UC San Diego

Blue whales (Balaenoptera musculus Linnaeus) eat almost exclusively euphausiids and must find dense aggregations above the local mean to satisfy their energetic needs. The paradox implied by the predator-prey relationship between these large baleen whales and euphausiids is how the whales can acquire sufficient food from such small, patchily distributed prey. This problem can be conceptualized as interactions on multiple spatial scales. Through analysis of the euphausiid mandible remains in whale fecal samples, I determined that the prey of blue whales from the northeast Pacific population is consistently and overwhelmingly dominated by Thysanoessa spinifera Holmes, a large neritic euphausiid. Based on the blue whales’ extremely limited diet, I hypothesized that blue whales and T. spinifera would be consistently co-located across a range of spatial scales. I investigated the spatial relationship between the predator and prey at multiple spatial scales: the North Pacific Ocean, the California Current System, a regional scale where there is long-term sampling by the CalCOFI program, and a local scale near blue whale aggregation centers. The distributions of blue whales and T. spinifera were only related at the two largest scales investigated. An apparent northward blue whale range expansion beginning around 1997 did not appear to be related to reductions in T. spinifera prey in more southern waters. As sampled, the regional and local scales did not capture the scale of interaction between these predators and prey, suggesting that a smaller scale of sampling was necessary closer to the ambit of a foraging whale. Focusing on a single bathymetric feature (Nine Mile Bank near San Diego, California) I tested the hypothesis that both prey euphausiids and baleen whales aggregate at such abrupt changes in topography. Instead of serving as a point of increased relative abundance, the bank actually represented an offshore limit for blue, fin, and humpback whales feeding on T. spinifera. T. spinifera adults were found deeper in the water than less energetically valuable smaller individuals. The results of this dissertation suggest that blue whales are highly selective predators that can locate and feed on vertical aggregations of T. spinifera located near steep topography.