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IP11 Investigations of the Columbia River and Estuary (USA)
(IP090) Dredge entrainment of Dungeness crab in the Columbia River Estuary.
Williams, G1, Pearson, W1, Skalski, J2, Larson, K, 1 Battelle Marine Sciences Laboratory, Sequim, WA, USA2 University of Washington, Seattle, WA, USA
ABSTRACT- Ongoing maintenance dredging, as well as proposed deepening, of the Columbia River Estuary (CRE) navigation channel has raised concerns about dredge entrainment effects on Dungeness crab. Sponsored by the Portland District of the U.S. Army Corps of Engineers, scientists at the Pacific Northwest National Laboratory provided direct measurements of Dungeness crab entrainment rates aboard the Corps dredge Essayons at multiple locations (River Miles -3 to 24) in the CRE in 2002. A modified Dredge Impact Model used summer 2002 entrainment rates and total volumes of materials dredged to project crab entrainment, adult equivalent loss, and loss to the fishery associated with maintenance dredging in the mouth and projected deepening in the upper river. In the mouth, the entrainment rate for all age classes over all sampling was 0.06 crab per cubic yard (cy) (i.e., one crab per approximately 16.6 cy dredged). In the upper river, entrainment rates of all crab age classes ranged from 0.0 at Miller Sands to 0.224 crabs per cy at Desdemona Shoals in June 2002. Total loss to the fishery estimated for maintenance dredging at the mouth was about 10,000 crabs per year. Worst-case projected fishery losses at upriver locations associated with construction and maintenance dredging over 10 years for channel deepening represented approximately 1% of the annual crab landings for the Washington and Oregon region around the Columbia River (5.3 million crabs from 1991 to 2001). Age 2+ crabs had the highest entrainment rate in most areas, whereas 0+ crab had the lowest. Furthermore, female crab had higher entrainment than did males in some locations. Taken together, the studies also showed that at salinities below 16 psu, crab entrainment rates fell logarithmically as bottom salinity declined. This relationship may be used to reduce crab entrainment by adjusting time of dredging.
Key words: Cancer magister, Dredge
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