MEPS 223:299-310 (2001)  -  doi:10.3354/meps223299

ABSTRACT: Gray whales Eschrichtius robustus forage in parts of Clayoquot Sound on several prey species in different habitats. Between June and September in 1996 and 1997 we carried out analyses of the density, biomass, and other measures of their primary prey species, and of whales¹ movement patterns in response to prey characteristics. The prey base consists of hyper-benthic mysids (family Mysidae), pelagic porcelain crab larvae (4 spp. of family Porcellanidae), benthic amphipods (family Ampeliscidae) and benthic ghost shrimp Callianassa californiensis. Whales foraged primarily for mysids, switching to porcelain crab larvae in August, and then to amphipods even later in the season when these organisms increased in body size. In 1997, whales rapidly switched from feeding on planktonic to benthic prey during mid-August. Sampling indicated low numbers of mysids and crab larvae at that time. Selection of amphipod prey was based on high biomass and a high proportion of individuals ≥6 mm in length. In parts of the study area gray whales did not return to forage on benthic amphipods when this size criteria was not met. A single whale departed from a ghost shrimp feeding ground because its search time for food was long, it achieve a low biomass removal rate, and it was not able to find sufficient food each day. We show that gray whales are dynamic and selective foragers that switch prey and foraging tactics rapidly to take advantage of short-term availability of energy.

KEY WORDS: Gray whale · Eschrichtius robustus · Prey selection · Prey-switching · Plankton · Benthos · Clayoquot Sound