Are whale prey euphausiids associated with steep bathymetric features in the California Current Ecosystem?

Blue whales are obligate euphausiid predators and may further focus their feeding activity on specific euphausiid size classes and species. Euphausiids are extremely patchy prey and whales must feed on patches of sufficient density in order to offset the energetic costs of lunge feeding. Blue whales are often observed over steep coastal bathymetry, including an area within the California Current Long Term Ecological Research (CCE-LTER) site called Nine Mile Bank. I hypothesized that prey euphausiids (including Thysanoessa spinifera and Euphausia pacifica) are found in greater densities near the bank feature than on either side of it, potentially explaining the bank-associated whale sightings. I also hypothesized that larger individuals of prey species would be found deeper in the water column than smaller individuals, which would allow whales to target the larger, more energetically rich individuals. I assessed the prey field through a combination of net collection and acoustic methods. Bongo net samples were collected along transects across the bank and vertically discrete samples were collected using a strobe light-equipped MOCNESS. Mulifrequency acoustic backscatter was analyzed using dB differencing methods to determine the distribution of euphausiid-like targets. Preliminary results suggest that the bank feature may instead act as a boundary, with prey euphausiids found in greater densities on- and inshore of the feature than on the offshore side.