RESPONSES OF A NONMIGRATORY STOCK OF BROWN TROUT, SALMO-TRUTTA, TO OVINE GROWTH-HORMONE TREATMENT AND SEAWATER EXPOSURE

The ability of ovine growth hormone (oGH) to enhance the hypo-osmoregu latory performance of a non-migratory stock of the brown trout (Salmo trutta) was investigated. Three groups of juvenile fish were intraperi toneally implanted either with a cholesterol pellet (sham) or with a c holesterol pellet containing 250 mug oGH (treated); control fish did n ot receive any implant. While still in fresh water, the gill Na+/K+-AT Pase activity of the oGH-treated group was four times higher than that of sham and control groups 2 weeks after implantation. Transfer to se a water (SW) resulted in marked increases in plasma electrolyte levels of the sham and control groups, whereas the oGH-treated group showed only minor perturbations in plasma electrolyte concentrations. Subsequ ent regulation of plasma electrolyte parameters to lower levels was sl ower or less effective in the non-treated groups than in the oGH-treat ed group. In the non-treated groups, SW exposure brought about signifi cant increases in gill Na+/K+-ATPase activity but only after a time la g of 7 days. An additional significant increase in gill Na+/K+-ATPase activity was also observed in the oGH group 7 days after SW exposure. By the end of the experiment, oGH-treated fish were significantly larg er than non-treated ones. The data indicate that after oGH administrat ion, juvenile non-smoltifying brown trout respond to SW exposure in th e same manner as a fully smolted salmonid.