Population structure of North Pacific humpback whales on their feeding grounds revealed by stable carbon and nitrogen isotope ratios

Humpback whalesMegaptera novaeangliaein the North Pacific Ocean are a migratory species known to have a complex population structure on both feeding and breeding grounds. We described the structure of this population using stable isotope analysis of skin samples (n = 1105) collected from free-ranging North Pacific humpback whales from 10 sampling regions in 2004 and 2005. We detected significant quadratic relationships between latitude and both δ13C (R² = 0.29) and δ15N (R² = 0.23) as well as between longitude and δ13C (R² = 0.43) and δ15N (R² = 0.16). A weak negative linear relationship was seen between increasing distance from shore and both δ13C (R² = 0.05) and δ15N (R² = 0.02). Sampling regions were significantly different for both δ13C (ANOVA,F 9,1094= 136.4, p < 0.001) and δ15N (F 9,1095= 71.5, p < 0.001). We performed classification tree analyses using δ13C and δ15N as predictor variables to assign membership to sampling regions. Results of initial classification and ANOVAs supported combining the 10 sampling regions into 6 feeding groups. When applied to these feeding groups, the classification tree was able to predict 57% of group membership correctly, with accuracy rates for individual groups ranging from a low of 19% to a high of 78%. These results indicate that stable isotope analysis can be used to distinguish unique feeding aggregations of humpback whales within the North Pacific Ocean. Ultimately, isotopic characteristics of these aggregations can be applied to animals sampled on breeding grounds to assign them to a feeding aggregation, enhancing the ability to describe habitat linkages and migration patterns of humpback whales.