Using a stock assessment framework to examine circle hooks: exploring changes in catchability and implications for management

A presumed conservation benefit of circle hooks is that they reduce catchability (q) and therefore bycatch of non-target species. While these changes may benefit a fish stock, they are difficult to incorporate in a stock assessment context, particularly for models that rely on fishery-dependent data, because few experiments exist that quantify the effects of circle hooks for a given species over appropriately large spatial scales. Consequently, to develop management advice, it may be necessary to model assumed changes in q within the adopted stock assessment model framework. Here we present a case study of bigeye tuna (Thunnus obesus Lowe, 1839) a highly migratory species managed by the International Commission for the Conservation of Atlantic Tunas (ICCAT) and explore the management implications of changes in q within a multi-fleet, age-structured assessment context. This study demonstrates that changes in q on the order of plus or minus 30% are sufficient to cause notable differences in the magnitude of common management reference points estimated by stock assessment models. Relative to a base model that assumed a constant q, models that incorporated a theoretical reduction in q produced higher stimulates of spawning stock biomass and maximum sustainable yield, and lower estimates of fishing mortality while a theoretical increase in q had the opposite effect. The magnitude of the change was dependent on the number of fisheries affected. We conclude that carefully designed studies are essential to quantify the effects of a proposed gear change and to inform the appropriate parameterization of stock assessment models.