Characterizing Uncertainty in Fish Stock Assessments: the Case of the Southern New England-Mid-Atlantic Winter Flounder

The reauthorization of the Magnuson-Stevens Act requires specification of scientific uncertainty associated with stock assessments. The scientific uncertainty associated with stock assessments of southern New England-mid-Atlantic winter flounder Pseudopleuronectes americanus is considered as a case study. Focus is placed upon the uncertainties associated with the assumptions, assertions, and choices (AACs) made in the stock assessment analysis. Two classes of AACs are discussed. The first class involves AACs that characterize the population dynamics of the stock; these AACs include the unit stock assumption, the problem of dealing with retrospective patterns, the method of averaging fishing mortality across cohorts to yield an annual value for fishing mortality, and the equilibrium structure of the stock. The second class of AACs is related to the choice of methods used to determine whether the stock is overfished; these AACs involve focusing on the maximum sustainable yield (MSY) proxy rather than MSY per se to determine overfishing levels. The MSY proxy approach is discussed and compared with heuristic calculations of the MSY approach. Arbitrary choices of instantaneous natural mortality and percent maximum spawning potential in the analysis can lead to an arbitrary decision on whether or not the stock is overfished. We conclude that there is considerable scientific uncertainty on the status of the southern New England-mid-Atlantic winter flounder stock. The uncertainty identifies critical unknowns in winter flounder population dynamics.