Evidence to inform spatial management of a western Pacific Ocean tuna purse seine fishery

Abstract

Fisheries can have profound impacts on co-occurring species exposed to incidental capture, particularly those with life history traits that make them vulnerable to elevated mortality levels. Fisheries spatial management holds substantial potential to balance socioeconomic benefits and costs to threatened bycatch species. This study analyzed observer program data for a western Pacific Ocean tuna purse seine fishery to estimate the effect of the spatial and temporal distribution of fishing on catch rates of target and at-risk species by fitting spatially-explicit generalised additive multilevel regression models within a Bayesian inference framework. Mean field prediction surfaces defined catch rate hotspots for principal market tunas, silky sharks, rays and whale sharks, informing the development of candidate area-based management strategies. Due to sample size limitations, odontocete and marine turtle catch geospatial patterns were summarized using 2D hexagonal binning of mean catch rates. Effort could be focused in two areas within core fishing grounds in the Solomon and Bismark Seas to reduce overlap with hotspots for silky sharks, rays and whale sharks without affecting target catch. Effort could also be shifted outside of core fishing grounds to zones with higher target tuna catch rates that would also reduce overlap with hotspots for at-risk species. However, two tuna warmspots overlapped silky and whale shark warmspots. Sparse and small marine turtle and whale shark hotspots occurred across the fishing grounds. Research on the economic and operational viability of alternative spatial management strategies is a priority. A small subset of sets had disproportionately large odontocete captures. Real time fleet communication and move-on rules and avoiding sets on dolphin schools might reduce odontocete catch rates. Management of informative operational predictors such as set association type and mesh size present additional opportunities to balance catch rates of at-risk and target species. A transition to employing output controls that effectively constrain the fishery would alter the spatial management strategy to focus on zones with the lowest ratio of at-risk bycatch to target tuna catch. Findings inform the design of alternative spatial management strategies to avoid catch rate hotspots of at-risk species without compromising the catch of principal market species.