the country.…”

“…In order for the estuary to be used as a fishing harbour, a new mouth was cut through the sand dunes in 1966 and the channel stabilised between three concrete breakwaters. The new channel is dredged to a minimum depth of 4 m, thus allowing the passage of purse-seine boats into the estuary (Bennett, 1993). The original mouth rapidly silted up and created a lagoon running parallel to the coast (Figure 1).…”

“…Fish fauna inhabiting this system have been studied by a number of authors in the past Bennett, 1993;Harrison, 1997), all of whom have highlighted the importance of this system to the inshore marine fish fauna of the region. Some (e.g., Bennett, 1993) have speculated on links between freshwater flow and the abundance of fish within the system but have not managed to significantly enhance understanding in this sphere owing to the limited temporal coverage of their sampling.…”