Abstract Winter flounder, Pseudopleuronectes americanus, is currently being evaluated as a stock enhancement candidate in New Hampshire, USA; however, little is known about the gonadal development or the sex ratio of cultured juveniles. To determine the size at gonadal differentiation, 327 cultured fish ranging from <20 to 110 mm total length (TL), in 10‐mm‐TL size classes, were examined histologically. Gonads had differentiated into testes and ovaries in fish ≥41 mm TL (98%), whereas the majority of fish (81%) smaller than 40 mm TL possessed undifferentiated gonads. A total of 313 cultured fish >40 mm TL were analyzed for sex ratio. In 2003, 67 females and 164 males were identified, yielding a sex ratio that was significantly skewed toward male (χ2= 40.7, df = 1, P < 0.001). This trend held true when cultured fish were sorted by age and length, with the exception of those fish 61–70 mm TL. This aberration probably was because of a small sample size in this length category. However, in both the 2004 and the 2005 cultured populations, flounder sex did not deviate from a 1:1 ratio (2004 χ2= 0.12, df = 1, P= 0.724 and 2005 χ2= 0.02, df = 1, P= 0.881). The 2003 data suggest that environmental or genetic factors may affect winter flounder sex determination; rearing manipulation studies in the hatchery are needed to confirm this hypothesis.
