LAUSR

Abstract Winter flounder, Pseudopleuronectes americanus, were collected from three fishery stocks in the United States (U.S.) during 2009–2013 to describe spermatogenesis. Spermatogenesis included rapid, synchronous transitions during autumn from mitotic spermatogonia to meiotic spermatocytes, then to haploid spermatids, and the release of spermatozoa into lobule lumen. As these transitions occurred several months prior to the spring spawning season, maturity of the testes are a poor indicator of spawning season, however, individuals can be readily evaluated prior to the spawning season for evidence of skipped spawning. Throughout this process, the gonad weight increased dramatically, with spawning-season maxima from 11 to 17% relative to individual somatic weight, suggesting considerable reproductive investment relative to other flatfishes. Fish condition also cycled seasonally, which indicated that males follow a capital breeding pattern that has been reported for female winter flounder. The timing of spermiogenesis was very similar in the fish among each U.S. stock (Southern New England [SNE], Georges Bank [GB], and the Gulf of Maine [GOM]) and published data from coastal Newfoundland, even though U.S. winter flounder spawned earlier in spring than fish in the Canadian stock. Male maturity varied in relation to both size and age, and over time, in patterns similar to those reported for female winter flounder, but at smaller and younger sizes than females. These intra-specific variations in reproductive seasonality, maturation, and skipped spawning suggest that winter flounder have the potential to adaptively respond to a dynamic environment in a region where ocean warming is occurring rapidly.