Stock Assessment of Subtidal Geoduck Clams (Panopea abrupta) in Washington

WDFW is mandated to perform biological stock assessment of the commercial geoduck resource and to make annual recommendations on the Total Allowable Catch (TAC) for each geoduck management region. Systematically spaced strip transect surveys are used to estimate the density of harvestable geoducks within commercial tracts, and a sample of geoducks is taken from these transects to estimate average weight. Biomass estimates on commercial tracts are the product of mean biomass per unit area and the total area of the tract. Regional biomass estimates are the sum of all surveyed commercial tract estimates within the region. Regional TACs are the product of the regional biomass estimate and the recommended harvest rate. An age-based equilibrium yield model was used to predict the long-term consequences of various harvest rates, using geoduck life history parameters which were estimated from existing WDFW data and literature sources. The model predicts yield and spawning biomass per recruit over a range of fishing mortality rates. Five commonly-used constant harvest rate strategies were simulated with the model, including two based on yield-per-recruit analysis and three based on spawning biomass per recruit analysis. An F_40% strategy is recommended as a risk-averse policy for geoducks. Under an F_40% strategy, the recommended annual TAC is 2.7% of the current commercial biomass within a region.