Research Document - 2000/122
Preliminary Review of Experimental Harvest Rates in the Depuration Fishery for Intertidal Clams.
By G.E. Gillespie
Abstract
This paper reviews eight beaches managed experimentally in the depuration fishery for intertidal clams in British Columbia. The beaches were surveyed annually between 1997 and 1999, and total allowable catches calculated by applying harvest rates of 0.50, 0.25 or 0 to the estimated legal biomass of Manila clams.
Three of four beaches managed using a 0.50 harvest rate exhibited drastic declines, and were removed from the fishery. The fourth beach maintained stock levels at actual harvest rates of approximately 0.40. One of two beaches assigned a 0.25 harvest rate declined and was removed from the fishery. The second maintained stock levels for at least one year of harvest at an actual rate of 0.23. Three control beaches exhibited different stock trends: one showed increased legal densities, one remained relatively unchanged at low stock levels, and one declined throughout the three-year program. From these preliminary results, it was apparent that the harvest rates used were too high, and that there is no single harvest rate that ensures sustainability for all beaches.
Because recruitment is sporadic in clam populations, little information regarding recruitment patterns was gathered in the first three years of the program. The three beaches that exhibited significant recruitment were in a similar geographic area, perhaps indicating that recruitment fails or succeeds over larger areas, as opposed to on a beach-by-beach basis. However, these beaches all had relatively large stocks of legal clams, perhaps indicating that a large proportion of the larvae produced from a beach remain in the vicinity and settle on the same beach where they were spawned. The two hypotheses are not exclusive; conditions required for good recruitment may occur over a larger area, with the magnitude of recruitment on individual beaches in that area related to spawning stock size.
The paper proposes a management framework using biologically-based reference points. The limit reference point is a density of 30 legal clams/m2, at which time the beach is closed for recovery. Harvest rates increase gradually with increasing legal density, from 0.10 at densities between 30 and 70 legal clams/m2 to 0.20 at densities between 70 and 130 legal clams/m2, and finally to 0.40 at densities greater than 130 legal clams/m2. Beaches closed can be re-opened at densities above 70 legal clams/m2. This framework allows moderation of harvest rates in response to stock characteristics, benefiting the stock when densities are declining, and allowing increased production when densities are increasing.
The paper recommends that the harvest rates used in the current framework be reduced, that alternatives to constant harvest rate management be considered, that protocols for estimating total landed weight and species composition be established, and that beaches managed using constant total allowable catches from a single baseline survey be re-assessed and this management framework be re-evaluated.
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