Isotopic tracking of foraging and long-distance migration in northeastern Pacific pinnipeds

Abstract

We investigated the impact of foraging location (nearshore vs offshore) and foraging latitude (high vs middle) on the carbon (δC) and nitrogen (δN) isotope compositions of bone collagen of northern fur seals (Callorhinus ursinus), harbor seals (Phoca vitulina), California sea lions (Zalophus californianus), and northern elephant seals (Mirounga angustirostris). Nearshore-foraging harbor seals from California had δC values 2.0‰ higher than female northern elephant seals foraging offshore at similar latitudes. Likewise, nearshore-foraging harbor seals from Alaska had values 1.7‰ higher than male northern fur seals, which forage offshore at high latitudes. Middle-latitude pinnipeds foraging in either the nearshore or offshore were C enriched by ∼1.0‰ over similar populations from high latitudes. Male northern elephant seals migrate between middle and high latitudes, but they had δC values similar to high-latitude, nearshore foragers. Female northern fur seal δC values were intermediate between those of high- and middle-latitude offshore foragers, reflecting their migration between high- and middle-latitude waters. The δC values of California sea lions were intermediate between nearshore- and offshore-foraging pinnipeds at middle latitudes, yet there was no observational support for the suggestion that they use offshore food webs. We suggest that their "intermediate" values reflect migration between highly productive and less-productive, nearshore ecosystems on the Pacific coasts of California and Mexico. The relative uniformity among all of these pinnipeds in δN values, which are strongly sensitive to trophic level, reveals that the carbon isotope patterns result from differences in the δC of organic carbon at the base of the food web, rather than differences in trophic structure, among these regions. Finally, the magnitude and direction of the observed nearshore-offshore and high-to middle-latitude differences in δC values suggest that these gradients may chiefly reflect differences in rates and magnitudes of phytoplankton production as well as the δC value of inorganic carbon available for photosynthesis, rather than the input of C-enriched macroalgal carbon to nearshore food webs.

Trophic inference in two sympatric sharks, Sphyrna lewini and Carcharhinus falciformis (Elasmobranchii: Carcharhiniformes), based on stable isotope analysis at Malpelo Island, Colombia

“…This species is considered as an important consumer of cephalopods in the eastern Pacific C compared with those offshore due to the influence of benthic primary production (France 1995, Tanaka et al 2008 or upwelling (Burton andKoch 1999, Graham et al 2010). The similarity of the results may be due to: (1) the high mobility of S. lewini throughout the ETP (Bessudo et al 2011b); (2) The results of this study support the hypothesis of Bessudo et al (2011aBessudo et al ( , 2011b, who suggest that during the night, Sphyrna lewini leaves Malpelo Island, probably to feed.…”

Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic using stable isotopes

“…Because stable isotopes in the environment are incorporated into primary producers and then transferred up the food chain (DeNiro andEpstein 1978, Minagawa andWada 1984), the isotopic values of tissues of higher trophic level organisms reflect differences in the stable isotope values of primary producers of the environment in which these organisms foraged (Schell et al 1989, Minami and Ogi 1997, Burton and Koch 1999, Kurle and Worthy 2002, Cherel and Hobson 2007, Pajuelo et al 2010. These spatial isotopic differences in primary producers create isotopically distinct regions that can be used to infer residency and movement patterns of organisms migrating among them (Rubenstein andHobson 2004, Graham et al 2010).…”

Geographic variation in the trophic ecology of an avian rocky shore predator, the African black oystercatcher, along the southern African coastline

“…This baseline signal is then transmitted along the food chain in a predictable manner and is ultimately reflected in organisms at higher trophic levels (Cherel & Hobson 2007). This effect has been widely used to investigate spatial and temporal aspects of the feeding ecology of oceanic predators (Burton & Koch 1999, Quillfeldt et al 2005, Cherel & Hobson 2007, but rarely used in largescale studies involving higher trophic level organisms of intertidal ecosystems (but see Atkinson et al 2005). δ 13 C has been particularly exploited at the lower trophic levels to distinguish between the 2 main potential sources of carbon in intertidal habitats: nearshore benthic and offshore pelagic primary production (Fry & Sherr 1984, France 1995, Post 2002.…”

“…Large scale δ 13 C patterns at the base of marine food webs in relation to spatial variability of CO 2 concentration, water temperature and growth rates of primary producers (Rau et al 1982, Goericke & Fry 1994) have been widely used to investigate animal migration patterns (see review in Hobson 1999) and foraging movements in marine predators (Burton & Koch 1999, Quillfeldt et al 2005). δ 13 C ratios can also be used to identify the ultimate sources of carbon for consumers when the 13 C signatures of food sources differ (Post 2002), and in the case of higher level predators, can help to determine the main component of the diet.…”