A novel method for determining post-release mortality, behavior, and recovery period using acceleration data loggers
Recent declines in global fish stocks have raised questions regarding the sustainability of both extractive and catch-and-release fishing activities, and prompted efforts to quantify the total impact of fishing pressure. While at-vessel mortality rates are relatively easy to obtain, accurately assessing post-release mortality and sub-lethal behavioral effects has proven difficult. Acceleration data loggers (ADLs) represent a useful tool in post-release studies, but have yet to be fully utilized. The goal of this paper is to demonstrate the utility of ADLs in identifying mortality events, and to provide an example of recovery period quantification methods using acceleration-based swimming metrics. To illustrate the application of this method, we use sample data from blacktip sharks (Carcharhinus limbatus) captured and released in the Florida recreational shark fishery. Mortality events were inferred from stationary depth traces and the eventual cessation of all tailbeat activity, while posture information confirmed that the tag was still attached to the animal. We also detail how ADL data from surviving individuals were used to calculate 58 metrics of fine-scale swimming behavior. Using nonlinear mixed modeling, we found 19 of these metrics displayed a significant logistic relationship with time post-release, indicative of a recovery period. Calculated recovery periods ranged from 5.1 to 19.5 h, with a mean of 10.54 ± 3.78 h. The low cost of ADLs and their capacity for multiple deployments allows for relatively large sample sizes at a fraction of the cost of satellite tag studies. ADLs provide definitive evidence of post-release mortality, and also allow for the quantification of sub-lethal effects which can be used to measure the impact of different gear types or handling methods, even in species for which mortality events are rare.