Scientific Uncertainty in Stock Assessments Evaluated

Steve Ralston (Fisheries Ecology Division) and Ray Conser (Fisheries Resources Division), both members of the Pacific Fishery Management Council’s Scientific and Statistical Committee (SSC), attended a meeting of the SSC’s groundfish and coastal pelagic species subcommittees to discuss new requirements of the Magnuson-Stevens Reauthorization Act (MSRA). The meeting, held September 1-2 at the Alaska Fisheries Science Center in Seattle, was organized to establish an analytical approach to representing scientific uncertainty in stock assessments used by the Council.

In particular, the MSRA requires that SSCs determine the level of scientific uncertainty in stock assessments so that catch levels associated with overfishing level (OFL) can be reduced to insure that overfishing does not occur. Reduction of the OFL to account for scientific uncertainty produces an allowable biological catch (ABC) and under the law it is the responsibility of the SSC to oversee that reduction.

The PFMC SSC’s approach to this problem will be to characterize the “within” level of uncertainty in assessments through standard statistical procedures (i.e., inversion of the Hessian matrix) and to quantify a “between” level of uncertainty by a retrospective view of past assessments conducted on a stock. If treated as additive, the probability of exceeding the overfishing level (P*) can be calculated and a policy choice made to establish a buffer between the OFL and the ABC.

Further analysis of these issues will be conducted over the next two months, and then presented to the PFMC at their November meeting in Costa Mesa, California.

(September 8, 2009)