Thyroid hormones in brown trout (Salmo trutta) reproduction and early development

Gravid brown trout (Salmo trutta) females were injected with various doses of a synthetic gonadotropin-releasing hormone analog (GnRHa), given with or without an injection of triiodothyronine (T₃), in order to investigate the potential of T₃ (a) to enhance the stimulatory effect of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were initiated until ovulation was detected 5–38 days later, endogenous plasma T₃ levels increased from an average of 3.6 to 11.6 ng ml⁻¹. Injection with 20 mg T₃ kg⁻¹ body weight, further elevated plasma T₃ levels at ovulation (16.0 ng ml⁻¹. Mean time to ovulation was reduced significantly in fish injected with 10 μg kg⁻¹ of GnRHa, whereas treatment with lower doses was ineffective. Injection with T₃ did not enhance the ovulatory response of brown trout to GnRHa. Unfertilized eggs obtained from T₃-injected females had a higher T₃ content, suggesting a transfer of T₃ from the maternal circulation into the oocytes. Maternal T₃ injection had no effect on egg fertilization rates, embryo survival to eyeing and hatching, or the prevalence of abnormal larvae at the time of hatching. Length and weight gain of the progeny during yolk absorption was also not influenced by maternal T₃ treatment. At the completion of yolk-sac absorption, progeny from females injected with T₃ had a higher prevalence of skeletal abnormalities than controls. The results suggest that in teleosts like brown trout, which have high endogenous circulating T₃ levels, treatment of females with T₃ does not enhance responsiveness to GnRHa and it has the potential for deleterious effects on their offspring.