Abstract
Comparative analysis of isotope values from different tissues can capture temporal variation in the trophic and foraging behavior of difficult to study large marine predators, reveal- ing either uniform or variable ecological roles over time. The isotopic values (δ 13 C and δ 15 N) of dermis, and muscle tissue of silky Carcharhinus falciformis and blue sharks Prionace glauca sam- pled in the northeast central Pacific were analyzed to quantify ontogenetic inter- and intra-tissue isotopic variation. Consistent differences in δ 15 N values occurred between dermis and muscle tis- sue for both species (2.5 ± 0.4 ‰ and 2.1 ± 0.3 ‰, respectively), while tissue differences in δ 13 C values were more variable between species (2.3 ± 0.6 ‰ and 0.7 ± 0.6 ‰, respectively), likely a result of tissue composition. The overall δ 15 N and δ 13 C values of dermis and muscle were highly correlated for blue sharks and for silky sharks with the exception of silky shark δ 13 C values. This pattern indicates that dermis isotope values are able to provide a proxy for muscle tissue, similar to that previously reported for fin, accepting dermis-specific diettissue discrimination factors. Tissue-specific ontogenetic isotopic variation for the silky shark, and the low regression slope value between dermis and muscle δ 13 C values, however, may suggest that dermis and muscle tissue have different isotopic turnover rates. These data demonstrate that dermis yields valuable isotope data to examine the trophic ecology and feeding/movement behavior of sharks, but further work is required to address dermis-specific turnover rates and diettissue discrimination factors.
Highlights
Compared to the instantaneous ‘snapshot’ of dietary information obtained from gut content analysis, stable isotope analysis (SIA) allows exami
The dermal collagen fiber layers that underlie the dermal denticles of shark skin, potentially provides another tissue that is easy to sample, and is likely more homogeneous in terms of structural composition compared to fin (Matich et al 2010, Hussey et al 2011, Carlisle et al 2012)
A skin tissue sample, including both dermis and dermal denticles, was excised from the anal fin region of each individual to minimize the effects of specific variation in skin thickness, and a small portion (~10 g) of white muscle tissue was excised adjacent to the vertebral column
Summary
Our understanding of the temporal and spatial variation in trophic roles and foraging dynamics of shark species has improved based on the application of carbon and nitrogen stable isotopes (Matich et al 2010, 2011, Hussey et al 2011, Kim et al 2012a). Considering the threatened status of many shark species (Dulvy et al 2014) and the requirement to limit mortalities, nondestructive or minimally invasive sampling methods are increasingly being adopted in field studies (Hammerschlag & Sulikowski 2011). Such tissue sampling from large sharks often includes blood (plasma and red blood cells), fin, and/ or a biopsy sample of skin, connective tissue, and muscle. The isotopic composition of dermis has received limited attention (but see Carlisle et al 2012, Jaime-Rivera et al 2013) despite the above advantages
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