Research Document 1997/144
Evaluation of harvest models for manilla clam fisheries in British Columbia
By G.E. Gillespie and J. Bond
Abstract
Simple harvest models were used to evaluate the effects of various harvest rates on yields and stock size in Manila clam (Venerupis philippinarum [= Tapes philippinarum]) fisheries. Models used included the Washington State Department of Fish and Wildlife management program, depuration fisheries in British Columbia, and estimated harvest rates from size-limit fisheries. Data were taken from surveys completed by First Nations for co-management pilot programs.
Because recruitment patterns are not known for Manila clams, we examined effects of various harvest rates on the stock estimated from stock surveys, essentially the initial legal-sized stock and one year's recruitment.
The Washington State model is designed to partition the estimated stock over a four-year harvest period. Size limit models (constant harvest rates of 0.5 to 0.7) have the higher yield, but decrease stock size more quickly than other models. If recruitment is relatively high, size limit fisheries deplete detected stock in 3-4 years, depending on harvest rate. Under medium or low recruitment, size limit fisheries deplete detected stock in 2-3 years. The model employing a constant TAC of 25% of the initial legal stock estimate depletes that stock in 2, 3 or more than 4 years, depending on recruitment level. A 50% TAC model depletes detected stock in 3, 2 or 1 year.
High harvest rates produce high yields, but are more quickly dependent on annual recruitment to maintain harvest levels. Harvests resulting from lower harvest rates show decreased total yield, but greater consistency.
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