A POTENTIAL FRAMEWORK FOR EVALUATING THE EFFICACY OF BIOMASS LIMIT REFERENCE POINT IN THE PRESENCE OF NATURAL VARIABILITY AND PARAMETER UNCERTAINTY: AN APPLICATION TO NORTHERN ALBACORE TUNA (THUNNUS ALALUNGA)

The international fisheries managed by ICCAT are typically regulated with the objective of harvesting, on average, the maximum sustainable yield (MSY). However, even in a fishery that generally realizes this goal, the stock biomass will fluctuate above and below the critical level defined as BMSY. Furthermore, the stock will be designated as “overfished” each time the biomass falls below BMSY. Since excursions below BMSY when fishing at FMSY will occur (e.g. due to fluctuations in recruitment), it may be sensible to permit biomass to decline below BMSY without initiating regulatory measures as long as it remains above an accepted biomass limit (Blim) if BMSY is considered merely a target. Alternatively, target fishing mortality rates could be adopted such that the expected probability of the stock excursion below BMSY due to natural variability and uncertainty in estimation is negligibly low, thus establishing a target Btarg > BMSY. This document presents a framework for evaluating potential Blim and Btarg and discusses the probability of rebuilding a depleted stock under various definitions of Blim, in the presence of natural variability and parameter uncertainty. To illustrate the method, this document includes an example based on the most recent stock assessment of northern albacore tuna.