Relative survival of gags Mycteroperca microlepis released within a recreational hook-and-line fishery: Application of the Cox Regression Model to control for heterogeneity in a large-scale mark-recapture study

• Gags Mycteroperca microlepis released in a recreational fishery were observed.
• Release conditions were related to subsequent mark-recapture rates.
• It was necessary to control for variable reporting and recapture rates.
• Relative survival was reduced for gags released in fair and poor condition.
• Overall estimated discard mortality was low in shallow water and increased with depth

From June 2009 through December 2012 fishery observers were placed on charter and headboat vessels operating in the Gulf of Mexico to directly observe reef fishes as they were caught by recreational anglers fishing with hook-and-line gear. The objective of this study was to relate injuries and impairments measured directly from gags Mycteroperca microlepis caught and released within the recreational fishery to subsequent mark-recapture rates. Due to the large spatial and temporal scales of the study design, it could not be assumed that encounter probabilities were equal for all individual tagged fish in the population. Also, changes in fishing effort following the Deepwater Horizon oil spill during 2010 in the Gulf of Mexico and drastically reduced recreational harvest seasons for gag during 2011 and 2012 were unanticipated during the design of this study. Therefore, it was necessary to control for potential covariates on encounter and recapture rates for gags tagged in different regions, different years, and different times of year. This analysis demonstrates the utility of the Cox regression proportional hazards model in comparing relative survival among gags released in various conditions while controlling for potential covariates on both the occurrence and timing of recapture events. A total of 3954 gags were observed in this study, and the majority (77.26%) were released in good condition (condition category 1), defined as fish that immediately submerged without assistance from venting and had not suffered internal injuries from embedded hooks or visible damage to the gills. However, compared to gags caught in shallower depths, a greater proportion of gags caught and released from depths deeper than 30 m were in fair or poor condition. Relative survival was significantly reduced (alpha <0.05) for gags released in fair and poor condition after controlling for variable mark-recapture rates among regions and across months and years when tagged fish were initially captured and released. Gags released within the recreational fishery in fair and poor condition were only 66.4% (95% C.I. 46.9–94.0%) and 50.6% (26.2–97.8%) as likely to be recaptured, respectively, as gags released in good condition. Overall discard mortality was calculated for gags released in all condition categories at 10 m depth intervals. There was a significant linear increase in estimated mortality from less than 15% (range of uncertainty, 0.1–25.2%) in shallow depths to 30 m, to 35.6% (5.6–55.7%) at depths greater than 70 m (p < 0.001, R2 = 0.917)