Inshore spawning of cobia (Rachycentron canadum) in South Carolina
Lefebvre, Lyndsey Stephanie
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Inshore spawning of cobia was investigated in Port Royal Sound (PRS) and St. Helena Sound (SHS), South Carolina. Cobia caught in PRS, SHS, and offshore were collected from anglers and South Carolina Department of Natural Resources employees between April and June 2007 and 2008. The mean gonadosomatic index (GSI) for females was 7.3, higher than that reported in previous studies, and females collected inshore had significantly higher GSI values then those collected offshore (7.8 and 5.6, respectively; p=0.0023). Histological analysis of ovaries demonstrated the majority of specimens were in the late developing stage and none were immature or recovering. Females with histological indications of prior spawns were collected inshore and offshore. The collection of two gravid females caught in PRS and SHS indicate spawning was occurring locally. As secondary evidence of spawning, ichthyoplankton surveys were conducted in both estuaries in May and June of 2008. For use in positively identifying and aging eggs collected, two growout studies were conducted using hatchery-reared cobia eggs to provide a temperature based time-series reference collection. A total of 924 eggs and 45 larvae were collected and identified as cobia from plankton stations located 12.1 to 20.6 km from the mouth of PRS and 5.1 to 14.3 km from the mouth of SHS. Visual identification of eggs based on morphology were supported with an analysis of covariance, which demonstrated that the relationship between egg and oil droplet diameters did not differ significantly between hatchery reared and plankton gathered eggs (p=0.35; R2=0.61). Using the estimated ages of eggs collected in the plankton survey, spawning times of wild cobia occurred between 1530 and 2145 hours. Based on these findings, PRS and SHS provide spawning habitat for cobia. Current management strategies for cobia in South Carolina may need revision in light of the new information.