Flexible foragers: Humpback whale diets in the California Current indicate variance in ocean climate and ecosystem conditions

Citation

Fleming, A, C Clark, J Calambokidis, and J Barlow. 2015. Flexible foragers: Humpback whale diets in the California Current indicate variance in ocean climate and ecosystem conditions. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.

Abstract

While large, migratory predators are often cited as indicator species for ecosystem processes and conditions, their utility as indicators is dependent upon an understanding of their response to environmental variability. In this study, we examined humpback whale response to environmental variability through stable isotope analysis of diet. Carbon and nitrogen stable isotope signatures were analyzed from 295 humpback whale skin biopsies collected over a dynamic twenty year period (1993-2012) in the California Current Ecosystem (CCE) from a single feeding population. The population showed significant inter-annual variability in isotope signatures (δ15N p<0.0001; δ13C p<0.0001) and our study period captured two major shifts in isotopic ratios of CCE humpback whales. These shifts appear to be the result of prey-switching behavior in response to oceanographic and ecological changes in the CCE. The 1993-2003 and 2010-2012 humpback whale signatures reflected carbon and nitrogen values associated with a diet dominated by krill. These periods were characterized by negative phases of the Pacific Decadal Oscillation (PDO), cool sea-surface temperatures, strong upwelling, and high krill biomass. In contrast, the isotopic signatures of 2004-2006 samples reflected values associated with a diet dominated by schooling fish and the PDO was positive, sea surface temperatures were warmer, seasonal upwelling was delayed and anchovy and sardine populations displayed increased biomass and range expansion. These findings suggest that humpback whales are flexible in their prey selectivity and exploit available prey patches dense enough to meet their metabolic needs. In doing so, their foraging behavior is a synoptic result of conditions across the CCE. This study integrates multiple lines of evidence over the course of an environmentally variable twenty-year period, to provide a more complete understanding of the relationships between climate, prey and predators. A better understanding of how predators respond to changing conditions greatly improves their utility as indicators of ecosystem change.