Whisker stable isotope values indicate long-term foraging strategies for female New Zealand sea lions

Abstract

Determining the foraging strategies for top marine predators is fundamental to understanding their role in the marine ecosystem and essential to gain insight into how species and populations may respond to environmental variability and human impacts. The long-term foraging strategies of individuals can be studied using stable isotope analysis of whiskers, which archive keratinous tissue. Here, stable isotope analysis (δ13C and δ15N) along the length of whiskers from female New Zealand (NZ) sea lions Phocarctos hookeri was used to investigate individuals’ long-term foraging strategies. Previous telemetry studies showed that individual female NZ sea lions have one of 2 distinct foraging strategies that are habitual within and between years. Furthermore, past stable isotope research showed that these 2 distinct foraging strategies, i.e. benthic (foraging on the sea floor) or mesopelagic (foraging at various depths in the water column), can be identified through δ13C and δ15N stable isotope values from blood and whiskers. In the present study, chronological serial stable isotope sampling of female NZ sea lion whiskers confirmed long-term consistency of individual foraging strategies. Thirty-one of 35 individuals showed constant benthic (n = 13) or mesopelagic (n = 18) isotopic values along the length of their whiskers. The remaining 4 individuals showed mesopelagic foraging strategies but with slight oscillations. Serial stable isotope analysis of whiskers is a powerful tool for investigating the ecological niche of top marine predators throughout their adult life. This tool can be used within the Auckland Islands’ NZ sea lion population to determine the proportion of the female population that are exposed to detrimental interactions with fisheries.