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7231614 
Journal Article 
Modeling geoduck, Panopea abrupta (Conrad, 1849) population dynamics. II. Natural mortality and equilibrium yield 
Bradbury, A; Tagart, JV; , 
2000 
1 
Journal of Shellfish Research
ISSN: 0730-8000
EISSN: 1943-6319 
NATL SHELLFISHERIES ASSOC 
GROTON 
63-70 
WOS:000088636000013 
The natural mortality rate of peoduck clams, Panopea abrupta (Conrad, 1849), was estimated from data collected at 14 previously unfished sites in Washington State in order to predict the potential yield of the commercial fishery under various harvest rate strategies. The instantaneous rate of natural 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, schedules of sexual maturity, weight-at-age, and fishery selectivity-were estimated from the literature and file data. These parameter estimates were used to drive an age-based equilibrium yield model that predicted yield per recruit (YPR) and spawning biomass per recruit (SPR) over a range of fishing mortality rates. The model produced values of the instantaneous fishing mortality rate (F) for five commonly used constant harvest rate strategies. The fishing mortality rate producing maximum YPR (F-max) ranged from 0.053-0.100 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 (averaged over all sites), corresponding to an annual harvest rate of 2.7% of the exploitable biomass. The model was most sensitive to estimates of M, whereas growth, fishery selectivity, and sexual maturity schedules had relatively 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.