THYROID-HORMONES IN BROWN TROUT (SALMO-TRUTTA) REPRODUCTION AND EARLYDEVELOPMENT

Gravid brown trout (Salmo trutta) females were injected with various d oses of a synthetic gonadotropin-releasing hormone analog (GnRHa), giv en with or without an injection of triiodothyronine (T-3), in order to investigate the potential of T-3 (a) to enhance the stimulatory effec t of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were init iated until ovulation was detected 5-38 days later, endogenous plasma T-3 levels increased from an average of 3.6 to 11.6 ng ml(-1). Injecti on with 20 mg T-3 kg(-1) body weight, further elevated plasma T-3 leve ls at ovulation (16.0 ng ml(-1)). Mean time to ovulation was reduced s ignificantly in fish injected with 10 mu g kg(-1) of GnRHa, whereas tr eatment with lower doses was ineffective. Injection with T-3 did not e nhance the ovulatory response of brown trout to GnRHa. Unfertilized eg gs obtained from T-3-injected females had a higher T-3 content, sugges ting a transfer of T-3 from the maternal circulation into the oocytes. Maternal T-3 injection had no effect on egg fertilization rates, embr yo survival to eyeing and hatching, or the prevalence of abnormal larv ae at the time of hatching. Length and weight gain of the progeny duri ng yolk absorption was also not influenced by maternal T-3 treatment. At the completion of yolk-sac absorption, progeny from females injecte d with T-3 had a higher prevalence of skeletal abnormalities than cont rols. The results suggest that in teleosts like brown trout, which hav e high endogenous circulating T-3 levels, treatment of females with T- 3 does not enhance responsiveness to GnRHa and it has the potential fo r deleterious effects on their offspring.