Abstract:
The three species of fish studied here, £troplus maculatus, Etroplus
suratensis and Puntius filamentosus are teleosts indigenous to Sri Lanka.
The three species are found in abundance in the Bolgoda estuary and
specimens for the present study were obtained from this estuary. The sal inity
of the lagoon water varies from fresh water to 100% sea water in the different
parts of the estuary and fluctuates periodically. Although these fishes are
commonly found in both fresh and brackish waters, no work has been
reported in respect of their osmoregulatory abilities and salinity tolerances.
This paper reports on some aspects of osmoregulation in E. maculatus,
E. suratensis and P. filamentosus. The two Etroplus species were able to
tolerate salinities ranging from 0-60% sea water, while P.filamentosus was
able to tolerate only the narrow range from 0-20%sea water. In all species,
sodium salts accounted for most of the osmotic concentration, which is
indicative of an evolution in fresh water. Metabolic rates were measured for
each group of fish in various salinities to determine the relationship between
the medium osmotic concentration and the osmotic work done. E.maculatus
showed a significant reduction in energy expenditure at 30% sea water, the
concentration isosmotic to its plasma osmotic concentration. E.suratensis
and P.filamentosus showed no significant changes in metabolic rates at the
different salinities tolerated by them. Salinity preferances were determined
for the three species using a horizontal salinity gradient chamber. The
Etroplus species were found to prefer salinities ranging from 10-20% sea
water, while P. filamentosus was found to prefer fresh water.
