Matching and mismatching stable isotope (δ[sup.13]C and δ[sup.15]N) ratios in fin and muscle tissue among fish species: a critical review

Using non-lethal tissue sampling for stable isotope analysis has become standard in many fields, but not for fishes, despite being desirable when species are rare or protected, when repeated sampling of individuals is required or where removal may bias other analyses. Here, we examine the utility of fish dorsal fin membrane as an alternative to muscle for analyzing δ[sup.13]C and δ[sup.15]N ratios in two reef fish species (blue cod Parapercis colias and spotty Notolabrus celidotus) that have differing feeding modes. Both species exhibited evidence of size-based feeding from fin δ[sup.15]N values, but not from muscle. Blue cod fin δ[sup.15]N increased steadily throughout the sampled size range (213-412 mm fork length), whereas spotty exhibited a distinct ontogenetic diet shift at approximately 120-140 mm fork length after which size-based feeding did not occur. Fin membrane was higher than muscle in δ[sup.13]C in both species and in δ[sup.15]N for blue cod, but fin δ[sup.15]N was lower than muscle in spotty. The δ[sup.13]C and δ[sup.15]N fin-muscle offsets were constant in spotty regardless of size, while in blue cod, δ[sup.13]C was constant with fish size, but δ[sup.15]N offsets increased with increasing fish size. Non-lethal sampling utilizing fin tissue can be employed to estimate stable isotope values of muscle in fishes, but it is necessary to assess relationships among tissues and the effects of fish size on isotope values a priori for each species studied. Our data indicated that fin membrane may be a more sensitive tissue than muscle for detecting size-based feeding in some fish species using stable isotopes. A critical literature review revealed inconsistencies in tissue types tested, little understanding of tissue-specific trophic shift or turnover rates, and pseudo-replicated analyses leading to erroneous postulating of 1:1 relationships between tissues.
Introduction In recent years, understanding spatial and temporal changes in the trophic position and foraging dynamics of reef fishes has been frequently achieved by utilizing stable isotopes of nitrogen and [...]