Efforts to understand stock structure of summer flounder (Paralichthys dentatus) in North Carolina, USA

Understanding the stock structure of the summer flounder is critical to attempts to manage this species. Currently such research is particularly urgent due to increased interest in commercial culture and stock enhancement of summer flounder as this creates pressure to transplant fish among geographic areas. Studies of summer flounder in the coastal waters of North Carolina are of particular relevance to the stock structure due to the existence of a zoogeographic boundary at Cape Hatteras, NC. The importance of this boundary is being investigated through mark–recapture studies of adults, field sampling of larvae and laboratory experiments on larvae and juveniles originating from different brood stocks. Twenty-three thousand summer flounder were marked in coastal waters and movement of recaptured animals relative to season and the zoogeographic boundary analysed. Seasonal occurrence of larvae relative to this boundary was compared and animals were characterised in terms of fin ray numbers and size and developmental stage at arrival at the coast. In the laboratory we reared larvae from two brood stocks; one originating from the northern portion of the summer flounders range, and the other from North Carolina. These animals were used to determine the importance of temperature to fin ray formation and to compare growth of the two groups of larvae relative to temperature. Additional laboratory experiments include comparisons of salinity tolerance of larvae during the settlement period. Our results support the existence of different groups relative to this zoogeographic barrier and suggest that extensive movement of summer flounder from one region to another for stock enhancement or culture should be prohibited.