Hormonal control of gonadal maturation and development of secondary sexual characteristics in coho salmon, Oncorhynchus kisutch, from Lakes Ontario, Erie, and Michigan

Abstract Pituitary and serum gonadotropin (GTH) levels and plasma 11-ketotestosterone levels were measured in sexually mature spawning adult and sexually precocious “jack” coho salmon ( Oncorhynchus kisutch ) from Lakes Ontario, Michigan, and Erie to determine whether the relatively low fecundity of the Lake Erie coho salmon has an endocrine aetiology, possibly associated with the apparent thyroid dysfunction in the Great Lakes salmon. Highest pituitary GTH content was found in Lake Ontario adult males and females; pituitary GTH content of coho salmon from Lakes Erie and Michigan were similar. Serum GTH levels were similar in adults and “jacks” from Lakes Ontario and Michigan and these were significantly higher than in the Lake Erie fish. Serum 11-ketotestosterone levels in adult males and “jacks” from Lake Ontario were up to 5.4-fold higher than in comparable animals from Lakes Erie and Michigan, whereas in the females serum 11-ketotestosterone levels in the Lake Erie coho salmon were significantly lower than in comparable fish from the other two lakes. The findings are discussed in relation to the interlake differences in gonadosomatic index, egg fecundity, and degree of apparent thyroid dysfunction.