PNNL

Abstract

To address concerns about crab entrainment during maintenance dredging at the Mo uth of the Columbia River, direct measurements of crab entrainment rates were co nducted during the summer of 2002 from River Mile 3 to River Mile +3. The entr ainment rate for all age classes over all sampling in the MCR was 0.0603 crabs p er cy. The sex ratio of the older crabs entrained in the MCR was significantly skewed to the females. A modified DIM was used to calculate the entrainment (E) , Adult Equivalent Loss (AEL) at Age 2+ and Age 3+ and the Loss to the Fishery ( LF) for the dredged volumes accomplished in 2002 and for the five-year average d redged volumes (both for the Essayons and the contractor dredges). For both set s of projections, the coefficients of variation on the E, AEL, and LF were all u nder 5%. For the MCR total dredged volume (4,600,378 cy) in the summer of 2002, the estimated AEL at age 2+ was 180,416 crabs with 95% confidence limits from 1 63,549 to 197,283 crabs. The AEL at age 3+ estimated for the summer 2002 in the MCR was 81,187 with 95% confidence limits from 73,597 to 88,777 crabs. The pro jected LF for summer 2002 in the MCR was 10,471 with 95% confidence limits from 9,537 to 11,405 crabs. For the five-year average total MCR dredged volumes (4,3 91,872 cy), the estimated AEL at age 2+ was 172,238 crabs with 95% confidence li mits from 156,135 to 188,341 crabs. The AEL at age 3+ estimated for the MCR was 77,507 with 95% confidence limits from 70,261 to 84,753 crabs. The projected L F was 9,997 with 95% confidence limits from 9,105 to 10,889 crabs. Because fema le crabs appeared in the entrainment samples at a higher rate than did males, ab out 82% of the AEL at Age 2+ in the MCR was comprised of female crabs. Salinity in dredged materials from the MCR was close to that of ocean water for most of the sampling from July to October 2002 with about 82% of the salinity measuremen ts above 32 o/oo. At the high salinities found in the MCR, entrainment rates di d not vary significantly with salinity. These results support the concept discu ssed in Pearson et al. (2002) that where bottom salinities are high most of the time, factors other than salinity are influencing crab distribution and entrainm ent rates. The results reported here coupled with those in Pearson et al. (2002 ) indicate that low salinity influences crab entrainment rates.