David B. Sampson
Oregon State University
| Email: David.Sampson@oregonstate.edu | OSU Disclaimer |
Dover sole have been an important target of West Coast trawlers since before World War II. Annual landings from U.S. waters averaged 12,792 mt during the 1970s, 18,393 mt during the 1980s, and 12,367 mt during the 1990s. According to the new stock assessment for West Coast Dover sole (Sampson and Wood, 2001), there has been relatively little change in the size of this stock since the early 1990s. Despite this, however, the Optimum Yield (OY) values adopted by the Council for 2002 (7,440 mt) and for 2001 (7,677 mt) are considerably smaller than the OY for 2000 (9,426 mt). Two main factors account for the reduced allowable harvest of Dover sole: changes in the target fishing rate and the "40-10" harvest control policy.
The annual Acceptable Biological Catch (ABC) values for 1991 through 1999 were based on an F35% target fishing rate. The F35% rate of fishing mortality is the rate that reduces the spawning potential per female to 35% of the theoretical level that would be observed in an unfished stock. At the F35% fishing rate the Dover sole ABC for 2002 would be 10,046 mt, but would decline to 9,277 mt for 2006 because of projected declines in the exploitable stock biomass. In June 2000 the Pacific Fishery Management Council (PFMC) endorsed the recommendation of the West Coast Groundfish Harvest Policy Workshop that F40% be used as the default target rate of fishing mortality for flatfish species. Modeling studies and expert opinion suggested that F40% is a reasonable approximation to the rate of fishing mortality that produces the maximum sustainable yield. For a given stock size, fishing at F40% yields a smaller catch than fishing at F35% because more fish must be left unharvested to obtain the increased spawning potential. The Dover sole ABC value for 2001 was based on an F40% fishing rate (7,151 mt from the Vancouver through Monterey INPFC regions plus 526 mt from the Conception region, half the historic average catch). The ABC for 2002, also based on F40%, is 8,510 metric tons, declining to 8,282 tons for 2006.
Although fishing at the F40% rate conserves more fish than fishing at F35%, results from the 2001 stock assessment indicated that the Dover sole stock probably would decline if exploited at the F40% fishing rate. For example, the diagonal line in the graph below shows the theoretical relationship between recruitment and spawning biomass at the F40% fishing rate. Most of the estimates of annual recruitment fall below and to the right of this line, implying that the spawning biomass left after fishing would not be sufficient to replace the recruits that originally produced that spawning biomass. Also, it appears that the biomass of Dover sole in recent years has been supported primarily by two very strong year-classes, born in 1988 and 1991. If those year-classes instead had been of average strength, recent harvest levels would have caused the stock biomass to decline. The biomass from those two year-classes has now largely been exhausted.
Preliminary values for the Dover sole ABC for 2002, recommended by the Groundfish Management Team at the September 2001 Council meeting, ranged from 8,510 to 6,142 mt, with the low value based on an F50% target fishing rate.
To comply with Federal regulations and the National Standard Guidelines, in 1998 the Council amended its groundfish fishery management plan and adopted the so-called "40-10 harvest control" policy, which specifies how to calculate the Optimum Yield for a stock. When a stock's biomass is below the size that produces the Maximum Sustainable Yield (MSY), which is usually assumed to be at 40% of the unexploited stock size, harvests are reduced so that the stock will more rapidly rebuild to the more productive MSY level (graph on next page). For example, suppose the ABC is 1000 tons when the stock is at 40% of the unexploited stock size. If the stock drops to 30% of unexploited the ABC will be 750 tons (3/4 of 1000 tons) and the OY will be 667 tons (2/3 of 1000 tons, equivalent to 88.9% of the ABC). If the stock is at 20% of unexploited the ABC will be 500 tons (1/2 of 1000 tons) and the OY will be 333 tons (1/3 of 1000 tons, equivalent to 66.7% of the ABC). If a stock's biomass is at or below 10% of the unexploited stock size, the OY is set to zero. The 2001 Dover sole stock assessment estimates that the spawning stock biomass at the start of 2002 will be 29% of the unexploited level. Therefore, the Dover sole OY for 2002 is 87% of the ABC.
The last assessment for Dover sole (Brodziak et al. 1997) was completed before the Council's adoption of the 40-10 harvest control policy and the requirement that assessments provide estimates of the unexploited stock size and the stock biomass that produces MSY. Because the 1997 assessment did not estimate the unexploited Dover sole stock biomass, the 40-10 harvest policy could not be applied when setting the OY values for 1998-2001.
The Dover sole Optimum Yield value adopted by the Council for 2002 is almost the same as the Council's 2001 OY value, but both are considerably smaller than the 2000 OY. The recent values are lower because of a more conservative target fishing rate and because the Council's 40-10 harvest policy mandates reduced harvests because the stock is below the level that produces the Maximum Sustainable Yield.
Brodziak, J., L. Jacobson, R. Lauth, and M. Wilkins. 1997. Assessment of the Dover sole stock for 1997. Appendix in: Status of the Pacific Coast groundfish fishery through 1997 and recommended biological catches for 1998: stock assessment and fishery evaluation. Pacific Fishery Management Council, Portland, OR.
Sampson, D.B. and C. Wood. 2001. Stock Status of Dover Sole off the U.S. West Coast in 2000. Appendix in: Status of the Pacific Coast groundfish fishery through 2001 and recommended biological catches for 2002: stock assessment and fishery evaluation. Pacific Fishery Management Council, Portland, OR.
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