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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 ±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 estimates 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.
The Bulletin of Marine Science is dedicated to the dissemination of high quality research from the world's oceans. All aspects of marine science are treated by the Bulletin of Marine Science, including papers in marine biology, biological oceanography, fisheries, marine affairs, applied marine physics, marine geology and geophysics, marine and atmospheric chemistry, and meteorology and physical oceanography.