Side-scan sonar estimates of the utilization of gray whale feeding grounds along Vancouver Island,Canada

Infaunal prey communities and gray whale feeding excavations were found at three sites along the west coast of Vancouver Island: Ahous Bay, Pachena Bay and Port San Juan. Side-scan sonar records indicated that whales disturbed up to 36% of the sea floor. The mean percentage of the bottom covered with feeding excavations was > 17% in each of the three feeding grounds. There was a positive relationship between the biomass of ampeliscid amphipod prey and the total quantity of prey consumed by gray whales from the three feeding grounds. There also was considerable annual variation in the size of feeding excavations, the total area covered by excavations, and the dispersion of excavations over the entire prey community during 3 years of observations at Pachena Bay. In general, two types of gray whale feeding records occurred in the Ampelisca tube mats. One record was composed of large complex excavations (mean > 20.5 m²): the other contained only small features(mean < 4 m²). Gray whales expanded the small feeding excavations into large, complex features by feeding along the edges of existing depressions. Edge feeding may increase the energy yield per dive as whales avoid low biomass areas inside existing excavations. It may also require less energy to lift the bottom by suction along the edge of an exposed tube mat. Gray whale feeding suspended over 1000 m³ of sediment per day. Since sediment grain size was coarser inside feeding excavations (mean = 0.160 mm) compared to outside (mean = 0.124 mm), gray whales may help to maintain the mud poor fine sand habitat of its primary benthic prey, Ampelisca, by winnowing away finer sediments. Certain infaunal prey types (e.g. onuphid polychaetes) are less resilient to gray whale predation than others (e.g. Ampelisca) as a result of their local distribution patterns and life history.