Behavioral ecology of summer resident gray whales (Eschrichtius robustus) feeding on mysids in British Columbia, Canada

Gray whales (Eschrichtius robustus) feed on inshore mysid swarms during the summer on the southern central coast of British Columbia. This study was undertaken to determine which aspect of prey dynamics was influencing the whales' feeding decisions. Individual resident whales were observed and behavior, diving/respiratory cycles, and movement patterns recorded. In situ observations by scuba divers and fecal samples confirmed that the whales feed on mysid swarms in kelp beds near the shore. Scuba divers collected mysids and took underwater photographs of the swarms at 11 sites on a weekly rotation to determine the population biology of these swarming crustaceans.; The mysid community consisted of nine species, but was dominated by Holmesimysis sculpta. Mysids reproduced throughout the summer months, and exhibited fast growth rates. They formed spatially and temporally stable swarms, which were large and extremely dense (mean = 440,000/m3).; Gray whales spent 77% of their time engaged in feeding activities, including searching for food (38%) and/or actively feeding (39%). Travelling accounted for 15% of their time, while only 8% of their time was spent socializing. Dives were short (mean = 2 min 22 sec), and they respired 2.9 times per surfacing with 15 seconds between blows. There were significant differences in respiratory parameters between activities. Feeding whales dove more frequently, respired fewer times, with shorter intervals between blows, and spent less time at the surface. I concluded that short, repeated dives were made to capture mysids from extensive planktonic swarms.; No relationship was found between whale feeding activity and the abundance of mysids, instead the number of whales feeding and the amount of time spent feeding were correlated significantly with the mean mysid body length. Feeding was negatively related to density because the densest swarms were formed of very small juveniles. Gray whales thus choose patches of mysids on the basis of prey size. Swarms formed of very small individuals either may not be captured effectively by the baleen or juveniles may be energetically deficient. These observations have important consequences for identifying critical feeding habitats and provide an interesting complexity to optimal foraging theory.