Differences in breeding Lake Superior pink salmon (Oncorhynchus gorbuscha (Walbaum)) associated with variation in thyroid hyperplasia

Abstract

Pink salmon (Oncorhynchus gorbuscha (Walbaum)) from eastern Lake Superior exhibit various degrees of thyroid hyperplasia. A link between goitrogenesis and growth is suggested by the increased prevalence of large lesions among large fish of both sexes. Despite males being larger than females, however, lesion prevalence did not differ between the sexes. More large lesions occurred among late migrants, especially males, indicating a seasonal increase in hyperplasticity. Delayed stream entry occurred in overtly lesioned fish of both sexes, evidence of a possible inhibition of maturation. Males and females differed in condition (weight-at-length) with lesion severity. Females with overt lesions showed reduced secondary sexual character development, more so than males. The pre- and post-spawning testis characteristics of males generally did not differ with lesion size, although the testes of spawned-out males with large lesions did have higher water contents. In females, the reduced water content of eggs in severely lesioned individuals was evidence of delayed maturation. Enlarged livers occurred in badly lesioned females, but not in males. In neither sex did severely lesioned fish exhibit shorter breeding life-spans. In overview, this study demonstrates that a wide range of effects is associated with differences in the severity of thyroid hyperplasia exhibited by Lake Superior pink salmon, these also being mediated by apparent differences in physiology between the sexes.