The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

Lake St. Lucia, the largest estuarine system in Africa (325 km²), was chosen as the field study area for a 3.5-yr ((1980)–83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded.The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and Zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution of juveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns.Turbidity and food type influences were difficult to separate but exceptions were the anchovyThryssa vitrirostris (Gilchrist and Thompson) and the soleSolea bleekeri Boulenger which occurred only in turbid water despite the widespread occurrence of their prey, andGenes acinaces Bleeker,G. rappi (Barnard), andG. fllamentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparidsRhabdosargus holubi (Steindachner) andR. sarba (Forsskal) were distributed according to turbidity and not their preferred foods.Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g.Liza dumerilii (Steindachner) andL. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g.Valamugil cunnesius (Valenciennes) andLeiognathus equula (Forsskal)) in 10–80 NTU, turbid-water species (e.g.Elops machnata (Forsskal) andThryssa vitrirostris) in > 50 NTU, and species indifferent to turbidity (e.g.Acanthopagrus berda (Forsskal) andTeraponjarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant ro?le, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries.