The effects of shelf-edge fishing on the demographics of the gag, Mycteroperca microlepis, population of the southeastern United States

Christopher C. Koenig and Felicia C. Coleman
Institute for Fishery Ecology (FSU/NMFS), Department of Biological Science, Florida State University, Tallahassee, FL 32306-1100. CCK phone: 850-644-4509, FCC phone: 850-644-2019, fax: 850-644-9829, koenig@bio.fsu.edu, coleman@bio.fsu.edu

Robert W. Chapman, Mark R. Collins, Patrick Harris, John McGovern, George R. Sedberry, and David M. Wyanski
Marine Resources Research Institute, South Carolina Department of Natural Resources, 217 Ft. Johnson Road, Charleston SC 29422. phone: 843-762-5045, fax: 843-762-5110, chapmanr@mrd.dnr.state.sc.us, collinsm@mrd.dnr.state.sc.us, harrisp@mrd.dnr.state.sc.us, mcgovernj@mrd.dnr.state.sc.us, sedberryg@mrd.dnr.state.sc.us, wyanskid@mrd.dnr.state.sc.us

Allyn G. Johnson
33728 Florida Avenue, Panama City, FL 23405; phone: 850-769-1908, allyn@digitalexp.com

Demographic changes in the gag, Mycteroperca microlepis, population (size structure, size at maturity, and sex ratios) of the southeastern U.S. have occurred in concert with increased fishing pressure. Comparison of relative sex ratios over the last two decades derived from hook and line catches during aggregation (Dec-Mar), post aggregation (Apr-Jul) and pre aggregation (Aug-Nov) periods in comparable areas of the Gulf and Atlantic indicate significant declines in the proportion of males. An age- or strictly size-mediated sex change process in gag is not supported by the data, whereas a social component to sex change is. Because sex change in gag occurs in temporal proximity to the aggregation period, and because large gag (> 900 mm TL) have also experienced a decline in the proportion of males, sex change probably depends on social information available only during aggregation. A mechanism is proposed for the observed decline in the relative proportion of males which involves disruption of the sex change process by aggregation fishing and subsequent loss of newly transformed males through fishing on shelf-edge reefs in post and pre aggregation periods. The relative benefits of marine reserves are discussed in light of the observed demographic changes.