Modeling geoduck, Panopea abrupta (Conrad, 1849) population dynamics. II. Natural mortality and equilibrium yield

The natural mortality rate of peoduck clams, Panopea abrupta (Conrad, 1849) , was estimated from data collected at 14 previously unfished sites in Wash ington State in order to predict the potential yield of the commercial fish ery under various harvest rate strategies. The instantaneous rate of natura l mortality (M) estimated by the catch curve method for geoducks of ages 28 -98 was 0.0226 y(-1). Other important life history parameters-growth, sched ules of sexual maturity, weight-at-age, and fishery selectivity-were estima ted from the literature and file data. These parameter estimates were used to drive an age-based equilibrium yield model that predicted yield per recr uit (YPR) and spawning biomass per recruit (SPR) over a range of fishing mo rtality rates. The model produced values of the instantaneous fishing morta lity rate (F) for five commonly used constant harvest rate strategies. The fishing mortality rate producing maximum YPR (F-max) ranged from 0.053-0.10 0 depending on the site growth parameters, but reduced SPR to 15-21% of the unfished level. F-values for the F-0.1 strategy ranged from 0.28 to 0.37, reducing SPR to 35-37% of the unfished level. Three harvest rate strategies that reduce SPR to either 35%, 40%, or 50% of the unfished level were also evaluated, with F-values ranging from 0.018 to 0.036. The F-40% strategy, currently adopted by Washington managers, was achieved with F = 0.028 (aver aged over all sites), corresponding to an annual harvest rate of 2.7% of th e exploitable biomass. The model was most sensitive to estimates of M, wher eas growth, fishery selectivity, and sexual maturity schedules had relative ly little effect on yield or SPR. Apparent shifts in recruitment during the past 30-45 y may have biased the estimate of M. Direct estimates of M and recruitment are therefore a high research priority if the model outputs are to remain useful.