FORECASTING GENETIC IMPACTS OF NET PEN FAILURES ON GULF OF MEXICO COBIA POPULATIONS USING INDIVIDUAL-BASED MODELING  

Tanya L. Darden*, John D. Robinson, Allen E. Strand, and Michael R. Denson
 Marine Resources Research Institute
 South Carolina Department of Natural Resources
 Charleston, SC 29412
 dardent@dnr.sc.gov

Global aquaculture production has expanded rapidly over the last half-century, with present production levels accounting for more than 50% of the global seafood demand.  Offshore cage-based fish farms are highly cost-efficient for production and grow out, and there is great interest in expansion of net pen operations in U.S. waters.  However, the development of net pen farming has raised concerns over the possible genetic and ecological impacts of escaped fish on wild populations.  We used a forward-time individual-based simulation model to investigate the potential impacts of net pen failures over the short term (50 years) on standing genetic variation of native cobia stocks in the Gulf of Mexico. These effects were assessed through simulations with varying net pen failure rates, broodstock sizes, and broodstock source populations.  In response to these parameter changes, we examined several genetic response variables focused on the levels of standing genetic variation in the population and the degree of temporal genetic differentiation within the native population. As expected, higher net pen failure rates resulted in greater genetic impacts on the native cobia population.  Additionally, the use of more genetically differentiated source populations resulted in larger influxes of non-native genetic variants and greater temporal genetic differentiation in the native population following net pen failure.  Our results highlight the importance of considering the appropriate source of broodstock in net pen aquaculture systems, quantify levels of introgression and genetic differentiation associated with alternative management practices, and provide recommendations for broodstock selection and maintenance.  A better understanding of the potential influences of net pen failure rate, broodstock number, and broodstock source on the genetic composition of the native stock will allow for the development of management protocols for responsible net pen aquaculture operations.