Abstract
Bycatch estimation is a challenging, yet crucial component of fisheries management and the protection of endangered species. In U.S. waters of the Gulf of Mexico and South Atlantic (south Florida to North Carolina), bycatch from the shrimp trawling industry is a risk to the conservation of the threatened giant manta ray. Due to uncertainty in overall fleet effort and extremely low observer coverage, traditional design-based or ratio-based estimates of bycatch are highly uncertain. We attempted to develop improved model-based bycatch estimates that integrated results from a published species distribution model (SDM) for giant manta ray with precise shrimp trawling effort distribution data supplied by NOAA. Conceptually, a species risk of bycatch is a combination of the probability of the species being in the location, fishing effort taking place in the location, and the catchability of the species. Combining SDM results and fishing effort metrics from electronic logbooks provided a weighted covariate by year, season, and management zone that more precisely predicted probability of giant manta ray bycatch in the South Atlantic. However, this covariate did not significantly improve estimation for the Gulf of Mexico, likely because shrimp effort and manta distribution in the Gulf of Mexico were spatially correlated. The improved model does show promise for future use, and with more data, bycatch estimation results should be more precise for not only giant manta ray populations, but other endangered species within the Gulf of Mexico and beyond. NOAA can use the estimates to inform shrimp fishery management policy decisions, creating a more sustainable future for both the fishery and protected species.