Isotopic evidence of distinct foraging ecology and movement pattern in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean

Frédéric Ménard (1) , Anne Lorrain (2, 3) , Michel Potier (4) , Francis Marsac (2)

1 IRD
2 THETIS - Thons tropicaux et écosystèmes pélagiques : taxies, interactions et stratégies d'exploitation
3 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)
4 IRD - Institut de Recherche pour le Développement

Ecologists primarily use δ15N values to estimate the trophic level of organisms, while δ13C, and even recently δ15N, are utilized to delineate foraging 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 sizes were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Size and latitude effects on δ15N and δ13C were investigated using linear models. Both latitude and size significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ15N. 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 size effect is significant for both species and indicates an increase in prey size through ontogeny. However, this effect is more pronounced in swordfish as a consequence of their different foraging 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 thesurface 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 δ13C, δ15N 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 δ15N values and to correct for any size or latitude differences in future food web studies.

Domaines: Sciences de l'environnement

Frédéric Ménard, Anne Lorrain, Michel Potier, Francis Marsac. Isotopic evidence of distinct foraging ecology and movement pattern in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean. Marine Biology, 2007, 153 (2), pp.141-152. ⟨hal-00452372⟩