Stable isotopes and elasmobranchs: tissue types, methods, applications and assumptions

Abstract

Stable-isotope analysis (SIA) can act as a powerful ecological tracer with which to examine diet, trophic position and movement, as well as more complex questions pertaining to community dynamics and feeding strategies or behaviour among aquatic organisms. With major advances in the understanding of the methodological approaches and assumptions of SIA through dedicated experimental work in the broader literature coupled with the inherent difficulty of studying typically large, highly mobile marine predators, SIA is increasingly being used to investigate the ecology of elasmobranchs (sharks, skates and rays). Here, the current state of SIA in elasmobranchs is reviewed, focusing on available tissues for analysis, methodological issues relating to the effects of lipid extraction and urea, the experimental dynamics of isotopic incorporation, diet-tissue discrimination factors, estimating trophic position, diet and mixing models and individual specialization and niche-width analyses. These areas are discussed in terms of assumptions made when applying SIA to the study of elasmobranch ecology and the requirement that investigators standardize analytical approaches. Recommendations are made for future SIA experimental work that would improve understanding of stable-isotope dynamics and advance their application in the study of sharks, skates and rays.

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

“…For Carcharhinus falciformis, the C ÷ N ratio ranged from 2.81 to 3.10 (bulk samples) and from 2.81 to 3.07 (lipid-free samples). We found significant statistical differences in C ÷ N ratio between bulk and lipid-free samples (Table 1), likely an effect of the lipid extraction process (Hussey et al 2012). However, since C ÷ N values were lower than 3.5 (Post et al 2007), all the analyses were performed using the δ 13 C and δ 15 N values of bulk samples.…”

“…Knowledge of diet, trophic position, movement patterns and habitat use of species have been recognized as critical factors in implementing successful conservation and management strategies of species and the environment (Hussey et al 2012). The diets and trophic interactions of elasmobranchs are sometimes difficult to determine using traditional methods alone (e.g., gut content analysis, direct observations, tagging), however, a complementary technique is stable isotope analysis (SIA).…”

“…This technique is based on the premise that heavy isotopes of an element are preferentially retained (e.g., nitrogen isotopes via protein amination/deamination, carbon isotopes via respiration) and that specific ratios of heavy to light isotopes are indicative of specific resource use (e.g., diet and habitat) (Wolf et al 2009) and trophic position (Post 2002). This biogeochemical method allows quantitative analysis of dietary composition and foraging patterns over a range of spatial and temporal scales (MacNeil et al 2005), as well as the examination of more complex questions pertaining to community dynamics, feeding strategies or diet, trophic position, and movement in aquatic organisms (Hussey et al 2012).…”

“…However, relatively little is known about their abundance, diet, and movement (Baum et al 2003, Heithaus et al 2008, Hussey et al 2012. Many shark species are in decline, mainly due to overfishing, bycatch, pollution, and habitat degradation (Baum et al 2003, Dulvy et al 2008.…”

Stable isotope analysis of vertebrae reveals ontogenetic changes in habitat in an endothermic pelagic shark

“…Therefore, the stable isotope compositions of different food webs provide the context within which a consumer's stable isotope composition can be used as tracers of habitat use, especially in migratory species [19][20][21]. This approach has been used to understand movements in marine organisms including marine mammals [22,23], seabirds [24], teleosts [25] and elasmobranchs [26]. The broad distribution of salmon sharks extends across several biogeographic provinces in the ENP [27].…”