Post-release survival of red snapper (Lutjanus campechanus) and red grouper (Epinephelus morio) using different barotrauma mitigation methods

Abstract

Captured fish that are released due to fishing regulations, catch-and-release efforts, or nontargeted bycatch are susceptible to mortality, particularly for species that are subject to barotrauma when retrieved from deep depths. To mitigate the effects of barotrauma, venting tools are commonly used to release air trapped in the gas bladder and enable fish released at the surface to return to their depth of capture. Recently, fishery managers have begun to promote an alternative method that involves recompression via the rapid descent of fish to their depth of capture or to a depth at which they can swim on their own. For some economically important species in the Gulf of Mexico, such as red snapper (Lutjanus campechanus) and red grouper (Epinephelus morio), discards account for more than 75% of total recreational catch. Because these two species are particularly susceptible to barotrauma, it is imperative to determine best practices for barotrauma mitigation. Here, we used mark-recapture to compare relative survival of both red snapper and red grouper after release using venting and recompression methods. We found that vented red snapper had similar tag return rates compared to those that were recompressed to a depth of 10 m. However, red snapper recompressed to 20 m or deeper had significantly higher return rates (up to 2.5 times higher) compared to those that were vented, indicating a lower discard mortality for individuals that were released using descender devices. The patterns were qualitatively similar for red grouper, with tag return rates 2 times higher for fish recompressed deeper than 20 m compared to vented fish. However, these results for red grouper were not statistically significant, potentially due to insufficient power (Type II error), a reduced relative tolerance to barotrauma compared to red snapper, or a combination of both. For both red snapper and red grouper, tag return rates were higher when fish were recompressed to deeper depths, but descending them all the way to the bottom was not necessary. Indeed, video observations further indicated that both species regained swimming abilities at depths shallower than depths of capture, although red grouper needed to be descended deeper than red snapper. The combined results of this study can inform fishery managers and anglers in the region on the apparent benefits of using recompression as a barotrauma mitigation method to improve post-release survival, and the species-specific needs of red snapper and red grouper.