Growth performance, osmoregulatory and metabolic modifications in red porgy fry, Pagrus pagrus, under different environmental salinities and stocking densities

The effects of two different environmental salinities [brackish water (BW), 12 parts per thousand, sea water (SW), 39 parts per thousand] and initial stock densities [low (LD), 1.0 g L-1; high (HD), 2.0 g L-1] on growth, osmoregulation, stress and energy metabolism of the fry Pagrus pagrus were investigated over a period of 45 days. Pagrus pagrus (n = 80, 5.51 +/- 0.25 g mean initial body weight) were randomly divided in eight groups. Growth, weight gain and specific growth rate increased in BW-acclimated fish compared with SW-acclimated fish. No differences were observed between the two stock densities tested at either environmental salinity. Plasma osmolality was lowest in BW-acclimated specimens, but the stock density had no effect on this parameter. Branchial Na+,K+-ATPase activity was positively correlated with environmental salinity, but unaltered at the renal level. Plasmatic parameters were enhanced by salinity and stocking conditions. At the hepatic level, triglyceride values were enhanced in BW-acclimated fish maintained at LD. Muscle metabolites (glycogen, glucose and lactate) increased in BW- compared with SW-acclimated fish; stock density had no influence. Our data suggest that changes in metabolic parameters could be correlated with the higher growth rates observed in P. pagrus acclimated to BW, while no significant effects due to the stocking density used were observed.