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Gray whale (Eschrichtius robustus) prey availability and feeding ecology in northern California, 1999-2000
The abundance, distribution, prey availability, and prey consumed by gray whales (Eschrichtius robustus) in northern California were investigated during 1999 and 2000. Systematic shore-based observations from four sites occurred year-round but were concentrated during summer months (May — October) of both years. Benthic and pelagic prey were sampled at three sites within the study area using SCUBA, plankton tows, and hand-held bottom grabs. Fecal slicks and mud plumes were Opportunistically collected during both seasons to assess prey consumption. Peaks in whale sighting rates (whales/survey hour) occurred in January and March-April during both years in conjunction with the annual southbound and northbound migrations. The highest annual sighting rates were observed in the summer months of both years. Based upon on-water and shore-based observations, approximately 15 — 25 gray whales summered in the area each year. The Klamath River area had the highest sighting rates during both years. The cumacean Diastylopsis dawsoni was the primary benthic prey resource available to and consumed by whales within the study area. Pelagic prey resources were more ephemeral in nature and collected in quantity over shorter time periods than benthic prey. Pelagic prey consumed includes the amphipod Atylus tridens (Patrick's Point), the euphausid Thysanoessa spinifera (Pt. St. George) and crab zoea larvae (Pt. St. George). This study is the first to qualify prey consumption and relate prey distributions to whale movement patterns at the southern end of the gray whales' summering range.