Figure 1. Animal sampling locations and the surrounding Bering Sea oceanographic domains.
Understanding how species utilize habitat and prey resources is a key aspect of foraging ecology and is necessary to assess impacts of anthropogenic and environmental influences on a population. A novel approach to quantifying both diet and habitat use for a wide range of species is to measure stable isotope ratios of nitrogen and carbon in different tissues. The stable nitrogen (15N/14N or δ15N) and carbon (13C/12C or δ13C) isotope ratios of a consumer’s tissue indicate trophic level and foraging location, respectively. Stable isotope analysis of different tissues provides assimilated dietary information over a range of time scales due to the dissimilar isotopic turnover rates of the different tissues. For example, δ15N and δ13C values of blood plasma reflect diet integrated approximately 1-2 weeks prior to collection, whereas, those of red blood cells (RBCs) represent the diet of the previous 1-2 months.
Figure 2. Foraging trips of adult female northern fur seals. Red, green, and blue lines indicate animals categorized as on-shelf, off-shelf, or mixed-shelf foragers, respectively. On-shelf foragers exclusively utilized <200-m depths, off-shelf foragers consistently traveled beyond the 200-m isobath, and mixed-shelf foragers utilized both on- and off-shelf habitat.
Figure 3. δ15N and δ13C values for plasma and RBC from individual adult female northern fur seals by foraging strategy.
Stable carbon and nitrogen isotope values have been used to describe foraging behavior and habitat use for a wide range of organisms including marine mammals. However, only a few studies have used stable isotopes to develop predictive habitat-use models. Northern fur seals (Callorhinus ursinus) from breeding colonies in Alaska are particularly well-suited subjects for evaluating predictions of foraging habitat of a top marine predator based on stable isotope analysis.
Fur seal foraging habitat in the eastern Bering Sea is defined by two distinct oceanographic regions: the North Pacific continental shelf, which includes waters <200 m in depth (on-shelf), and the Bering Sea deep-sea basin (off-shelf) (Fig. 1). Both diet and telemetry studies indicate fur seals from different breeding colonies have distinct foraging strategies: on-shelf foragers exclusively utilize the on-shelf habitat, off-shelf foragers travel beyond the 200-m isobaths to the off-shelf waters, and mixed-shelf foragers make foraging trips to both on- and off-shelf habitat.
Scientists from the National Marine Mammal Laboratory’s Alaska Ecosystems Program (AEP) constructed general linear mixed models using carbon (δ13C) and nitrogen (δ15N) stable isotope values of blood to predict on- or off-shelf habitat use by adult female northern fur seals from breeding colonies in Alaska. To assess the feasibility of the models, they compared individual δ13C and δ15N values from red blood cells (RBC) and plasma with individual satellite location and time-depth recorder dive data collected in 2006. Independent data collected in 2008 was used to test the performance of the model and determine whether it could account for annual differences in stable isotope values.
During both years, animals from each breeding site made foraging trips to distinct habitats in the Bering Sea that allowed for categorization of their foraging strategies (Fig. 2). For plasma and RBCs, δ13C and δ15N isotope values of animals foraging on-shelf were higher in comparison to off-shelf foragers in both 2006 and 2008 (Fig. 3). In both 2006 and 2008, the models accurately predicted on- and off-shelf foraging habitat of adult female fur seals using δ13C and δ15N isotope values from both plasma and RBCs. The model did not perform as well in predicting habitat use for animals which foraged in both on-shelf and off-shelf habitat; however, sample sizes for these animals were small.
The use of stable isotope analysis presented in this study represents a powerful approach to increase our understanding of foraging behavior for northern fur seals and other marine predators. These models are the first to accurately predict individual foraging habitat of northern fur seals using blood isotopes, and they were validated with an additional year of data. Because stable isotopes can be collected from large numbers of animals, do not require animals to be recaptured, and are relatively inexpensive to analyze, this approach has the potential to quantify habitat use patterns at a population level.
