Quantifying Delayed Post-Release Mortality in the Gulf of Mexico Recreational Red Snapper Fishery Using Acoustic Telemetry

Red snapper (Lutjanus campechanus) is considered an ecologically and economically important reef fish species in the Gulf of Mexico. Populations are highly regulated but have failed to recover as expected and stocks remain low. One possible reason for the slow recovery is that red snapper are susceptible to pressure-related injuries (barotrauma) and estimating delayed mortality rates in deep water environments is difficult. Delayed mortality may occur from reduced fitness, reduced predator avoidance ability, or inability to forage. To determine the extent of delayed post-release mortality, we tagged 40 red snapper with Vemco© V9AP accelerometer tags and released them at an oil platform at 50m depth. Unlike previous acoustic tagging experiments, fish were rapidly (1-3 min) tagged externally without anesthesia. Procedures were performed to best replicate normal fishing practices and minimize artifacts associated with tagging related surgeries (i.e. venting and use of only “survivors”). Acoustic receivers attached to the platform at depths of 30m and 50m recorded acceleration ((m/s)^-2) and depth (m) data for approximately 45 days. Two trials were performed with 20 fish tagged in summer and 20 fish tagged in winter. Three treatments for each trial included: 1) surface, non-vented; 2) weight descended hook; and 3) surface-released control. Control fish were captured and held for two weeks to allow recovery from barotrauma symptoms prior to experimental release. There was a significant difference in delayed mortality between the summer and winter trials with more tagged fish surviving past day five during the winter. Summer tagged fish displayed rapid detection drop-offs compared with winter tagged fish, which registered far more detections over a longer time period. There was also a difference in receiver detections among release treatments for the winter trial, with the weight descended hook treatment registering fewer mean detections fish^-1 day^-1 than surface released treatments. Acceleration rates differed depending on season and release treatment. These results suggest that temperature is critical in determining post-release survival or mortality. The method of release may also dictate the fate of discarded fish. Results from these experiments enhance our understanding of post-release mortality and behavior of red snapper and provide useful information that documents the fate of regulatory discards. Estimates of delayed post-release mortality from this study can be integrated into stock assessment models to achieve better calculations of overall mortality and ultimately more accurate estimates of population size.