USE OF OYSTER SHELL TO ENHANCE INTERTIDAL HABITAT AND MITIGATE LOSS OF DUNGENESS CRAB (CANCER-MAGISTER) CAUSED BY DREDGING

Juvenile Dungeness crab (Cancer magister) recruit to intertidal areas in estuaries along the Pacific Northwest coast of the United States in May and June of each year and survive best through their first summer in shell or eelgrass habitat. Experiments were initiated in Grays Har bor, Washington, to investigate the potential of using shell to enhanc e intertidal crab habitat as a means to augment the crab resource and mitigate losses from the subtidal population that occur during dredgin g. Experimental plots (225 M2) were constructed prior to crab settleme nt at each of three intertidal locations using three configurations of oyster shell (heavy layer, light scattering, and small piles of shell ). Resulting crab densities were comparable with those found in natura lly occurring shell with high numbers (20-60 crab.m-2) observed during settlement that declined to a relatively stable density of 10 crab.m- 2 in July and August. Crab survival was highest in both heavy and pile configurations, but the heavy shell configuration remained intact the longest. This enhancement experiment has become the impetus for a lar ge-scale (8 ha) mitigation program in 1992 as part of a dredging proje ct completed in 1990 in Grays Harbor.