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

Abstract

Humpback whales Megaptera novaeangliae in 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) col- lected from free-ranging North Pacific humpback whales from 10 sampling regions in 2004 and 2005. We detected significant quadratic relationships between latitude and both δ 13 C (R 2 = 0.29) and δ 15 N (R 2 = 0.23) as well as between longitude and δ 13 C (R 2 = 0.43) and δ 15 N (R 2 = 0.16). A weak negative linear relationship was seen between increasing distance from shore and both δ 13 C (R 2 = 0.05) and δ 15 N (R 2 = 0.02). Sampling regions were significantly different for both δ 13 C (ANOVA, F9,1094 = 136.4, p < 0.001) and δ 15 N (F9,1095 = 71.5, p < 0.001). We performed classification tree analyses using δ 13 C and δ 15 N 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 hump- back 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.