Ecologists primarily use δ
15N values to estimate the trophic level of organisms, while δ
13C, and even recently δ
15N, are utilized to delineate feeding habitats. However, many factors can influence the stable isotopic composition of consumers,
e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable
isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (
Thunnus albacares) and swordfish (
Xiphias gladius) of various body lengths were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along
a latitudinal gradient (23°S to 5°N). Body length and latitude effects on δ
15N and δ
13C were investigated using linear models. Both latitude and body length significantly affect the stable isotope values of the
studied species but variations were much more pronounced for δ
15N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating
along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a
relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The body length effect is
significant for both species but this effect is more pronounced in swordfish as a consequence of their different feeding strategies,
reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger
size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in the surface
waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish
tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus,
in contrast to δ
13C, δ
15N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation,
both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed
in this study allow us to make predictions of δ
15N values and to correct for any body length or latitude differences in future food web studies.
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