IMPACT OF A CONTROLLED-RELEASE FORMULATION OF RECOMBINANT BOVINE GROWTH-HORMONE UPON GROWTH AND SEAWATER ADAPTATION IN COHO (ONCORHYNCHUS-KISUTCH) AND CHINOOK (ONCORHYNCHUS-TSHAWYTSCHA) SALMON

Experiments were undertaken to evaluate a sustained release recombinan t bovine growth hormone (rbGH) formulation (Posilac(R)) in salmon. The investigation was divided into two parts. The first series of studies was used to establish optimal dosages and clearance rates for the exo genous protein in size-selected coho salmon, and to examine the effect of rbGH treatment upon smoltification. The second series of studies a ttempted to mimic the production setting, where chinook salmon were ra ndomly selected from a stock of animals, and taken through smoltificat ion to grow-out. In the first experiment, coho salmon were provided wi th a single injection of one of three doses (420, 1260, 4200 mu g/g bo dy weight equivalent) of the rbGH preparation (n greater than or equal to 30/group), or were injected with a saline solution containing the high dose equivalent of rbGH. Controls were treated with BSA, given on e of three (10, 30 or 100 mu l) placebo carrier preparations without r bGH, or left untreated. Animals receiving rbGH by sustained-release ex pressed highly significant (P < 0.01) growth acceleration over a 20-we ek period of observation, when compared with the five control treatmen ts. There were no significant differences in growth between the five c ontrol treatments. At the termination of the trial, control animals ha d doubled their weight and were 25% longer than starting values. In co ntrast, salmon receiving rbGH via sustained release were 5-7.5 times t heir starting weight and exhibited a 60-100% increase in length. Coho salmon injected once with the saline solution of rbGH grew significant ly (P < 0.05) larger than control groups over the first 16 weeks of ob servation. However, at trial termination, control and saline rbGH anim als were not statistically different in weight and length. Following t he growth trial. fish were subjected to a 24-h seawater challenge. Pla sma samples collected prior to and following challenge. indicated that rbGH-treated fish were better able to adapt to the hypersaline enviro nment, as indicated by their increased ability to regulate plasma sodi um (P < 0.05). Plasma profiles for rbGH determined that the protein wa s released from the Posilac(R) preparation for greater than or equal t o 20 weeks following injection. In the second experimental series, chi nook salmon were provided with the high dose Posilac(R) formulation (e quivalent to approximately 6600 mu g rbGH/g body weight), whereas cont rols received the placebo carrier alone (n = 856 for each group). The growth response was followed through transfer to sea water, and beyond for a period of 32 weeks. Significant differences (P < 0.01) between group weights and lengths were first observed following 12 weeks of ob servation, and maintained until trial end. Examination of size frequen cy distribution illustrated a significant (P < 0.05) broadening of the curves for rbGH-treated fish from week 8 onwards, indicating an overa ll increase in size variation. (C) 1997 Elsevier Science B.V.