STIMULATION OF PARR SMOLT TRANSFORMATION BY HORMONAL TREATMENT IN ATLANTIC SALMON (SALMO-SALAR L)

Atlantic salmon reared under natural conditions in a fresh water (FW) hatchery at Le Conquet (Brittany, 48-degrees-N) were treated with diff erent hormones in order to trigger or advance the parr-smolt transform ation and to improve seawater (SW) adaptability. In the first experime nt, juveniles were implanted with ovine growth hormone (oGH) pellets ( 7 mug/g body weight for pre-smolts, 25 mug/g for parr) and compared wi th sham-operated and control salmon at different times of the year. Di fferent results were observed following direct exposure to full salini ty (35 parts per thousand) SW depending on the date of treatment. Prio r to smoltification (November, December and February), pre-smolts trea ted with oGH had higher gill Na+,K+-ATPase activity in FW, lower plasm a osmolarity after SW transfer, and higher growth rate in SW compared with sham and control fish. However, for pre-smolts and smolts treated during April, May or June, there was no difference between treated an d untreated fish. At any time of year, oGH-implanted parr survived SW transfer better than sham and control fish. Using a recombinant trout growth hormone (rtGH) at lower doses (0.28 and 1.4 mug/g), a high perc entage survival was observed after direct SW transfer for both pre-smo lts (100% after 30 days in SW compared with 86% in sham) and parr (65% compared with 0%). rtGH is at least 10 times more efficient than oGH for increasing gill Na+,K+-ATPase and salinity tolerance. Treatments w ith cortisol (8 mu/g in silastic pellets), 3,5,3'-triiodo-L-thyronine (T3, 20 mg/kg of food during 6 weeks), ovine prolactin (oPRL, 7 mug/g) and oGH (7 mug/g) were used on pre-smolts in October (8 months old). After direct SW exposure, only oGH-treated fish (alone or in combinati on with other hormones) were able to adapt, survive and grow. Both oPR L and oGH treatment increased gill Na+,K+-ATPase activity in fish in F W; however, there was no improvement of SW adaptability in oPRL-implan ted salmon.