Characterizing the genetic population structure of and genetic effects of cold winters on spotted seatrout (Cynoscion nebulosus) in the southeastern United States

Spotted seatrout, a recreationally important fish in SC, have suffered substantial population declines during the cold winters of 2000, 2009, and 2010 when water temperatures dropped substantially below long-term averages. The repetitive occurrence of these winter-kills over a short timescale has prompted the SCDNR to investigate stock enhancement as a management option. A current evaluation of the genetic population structure of spotted seatrout in the southeastern U.S. is necessary prior to stocking. I assessed the genetic population structure across eight estuaries along the southeastern U.S. from NC to GA using 13 microsatellite markers. Results suggest significant population differentiation between fish in NC compared to SC and GA, with New River, NC serving as an area of integration between northern and southern spotted seatrout. Although I saw a significant break in gene flow between these areas, the overall pattern throughout the sampling range represents an increase in genetic differentiation related to the degree of geographic separation. The isolation-by-distance pattern supports fine-scale differences in the genetic composition of adjacent estuaries and suggests that stocking should be confined within 100 km of the locality of broodstock collection to maintain the natural gradient of genetic variation along the southeastern U.S. coast. The genetic diversity and effective population size of Charleston Harbor spotted seatrout were also evaluated temporally in relation to winter-kills. Spotted seatrout catch per unit effort significantly influenced the genetic diversity, although only minor changes were observed and the effective population size remained high. It appears that short overlapping generations allow spotted seatrout to genetically recover during population growth and maintain moderate levels of genetic diversity.