Potential Impacts of Minimum Weight Regulations for Pacific Bluefin Tuna

Simulation was used to estimate first-order effects on yield, total biomass, and spawning stock biomass for five minimum weight restrictions in the Pacific Bluefin tuna fishery. Projections included five levels of tolerated catch below the minimum size limit, thus producing 25 management scenarios. Data from the 2012 assessment were used in the simulations and the most recent five year averages for fishing mortality-at-age and numbers-at-age were used as the reference period for the projections. The results are best-case scenarios and represent what might happen given model assumptions, perfect implementation and no transfer of effort. In all scenarios, substantial long-term increases in biomass and yield were predicted. Maximum yield per recruit occurred with a minimum weight of 20 kg. A doubling of yield and a 10x increase in spawning stock biomass are theoretically possible. Short-term losses in yield occurred for one to three years following implementation. Work is needed to determine optimal management strategies whereby minimum size can be gradually increased as biomass rebuilds to minimize short-term losses in yield. Work is also needed to evaluate schemes in which a portion of the small tuna taken for grow-out operations are returned to the wild to mitigate removal of small tunas.