We describe the total lipid content, lipid class composition and fatty acid profiles of adult forage fishes (anchovy, sardine and sprat) sampled in the NW Mediterranean Sea in 2010 and 2011. Inter-and intra-species differences were mostly related to sampling period with limited effect of gender or total length. As an assemblage, total lipid content and relative levels of triacylglycerols and fatty acids 16:1n7, 20:5n3 and 14:0 in forage fish were highest in summer and autumn, indicating better feeding conditions and a more pronounced diatom-supported food web. In contrast, total lipid content was lowest at the end of winter and spring, and coincided with high levels of 22:6n3, indicating a more herbivorous diet based on dinoflagellates. Resource partitioning and niche separation, as inferred from fatty acid profiles, were apparent between species. Sardine showed a more diverse, temporally separated feeding strategy than anchovy, and dietary overlap was higher in winter than summer with sardine having higher markers of copepods, 22:1n11 and 20:1n9. Sprat collected in winter occupied a separate niche area to both sardine and anchovy with higher total lipid content and carnivory biomarker 18:1n9. Our results show that the lipid dynamics of forage fishes can be used to gain quantitative insights into sub-system level changes in species interactions, including prey and predator productivity.

The lipid composition of somatic and reproductive tissues was determined for female skipjack tuna Katsuwonus pelamis caught in the western Indian Ocean between latitude 10° N and 20° S and longitude 40° and 70° E. The highest total lipid (TL) contents were in the liver and gonads, with white muscle levels approximately three-fold lower. Three lipid classes dominated: triacylglycerols (TAG), sterol esters and wax esters (SE-WE) and phospholipids (PL). Collectively, these accounted for between 70 and 80% of TLs. Changes in lipid concentrations were evaluated over the maturation cycle. Immature fish had the lowest gonad and liver TL levels; concentrations of TL, TAG, SE-WE and PL accumulated from immature to mature (spawning-capable) phase, reflecting sustained vitellogenic activity of the liver and a transfer of lipids to developing oocytes from the onset of vitellogenesis. Gonado-somatic and hepato-somatic indices were positively correlated with each other and positively related to TL in the gonads and liver. Fulton's condition index and lipid concentrations in muscle did not vary significantly over the maturation cycle; fat content in the main storage tissues was undepleted as the ovary developed. Hence, K. pelamis apparently supports reproduction directly from food intake over the breeding season. In the gonads, reserve lipids (SE-WE and TAG) and sterols were related to batch fecundity but this was not the case for somatic and hepatic tissues. These results suggest that K. pelamis utilizes an income breeding strategy.

In an area that has barely been modified by fish aggregating devices (FADs) (e.g., the Mozambique Channel, known to be naturally enriched with logs, with few FADs), we found that the condition of skipjack tuna (Katsuwonus pelamis) associated with floating objects was lower than those in free-swimming schools. As this result was found in an area that mimics the environmental state prior to the use of FADs, it questions the interpretation of previous studies where such a difference has been interpreted to reflect the impact of extensive FAD deployment on tunas. It is possible that before the use of FADs, tunas associated with logs were also in poorer condition than tunas in free-swimming schools. Our results suggest that the evolutionary reason for which tunas associated with floating objects might not relate to short-term trophic benefit, and alternative hypotheses (e.g., the meeting point hypothesis) are discussed to explain this result.Résumé : Dans une région très peu modifiée par des dispositifs de concentration de poissons (DCP), le canal du Mozambique (naturellement riche en billes de bois, mais comptant peu de DCP), nous avons observé que l'embonpoint de listao (Katsuwonus pelamis) associées à des objets flottants était plus faible que celui de leurs congénères nageant en bancs libres. Puisque ce résultat porte sur une zone analogue au milieu naturel avant l'utilisation de DCP, il remet en question l'interprétation d'études antéri-eures voulant que cette différence reflète l'incidence d'un vaste déploiement de DCP sur les thonidés. Il est possible que, avant l'utilisation de DCP, les thons associées aux billes de bois présentaient également un moins bon embonpoint que les thons nageant en bancs libres. Nos résultats semblent indiquer que la raison évolutive pour laquelle les thonidés sont associées à des objets flottants ne serait peut-être pas être reliée à des avantages trophiques à court terme, et d'autres hypothèses (p. ex. l'hypothèse du point de rencontre) sont invoquées pour expliquer ces résultats. [Traduit par la Rédaction]