ObsCovgTools: Assessing observer coverage needed to document and estimate rare event bycatch

Observer program design and evaluation often overlook the challenges of documenting rare-event bycatch. To support and facilitate consideration of threatened, endangered, and protected species bycatch in evaluating observer programs and assessing fisheries impacts, we developed a software tool to assess observer coverage with respect to several objectives for documenting or estimating rare-event bycatch. The ObsCovgTools package for the R programming language, also available as an online application, predicts observer coverage performance for a given total fishery effort in relation to three metrics: (1) the conditional probability of observing any bycatch given that bycatch occurred in the fishery and the probability of any bycatch in the total fishery effort, (2) the upper confidence limit for total bycatch when none is observed, and (3) precision (coefficient of variation) of the bycatch estimate. We describe the tool; explore how specific observer coverage targets for these metrics vary with total effort, BPUE, and dispersion index; and apply it to evaluate observer coverage in the California drift gillnet fishery. Our results underscore the importance of considering effort as well as percentage in assessing how well an observer program documents bycatch. We caution that rare species interactions may not be documented in many observer programs, and should be anticipated through a complementary risk assessment approach. The tool’s modular design and open source programming approach encourage adaptation and augmentation to address additional objectives or complexities in sampling design or estimation.