Blue whales feeding on high concentrations of euphausiids around Monterey Submarine Canyon

Abstract

An unusually high concentration of blue whales, Balaenoptera musculus, fed on euphausiids that were concentrated in deep scattering layers and daytime surface swarms in Monterey Bay, California, during the fall of 1986. Blue whales were closely associated with deep scattering layers that were elongated along the southeast edge of the Monterey Submarine Canyon throughout most of November 1986. These scattering layers contained euphausiids, primarily Thysanoessa spinifera, which accounted for 45.4% of the total biomass of zooplankton tows taken through deep scattering layers. Tows taken above or outside the layers consisted of only 4.0% euphausiids. The greatest concentrations of blue whales were observed in areas of thick scattering layers that contained high krill biomass. In addition, surface-lunging blue whales fed on daytime surface swarms of Thysanoessa spinifera near the head of Monterey Submarine Canyon on 11 and 12 November 1986. Surface tows taken in whale feeding areas were dominated by T. spinifera, which accounted for 64.4% of the total zooplankton biomass, while surface tows taken outside whale areas consisted of only 0.3% euphausiids. Euphausiid densities in surface swarms were slightly higher and more variable than in deep layers. Thysanoessa spinifera size frequency distributions showed that surface swarms may be similar to the deep layer in some areas, but they may contain larger, sexually mature individuals in other areas. Blue whale fecal samples confirmed that the whales were feeding on euphausiids. The disappearance of the blue whales from Monterey Bay was accompanied by a decline in krill biomass along the southeast edge of the submarine canyon. This coincided with the cessation of a prolonged upwelling period that persisted in Monterey Bay throughout November 1986.