Experimental Animals and Treatments
Two experiments were conducted. In both, Atlantic salmon parr belonging to the Umealven (Ume River) stock were kept at the Norrfors hatchery (63°N, 20°E) in 2 X 2m tanks under standard hatchery conditions, with through-flowing river water at ambient temperature and under natural photoperiod. They were fed daily with commercial salmon food (Ewos Ltd., Westfield, Scotland). Two-year-old male parr that had matured in the previous autumn and still had running milt were castrated or sham operated on 25-28 March 1988 (experiment 1) and on 17-19 April 1989 (experiment 2) under —0.1% 2-phenoxyethanol (Flu-ka Chemie AG, Buchs, Switzerland) anesthesia. buy antibiotics online
Male parr of this salmon stock that have matured after their second summer generally remature the following year, although not before the testes have regressed to a gonadosomatic index (GSI = 100 X [gonad weight/total body]) of ^ 1% in June. The testes were excised through a single 1.5- to 2.0-cm-long lateral incision made into the abdominal cavity caudal to the pelvic fins. The incision was closed with sutures. Sham operations were performed in the same way, except that the testes were not removed. Fish receiving operations for experiment 2 were at the same time implanted, into the abdominal cavity, with Silastic (Dow Coming, Midland, MI) capsules (length 10 mm, outer diameter 2.4 mm) filled with crystalline T (Sigma Chemical Co., St.