Further investigations of simple “fixed proportion” candidate management procedures for North Atlantic Bluefin tuna using operating model package version 3.3.0.
Simple constant (intended) proportion CMPs are applied to the 16 conditioned Operating Models (OMs) in version 3.3.0 of the Package. Ranges of the two control parameters for west and east proportions are selected to give reasonable trade-offs between catch and stock recovery (where needed). For nearly all of the OMs this is readily achieved. However, problems arise for some of the abundance factor B OMs (with lower west stock abundances matching those in the most recent assessment) coupled to an unchanging Beverton-Holt stock-recruitment function for the west stock (recruitment factor 2). For one of these scenarios, even with no catch from the west area, an already depleted west stock declines under catches in the east that are well below present levels, because sufficient bluefin of western origin migrate to the east area and can be harvested there. The key immediate question then becomes whether such OM scenarios are sufficiently plausible that they need to be taken into account when seeking an MP which evidences robustness over a range of reasonably plausible levels of uncertainty.