Release mortality of endangered Warsaw grouper Hyporthodus nigritus: A state-space model applied to capture-recapture data

Warsaw grouper Hyporthodus nigritus is a large-bodied, deep-water, demersal fish listed on the IUCN Red List as Critically Endangered. In the USA, harvest of Warsaw grouper in recent decades has been heavily limited or prohibited. However, Warsaw grouper is part of a multispecies snapper-grouper fishery, making this species vulnerable to incidental capture (and release) as bycatch, even if not targeted for harvest. Therefore, release mortality is a primary concern for the recovery of this endangered species, especially given the potential barotraumatic effects of capture. Here, we used a long-term (11 yr) data set containing capture-recapture information on Warsaw grouper from a shallow site offthe Atlantic coast of Florida. The data were collected through a citizen-science program, the South Carolina Marine Game Fish Tagging Program. We fit a state-space model to these data that accounts for-and provides estimates of- release mortality as a function of length. These estimates quantify the combined effect of imme - diate and delayed mortality. We found that release mortality increased with length, from expected values lower than 10% to values exceeding 70%. At a total length of 700 mm, near our observed mean, the expected release mortality was 34% (95% credible interval of 3-57%). In general, release mortality estimates were lower than might be assumed (~100%) for a deep-water grouper. We suggest possible explanations for this result, such as the relatively shallow depth (~49 m) of this study and the careful treatment of released fish, and conclude by discussing implications for future research and conservation.