From wind to whales: trophic links in a coastal upwelling system

Blue whales Balaenoptera musculus meet the highest prey demands of any predator that has ever existed by feeding exclusively upon dense but patchy schools of pelagic euphausiids. We examined the role that seasonally high primary production supported by coastal upwelling combined with topographic breaks off California play in creating, collecting, and maintaining euphausiids at densities sufficient to allow exploitation by whales. We used concurrent ship-and mooringbased oceanographic, hydroacoustic, and net sampling, whale-sighting records, visual surveys, and time-depth recorder deployment to examine temporal and spatial linkages between (1) intensity of upwelling, (2) primary production, (3) development, density and distribution of euphausiids, and (4) the distribution, abundance, and foraging behavior of blue whales in Monterey Bay, California between 1992 and 1996. Blue whales fed exclusively upon adult euphausiids Thysanoessa spinifera and Euphausia pacifica that were larger than those generally available in the Bay. Foraging whales dove repeatedly to dense euphausiid aggregations between 150 and 200 m on the edge of the Monterey Bay Submarine Canyon. Euphausiid aggregations where whales were foraging averaged 153 g m -3 , approximately 2 orders of magnitude greater than mean euphausiid densities in the Bay (1.3 g m -3 ). High euphausiid densities are supported by high primary production between April and August (249 mg C m -3 d -1 ) and a submarine canyon that provides deep water down-current from an upwelling region. Peak euphausiid densities occur in late summer/early fall, lagging the seasonal increase in primary production by 3 to 4 mo. This lag results from the temporal development of euphausiids spawned around the spring increase in primary production and the shoreward collapse of productivity due to decreased upwelling in late summer. The migratory movements of the California blue whale probably reflect seasonal patterns in productivity in other foraging areas similar to those we describe for Monterey Bay.