Top Predators Forage fauna Zoo Phyto Data on trophic ecology of top predators Stomach Content Analyses Stable Isotope Analyses (Acoustics) To investigate feeding ecology & trophic structure in high-sea ecosystems To document the diversity of the forage fauna (micronekton)

stomachs are frozen Stomachs are taken from freshly caught fishes and analysed in the lab Diet indices on prey Occurrence Mean Number Mean Reconstituted Weight Diet Feeding range Trophic overlap Prey size distributions Size ratios Stomach content data

INDIAN OCEAN: 2 databases –YugNIRO 1962 to 1990 (Soviet Union) Low level of prey identification – Heterogeneity of data –STOMAC 2000 to 2010 (IRD) Small spatial and temporal scales – High identification prey level YugNIRO (Indian Ocean)STOMAC (West Indian Ocean) > 20,000 stomachs 6 main predators 300 prey taxa 5 functional groups

6 main predators Yellowfin tuna (Thunnus albacares) Bigeye tuna (Thunnus obesus) Skipjack (Katsuwonus pelamis) Dolphinfish (Coryphaena hippurus) Longnose lancetfish (Alepisaurus ferox) Swordfish (Xiphias gladius)

Epi-Pel Epi- Meso Meso Bathy Meso- Bathy Functional prey groups vs >300 prey taxa depth

+Δ N 15 N = n 15 N = n + 4 Δ N Δ N = Trophic enrichment 15 N baseline 15N predator Stable isotope ratios of N in consumers reflect those of their prey as they are enriched in a predictable manner 15 N serve as indicators of consumer trophic position Stable Isotope Analyses ( 15 N, 13 C) a tool to investigate trophic relationships (trophic tracers) You are what you eat, plus a few per mil "

PRINCIPE : les processus biologiques, physiques et chimiques induisent une répartition différentielle des éléments légers et lourds Cause : essentiellement dus aux différences de masse - variation de certaines propriétés physico-chimiques - modification des vitesses des réactions chimiques réactivité différentielle changement du rapport entre éléments lourds et légers

Nitrogen Isotope Fractionation Rate of 14 NH 4 + excretion is greater than the rate of 15 NH 4 + excretion δ 15 N predator = ~3.0 + δ 15 N prey () NO 3 -, NH 4 + N 2 14 NH 4 +

Multispecies trophic models of ecosystems depend on accurate depiction of trophic links Trophic level estimation using 15 N Stable Isotope Analyses (d 15 N, d 13 C): a tool to investigate trophic relationships Latitudinal effect on d 15 N : differences in nitrogen dynamics existing at the base of the food web propagate along the food chain up to top predators Isoscapes…

15 N isoscapes for bigeye (n = 196) and yellowfin (n = 387) tuna Values were normalized against the average within the study region (Graham et al.) Residuals between the interpolated 15 N values for the two species bigeye yellowfin