The influence of turbidity on fish distribution in Natal estuaries.
Studies In other parts of the world have proved that turbidity affects aquatic life and work In Australia and North America has shown that the distribution of some fish species may be determined by the level of turbidity present. This, coupled with the fact that: (i) Natal estuaries are important as nursery areas for the juveniles of many marine fish species, (ii) the estuaries exhibit a wide range of turbidities and (iii) little was known of the effects of turbidity on the fish populations in estuaries, led to this study being undertaken. Turbidity and its effects on fish distribution In Natal estuaries was investigated from January 1980 to June 1983. Lake St. Lucia, which is predominantly turbid but also has clear water areas, was the main study area. Six other estuarine systems sampled were chosen to cover a wide range of estuarine types and turbidities. Field sampling was undertaken to determine which species were present under different turbidities, simultaneously physical factors which were potentially affecting fish distribution were also monitored. In addition to this, laboratory equipment which enabled a turbidity gradient to be established In a choice chamber tank was used to test the turbidity preferences of 10 common estuarine species for which field data were available. These tests allowed the elimination of all physical factors except turbidity. Of the physical parameters monitored In the field, turbidity, temperature and food availability In the benthos, were determined as being important In affecting fish distribution within estauries. However, comparison of fish distribution data for twenty species, with these factors showed that turbidity was exerting the major influence. It was also found that fish species occurred In one of five groups, inhabiting either clear, 'clear to partially turbid', intermediate or turbid waters or they were indifferrent to turbidity. Laboratory results for eight of ten species tested showed significant aoreement with the field data. The results of this study have shown that turbidity is the most important factor determining the distribution of juvenile marine fish In estuaries and that the greatest number of species are present in , waters which are not clear. The attraction to and presence in such systems appears to be related to the fact that turbid estuaries 'provide protection from fish and bird predators while also acting to reduce intraspecific predation. Of factors attracting juvenile fish into estuaries, turbidity is probably the single most important acting in this respect.