δ15N and δ13C diet–tissue discrimination factors for large sharks under semi-controlled conditions

Abstract

Stable isotopes ( δ 15N and δ 13C) are being widely applied in ecological research but there has been a call for ecologists to determine species- and tissue-specific diet discrimination factors (∆ 13C and ∆ 15N) for their study animals. For large sharks stable isotopes may provide an important tool to elucidate aspects of their ecological roles in marine systems, but laboratory based controlled feeding experiments are impractical. By utilizing commercial aquaria, we estimated ∆ 15N and ∆ 13C of muscle, liver, vertebral cartilage and a number of organs of three large sand tiger ( Carcharias taurus) and one large lemon shark ( Negaprion brevirostris) under a controlled feeding regime. For all sharks mean ± SD for ∆ 15N and ∆ 13C in lipid extracted muscle using lipid extracted prey data were 2.29‰ ± 0.22 and 0.90‰ ± 0.33, respectively. The use of non-lipid extracted muscle and prey resulted in very similar ∆ 15N and ∆ 13C values but mixing of lipid and non-lipid extracted data produced variable estimates. Values of ∆ 15N and ∆ 13C in lipid extracted liver and prey were 1.50‰ ± 0.54 and 0.22‰ ± 1.18, respectively. Non-lipid extracted diet discrimination factors in liver were highly influenced by lipid content and studies that examine stable isotopes in shark liver, and likely any high lipid tissue, should strive to remove lipid effects through standardising C:N ratios, prior to isotope analysis. Mean vertebral cartilage ∆ 15N and ∆ 13C values were 1.45‰ ± 0.61 and 3.75‰ ± 0.44, respectively. Organ ∆ 15N and ∆ 13C values were more variable among individual sharks but heart tissue was consistently enriched by ~ 1–2.5‰. Minimal variability in muscle and liver δ 15N and δ 13C sampled at different intervals along the length of individual sharks and between liver lobes suggests that stable isotope values are consistent within tissues of individual animals. To our knowledge, these are the first reported diet–tissue discrimination factors for large sharks under semi-controlled conditions, and are lower than those reported for teleost fish.