Interactive effects of salinity on metabolic rate, activity, growth and osmoregulation in the euryhaline milkfish (Chanos chanos)

The euryhaline milkfish (Chanos chanos) is an excellent subject for studies of the physiological and behavioral processes involved in salinity adaptation. In this study, energy partitioning for metabolism, activity and growth, maximal activity performance and blood osmotic concentrations were assessed at two activity levels in juvenile milkfish fed equal rations and maintained at a relatively constant temperature (26+/-2 degrees C) and at salinities (15, 35 and 55 parts per thousand) that represented a wide range of osmoregulatory challenges. Changes in the measured parameters were not consistently related to the magnitude of the trans-integumentary osmotic gradients, Routine oxygen consumption rates were high in 35 parts per thousand salinity (mean +/-1 S.E.M. 167+/-8 mg O-2 kg(-1) h(-1)) and comparably low in 15 and 55 parts per thousand salinity (133+/-6 and 127+/-3mg O-2 kg(-1)h(-1), respectively). Routine activity levels (relative swimming velocity) were highest in 35 parts per thousand salinity (0.96+/-0.04Ls(-1)), where L is standard length, intermediate in 15 parts per thousand salinity (0.77+/-0.03 L s(-1)) and lowest in 55 parts per thousand salinity (0.67+/-0.03Ls(-1)). Growth was significantly higher in 55 parts per thousand salinity (3.4+/-0.2 % increase in wet body mass per day) than in 35 parts per thousand salinity (2.4+/-0.2% increase per day) and intermediate in 15 parts per thousand salinity (2.9+/-0.5 % increase per day), Maximum swimming velocities decreased with increases in salinity, from 9.9+/-0.7Ls(-1) in 15 parts per thousand salinity to 6.6+/-0.5Ls(-1) in 55 parts per thousand salinity, Sustained swimming activity above routine levels for 2h resulted in an increase in blood osmotic concentrations in milkfish in 55 parts per thousand salinity, but osmoregulation was re-established during the second 2h of activity. Thus, patterns of variation in metabolic rate and growth were largely parallel to variations in routine activity although, comparing 15 and 55 parts per thousand salinity, elevated maintenance costs for osmoregulation at the high salinity were detectable. Reduced osmoregulatory abilities and reductions in maximal swimming performance suggest that high salinity may constrain activity. The results demonstrate that investigations of salinity adaptation in euryhaline fishes should take into account the interactive effects of salinity on physiology and behavior.