Estimated Entrainment of Dungeness Crab During Dredging For The Columbia River Channel Improvement Project

Abstract

The scientific literature, analyses of salinity intrusion scenarios, and the summer 2002 site-specific data on entrainment and salinity all indicate that bottom salinity influences crab distribution and entrainment, especially at lower salinities. It is now clear from field measurements of entrainment rates and salinity during a period of low river flow (90-150 Kcfs) and high salinity intrusion that entrainment rates are zero where bottom salinity is less than 16 o/oo most of the time. Further, entrainment rates for 2+ and older crab decline in a clear and consistent manner as salinity decreases. More elaboration of the crab distribution-salinity model is needed, especially concerning salinity and the movements of 1+ crab.vii viii CONTENTS

Estimated Entrainment of Dungeness Crab During Maintenance Dredging of the Mouth of the Columbia River, Summer 2002

“…All of the regressions of the natural logarithm of the entrainment rates for various combinations of age classes against two measures of salinity were not significant. These results support the concept discussed in Pearson et al (2002) that where bottom salinities are high most of the time, factors other than salinity are influencing crab distribution and entrainment rates. The results reported here, coupled with those in Pearson et al (2002), indicate that low salinity influences crab-entrainment rates.…”

“…This document also reports on the salinities measured in the samples of dredged material taken in the MCR. An analysis of the influence of salinity on crab-entrainment rates based on both the MCR and upriver 2002 sampling appears separately in Pearson et al (2002), which includes results from entrainment measurements at locations upriver from the MCR.…”

“…This document describes the field studies of crab entrainment during 2002 maintenance and operations dredging at the mouth of the Columbia River (MCR) from River Mile (RM) -3 to RM+3 (Figure 1). A separate report by Pearson et al (2002) focuses on issues related to the proposed Columbia River Channel Improvement Project and presents results of field studies conducted from RM3 to RM24. Pearson et al (2002) also presents a general background on Dungeness crab biology, the influence of salinity on crab distribution, and the use of a modified Dredge Impact Model (DIM) to estimate entrainment of Dungeness crab.…”

“…This document reports results from sampling in the MCR between RM-3 and RM+3. Pearson et al (2002) reports the results from the sampling in locations upriver from the MCR (above RM+3). The Essayons is equipped with a special basket sampler, into which a portion of the dredged materials entering the vessel's hopper can be diverted.…”

“…To support the calculations of the volume of dredged material sampled by the basket sampler, measurement of the fluid flow in the pipe leading to the basket sampler was conducted aboard the Essayons in September 2002 during dredging operations. Pearson et al (2002) report the methods and results from the flow measurements, and the results of those flow measurements were used here in the calculation of entrainment rates. During 2002, the Essayons dredged over 3 million cubic yards (cy) of material from the MCR; researchers were aboard the Essayons from July to October 2002, when approximately 2.7 million cy were dredged.…”