Research Document - 2005/052

Evaluation of Assessment and Management Frameworks in the British Columbia Depuration Fishery for Intertidal Clams

By Gillespie, G.E., W.C. Hajas, J.S. Dunham

Abstract

We reviewed the experimental program in the depuration fishery for intertidal clams in British Columbia. The program consisted of beaches that were regularly surveyed between harvests to evaluate the effectiveness of various harvest rates in ensuring sustainable populations and harvest opportunities. Five beaches were evaluated: three unharvested controls (Mill Bay, Royston and Wall Beach) and two harvested beaches (Booth Bay and Goldstream). Two other beaches, China Cloud Bay and Long Bay, were afforded unrestrained harvest opportunities followed immediately by surveys to establish harvest rates in non-quota clam fisheries. Simple production models currently used to set harvest thresholds were re-evaluated, and population models were used to project biomass, abundance and quotas for several years following surveys.

Population responses to experimental harvest rates changed little from the previous evaluation. Booth Bay remained a highly productive beach even at high harvest rates (25-53%). Population levels and production at Goldstream remained relatively stable at harvest rates of 10-20%. The controls exhibited a range of responses, with a declining trend at Wall Beach and increasing trends at Mill Bay and Royston. None of the new information suggested that target and threshold limits currently used to set harvest rates required changing. Information from China Cloud and Long Bays indicated that harvest rates were approximately 11-13%, considerably less than previous estimates of unrestrained harvest rates.

Relatively low vulnerability of small clams to survey methods was noted from Industry surveys, and vulnerability and sampling error required reconciliation of survey data before the population model was used. The population model could be used to project quotas for approximately three years. Only the use of the median values (0.50 quantiles) of the projected populations yielded practical projections; more precautionary quantiles (0.05 and 0.25) resulted in rapid reductions in quotas. Our opinion is that the model allows for at least one year of projected quota in most cases before declines in quotas would motivate Industry to re-survey.

The paper recommended that current reference points for setting harvest rates be maintained, that the population model can be used to project population characteristics and quotas, and that issues surrounding vulnerability of small clams in Industry surveys be further evaluated.

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