Sex-Linked Growth Divergence of Summer Flounder from a Commercial Farm

In the wild, it is known that adult female summer flounder, Paralichthys dentatus, grow at a faster rate and to a larger body size than males. The age or size at which this divergence in growth rate occurs is not known. At GreatBay Aquafarms, Inc., the sex ratios of three production lots of juvenile fish was examined to determine if females grow faster than males under commercial cultivation and to identify the age and/or size at which sex-linked growth divergence begins. Production lots 99-2, 99-1, and 98-3, ages 8, 12, and 15 months (post-hatch), respectively, were sampled. The gonadal tissue from fish smaller than 60 mm was examined by histology. Fish larger than 12 cm were dissected, and gross observation was used to determine sex. With the exception of two samples that appeared to be males, gonadal tissue from fish under 60 mm appeared undifferentiated. In production 99-2 (8 months) no significant association between the percentage of female fish and body size was found. However, in productions 99-1 (12 months) and 98-3 (15 months) percentage of female fish increased significantly with body size group. At the time of sampling, the average weight of females from lots 99-2, 99-1, and 98-3 was 1.05, 1.1, and 1.4 times heavier, respectively, than males. Results from this study suggest that sex differentiation in summer flounder occurs at 6 to 12 cm, and indicates clear value of all-female production for the commercial cultivation of summer flounder.