Physiological Performance of Young-of-the-Year Atlantic Croakers from Different Atlantic Coast Estuaries: Implications for Stock Structure

Geographic variation in life history traits and population dynamics of Atlantic croakers Micropogonias undulatus found north of Cape Hatteras, North Carolina, suggests the possibility of two stocks along the U.S. Atlantic coast. The basis for this variation (i.e., genetic versus ecophenotypic) is unclear. Young-of-the-year Atlantic croakers were collected from three Atlantic coast estuaries (Delaware Bay, Delaware; Cape Fear River, North Carolina; and Indian River Lagoon, Florida) that represent the center and extreme distributional limits of the species' U.S. Atlantic coast range. Intrinsic growth capacity and cold tolerance were measured under common laboratory conditions to test for adaptive genetic variation in these traits. Results were used to evaluate the two stock hypothesis for the Atlantic croaker and to examine the integrity of Cape Hatteras as a possible genetic stock boundary. Growth capacity, feeding rate, growth efficiency, and cold tolerance were similar across geographic locations. Survival curves for Delaware and Florida Atlantic croakers were indistinguishable at each of four low temperatures tested (1, 3, 5, and 7°C), with neither group capable of surviving temperatures of 3°C or less. The suggested lack of adaptive variation we found in these physiological traits supports the hypothesis of a single genetic stock of Atlantic croakers along the Atlantic coast. Although severe winter temperatures may select for fast growth and cold-tolerant genotypes in northern estuaries, gene flow is apparently sufficient to preclude local genetic adaptation.