Dungeness crab depth distribution: effects of sea otters

Abstract

The distribution and abundance of Dungeness crabs in the Glacier Bay area were observed with a submersible in five bays with and three bays without sea otters. A matrix design was used with three levels of sea otter occupation and three depth categories. Goals of this study were to determine: 1) the depth distribution of crabs; 2) if depth was a refuge from sea otter predation; and, 3) the habitat of ovigerous female aggregations. Scuba was used to calibrate submersible counts and collect substrate samples; crab pots were used to confirm submersible sightings. Abiotic and biotic variables were analyzed to interpret distribution data and aggregation sites. A regional, long-term crab survey dataset was also examined. Sea otters may have decreased crab abundance in shallow waters. Two aggregations of ovigerous Dungeness crabs were observed in shallow water with sand substrate. However, only 1% of the 33 km of transects were classified as sand, suggesting that sand may be a limiting resource. No conclusions could be made about the independent effects of sea otter presence or depth due to strong interaction. Submersible observations, crab pot surveys, and marine topography together however, point towards a shift in crab depth distribution with sea otter presence