Abstract
Between 1997 and 2011, The National Marine Fisheries Service conducted 50 depletion experiments to estimate survey gear efficiency and stock density for Atlantic surfclam (Spisula solidissima) and ocean quahog (Arctica islandica) populations using commercial hydraulic dredges. A model formulated for this purpose, the Patch Model, was used to estimate gear efficiency and organism density. The range of efficiencies estimated is substantial, leading to uncertainty in the application of these estimates in stock assessment. A simulation protocol was developed to examine sources of uncertainty in Patch Model estimates. Analysis of simulations showed that uncertainty in the estimates of gear efficiency from depletion experiments was reduced by higher numbers of dredge tows per experiment, more tow overlap in the experimental area, a homogeneous as opposed to patchy distribution of clams in the experimental area, and the use of gear of inherently high efficiency. Stock density was of lesser importance, though still contributing to estimated uncertainty. Simulations suggest that adapting the experimental protocol during the depletion experiment by adjusting tow number and degree and dispersion of tow overlap may substantively reduce uncertainty in the final efficiency estimates. Simulations also suggest that the pattern of population dispersion in the experimental area is, and will likely remain, an important source of uncertainty, which may, however, be mitigated by updating experimental design during the course of the experiment.
Published Version
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