Abstract

An intertidal oyster shell habitat has been created and used to mitigate the subtidal dredging impact on Dungeness crab (Cancer magister Dana) population resulting from widening and deepening of the Grays Harbor navigation channel (47 degrees N, 124'W, USA). This paper addresses the effect of this artificial habitat on soft bottom species, focusing in particular on: (1) growth rate of the suspension feeder bivalve Mya arenaria L.; (2) both settlement and survival of the bivalve Macoma balthica (Linne); and (3) the effects of habitat heterogeneity, clam density, and crab density on the foraging rate of juvenile Dungeness crabs preying on Macoma balthica. Epibenthic shells did not affect the growth rate of Mya arenaria transplanted into oyster shell habitat. Recruitment of the bivalve Macoma balthica was not affected by shell either, but mortality rate was higher in areas covered by shell when compared with open mud. Laboratory experiments showed a positive density-dependent (Type III) functional response, indicating that Macoma balthica finds refuge at low cram density. At increased crab density in shell habitats interaction among juvenile crabs affects consumption rate of clams. Tethering experiments showed that juvenile crab mortality was higher in open mud, intermediate at the border of the shell plots, and lower in the center. This pattern of crab mortality suggests that juvenile Dungeness crabs only affect local clam populations in shell covered areas. These results indicate that although the artificial shell habitat successfully enhances settlement and survival of juvenile Dungeness crabs, it affects the ecology of some non-target species, in part through intensified predator-prey dynamics due to the increased local densities of young-of-the-year crabs.