Environmental associations with broad-scale Japanese and Taiwanese pelagic longline effort in the southern Indian and Atlantic Oceans

Tunas are globally important to fisheries because of their broad distribution and high market value. However, these characteristics also complicate their management, owing to difficulties estimating population size and sustainable harvest levels. Fishing effort data can be difficult to interpret, as the magnitude and distribution of effort are the result of multiple factors, including environmental variation and targeting different species. Yet, resolving patterns in fishing effort may provide information enabling the setting of sustainable harvest levels, the management of bycatch and the projection of potential responses to environmental change. To better understand the allocation of fishing effort, we evaluated Japanese and Taiwanese longline effort in the southern Indian and Atlantic Oceans and their relationships to the environment using generalized additive models. The variables evaluated included sea surface temperature (SST), sea ice extent, climate indices, and seasonal cycles, and the results were interpreted in physical, ecological, and management contexts. Our findings indicated hierarchical relationships are impacting fishing effort, with a seasonal cycle an important component of most models, which we hypothesize relates to variation in species targeting. This was often followed by finer-scale environmental indices, including isotherms and sea ice, indicating region-specific dynamics impacting the magnitude and distribution of fishing effort. Additionally, effort in some regions was associated with climate indices, highlighting interannual variation. This first quantitative description of the environmental associations of multi-species tuna fleets in the Southern Ocean is a step towards an improved understanding of fleet behaviour.