Assessing Mortality of Released or Discarded Fish Using a Logistic Model of Relative Survival Derived from Tagging Data

The mortality of released or discarded fish in commercial and recreational fisheries and in fisheries-independent research is a critical area of study for contemporary fisheries science. Key questions involving post-release survival include the following: What fraction of the bycatch discarded in a commercial fishery survives? What is the hooking mortality of fish released by recreational anglers, and how does this vary with the treatment and condition of the fish? What fraction of tagged fish do not survive the stress of capture and tagging? What percentage of hatchery-raised fish survive after stocking? To address these and related questions, we develop a two-step approach to estimating absolute post-release survival rates. First, fish are tagged and assigned to discrete classes based on their condition (from best to worst) at release. The relative survival of fish released in different conditions is estimated from tag returns by fitting a logistic model. Then, assuming that fish in the best condition survive to the same degree as fish that were not captured, absolute survival rates in the other categories are determined. Applying this method to one example from field research, we estimate that 69% of blacktip sharks Carcharhinus limbatus and 60% of bonnetheads Sphyrna tiburo survived the stress of gill-net capture, tagging, and release in a 1992-2004 study in Florida. Our method has broad application to determining the condition- or treatment-specific survival rates of released fish of many types in fisheries and research projects in which tag-and-recapture methods can be used.