STOCK ENHANCEMENT OF COBIA Rachycentron Canadum IN SOUTH CAROLINA

Cobia Rachycentron canadum is a large, fast-growing coastal migratory pelagic fish with a distribution that includes most of the world's warm oceans.  In southeastern South Carolina, cobia enter high salinity estuaries to spawn in late spring and early summer.  These inshore spawning fish form a discrete population segment (DPS) that is genetically unique to three estuaries: St. Helena Sound, Port Royal Sound, and Calibogue Sound.  

As coastal populations have expanded, interest in fishing for cobia has increased and caused concerns about the sustainability of the fishing effort on the DPS.  As a result, the South Carolina Department of Natural Resources developed a stock enhancement program to evaluate the feasibility of supplementing wild cobia populations with hatchery-raised animals.   Hatchery juveniles (typically 60-100 mm) were produced using local wild broodstock and released in the Port Royal Sound system from 2005-2009 and 2012.  Releases by year class have been small in scale and ranged from 1,392-53,673 fish.

Following stocking, genetic tissue and otoliths were collected through fisheries dependent sampling.  Samples were genetically evaluated and compared with known broodstock genotypes to determine hatchery or wild origin.  Year class was assigned by otolith and percent contribution to year class by hatchery fish was determined (number of hatchery fish from year class/ total fish from year class *100).

Hatchery contributions to a specific year class within the estuary have ranged from 0% to 80% for a given collection year.  Releases from the 2007 year class (n=53,673) have continued to make a strong contribution to that year class within the estuary over time (31-80%) and continue to be collected through the 2015 collection year.  Hatchery fish have also exhibited site fidelity similar to their externally-tagged wild counterparts, returning to the same estuaries multiple years after release.  As the population of the DPS has continued to decline, a stock enhancement program utilizing genetically robust broodstock management may serve as a useful tool for managers alongside traditional creel and size limits.