Using isoscapes to trace the movements and foraging behavior of top predators in oceanic ecosystems

The stable isotope composition of animal tissues can provide intrinsic tags to study the foraging and migratory ecology of predators in the open ocean. Chapter 13 (this volume) demonstrated that by comparing the isotope values of an animal and its local prey or environment, the animal's movements can be estimated, given that isotopic variation exists between habitats. The utility of using geographical variations in stable isotopes values, or isoscapes to study the movements of marine predators has been limited because of our lack of knowledge on the spatial variation of the carbon, nitrogen, and oxygen isotope values in the open ocean. In this chapter, we review the spatial patterns in the carbon and nitrogen values of primary producers in the oceans and broadly discuss mechanisms that set the isotopic composition at the base of marine food webs. We then discuss how spatial patterns in baseline and predator isotope values can be used to examine the movements and foraging behavior in two groups of marine predators, pinnepids and tropical tuna. These two case studies demonstrate that ocean isoscapes are a promising tool to investigate population-level movements and foraging behavior of elusive predators, but this method has limitations and will not achieve the fine-spatial resolution obtained with electronic tags and instrumentation. Furthermore, the construction and application of ocean isoscapes is still in its early development and requires knowledge about the physiology and behavior of the predator, an understanding of the temporal and spatial stability of the isotopic baseline, and validation with independent datasets.