Diel Activity of an Estuarine Population of Dungeness Crabs, Cancer Magister, in Relation to Feeding and Environmental Factors

Abstract

Dungeness crabs, C. magister, were collected at subtidal and intertidal sites during 4 quarterly 3-day diel sampling periods in June and Sept. 1980, and Jan. and April 1981. During each period, trawls were made during day and night, and high and low tide conditions. Crabs were significantly more abundant at the subtidal site. Crabs left the flats during low tide, causing a slight but insignificant increase in low tide densities in the channel. Crabs were more abundant by day at the subtidal site, but more abundant by night at the intertidal site. This phenomenon was statistically significant in Jan., and over all data combined, and was a response to the availability of food, particularly the shrimps, Crangon spp. Crab and shrimp densities on the flats were highly positively correlated in Sept. and Jan., but negatively correlated in June. Reduced catches during spring sampling were associated with very low salinities at the intertidal site and low tides at the subtidal site. These findings have application in the management of estuarine dredging practices.