Do juveniles of the estuarine-dependent dusky kob, Argyrosomus japonicus, exhibit optimum growth indices at reduced salinities?

Abstract

The natural distribution of early juvenile (<150 mm TL) dusky kob Argyrosomus japonicus (Sciaenidae) along the east coast of South Africa appears to be restricted to the reduced salinity upper reaches of estuaries. This study experimentally tested the effect of salinity on growth indices in an attempt to explain the estuarine distribution of early juvenile A. japonicus. Growth under laboratory conditions was used as an indicator of relative energy expenditure for osmoregulation at salinities 5 (hypoosmotic), 12 (isosmotic) and 35 (hyperosmotic), as energy used for osmoregulation becomes unavailable for growth. Juveniles (81 ± 1.5 mm SL and 9.6 ± 0.5 g) survived and grew at all three salinities over a nine-week period. Contrary to what was hypothesised, growth of juveniles increased with increasing salinity (average weight 35.57 ± 1.50 g, 42.31 ± 2.70 g and 48.86 ± 1.30 g for the 5, 12 and 35 treatments respectively), with specific growth rate of juveniles at the hypoosmotic salinity (2.05 ± 0.15) significantly lower than at isosmotic (2.52 ± 0.09) and hyperosmotic salinities (2.59 ± 0.09). Food conversion ratio was significantly higher at the hypoosmotic (1.30 ± 0.28) than the hyperosmotic salinity (0.94 ± 0.05), but there was no difference between the isosmotic (1.01 ± 0.08) and hyperosmotic salinity. Salinity did not affect condition factor (range: 2.67 ± 0.03–2.75 ± 0.10) or feed intake (range: 2.43 ± 0.17–2.75 ± 0.53). The results, in part, indicated that early juvenile A. japonicus expend more energy at salinities below isosmotic levels and that salinity alone does not explain their natural distribution within estuaries. It is possible that high conductivity in the upper reaches of certain South African estuaries may mitigate the negative effect of reduced salinity on growth and hence the estuarine distribution of early juveniles.