The pace of harvest and recovery in geoduck clam stocks fifty years into the fishery

Abstract

The commercial fishery for subtidal Pacific geoduck clams (Panopea generosa) in Washington State, USA, is substantial both by the amount of biomass extracted (2 million kg in 2019) and by the economic value (US $50 million annual). Management for this fishery, which began in 1970, is challenged by this species’ long lives, cryptic behavior, and recruitment variability. Current management, including a 2.7 % annual harvest rate, is based on an equilibrium yield model most sensitive to a natural mortality parameter estimated using sampled age distributions. We collected a large sample for aging from four new sites in Puget Sound to update the yield model and explore its applicability to individual management regions. Based on the new age distributions, natural mortality varies among regions, but is similar overall (0.0231 yr−1) to the value used in original formulation of the yield model (0.0226 yr−1), indicating that a change to the harvest rate is not needed if following the outputs of this model. However, concerns remain about the sustainability of the current harvest rate, particularly the time for harvested tracts (delineated harvest areas ranging in size from 0.06 to 1.45 km2) to return to their preharvest density. We calculated tract recovery rates from serial post-harvest density surveys, compared them to the expected recovery rate derived from the age-based equilibrium model, and investigated possible mechanisms behind fast or slow tract recovery. Using multiple scuba surveys spaced over four decades from 38 tracts in the South Puget Sound, we estimate that tracts recover on average at 0.03 geoducks per square meter per year. The projected average time to recovery is 55 years, compared to a projection of 39 years when the yield model was developed. There is some support for the hypothesis that tract recovery rates have slowed in recent decades. Although there were strong spatial patterns, with higher recovery in central basins compared to within inlets, there was only weak support for the hypothesized mechanisms behind tract recovery such as mean current speed. Our inability to clearly relate recovery to physical variables such as habitat, paired with extreme interannual variability in recruitment suggested by the age distributions, supports the hypothesis that the strength, timing, and spatial extent of episodic recruitment events is the primary driver of tract recovery. Currently, harvestable biomass estimation and fishery management assume that all previously harvested tracts will eventually recover to preharvest densities. However, in the South Puget Sound, only 40 % of tracts are likely to recover on a 50-year time frame relevant to management. Despite our validation of the key parameter used in the yield model, we advocate switching geoduck management to a tract-recovery-based strategy in which harvest is paced to known tract recovery, and biomass estimation is restricted to only those areas likely to hold renewable resource. This study highlights the importance of evaluating recovery patterns of stocks once data over appropriate timescales are available, and comparing to the management model to determine if fishery goals are being met.