Albacore tuna (Thunnus alalunga) is a highly commercial fish species harvested in the world's Oceans. Identifying the potential links between populations is one of the key tools that can improve the current management across fisheries areas. In addition to characterising populations' contamination state, chemical compounds can help refine foraging areas, individual flows and populations' structure, especially when combined with other intrinsic biogeochemical (trophic) markers such as carbon and nitrogen stable isotopes. This study investigated the bioaccumulation of seven selected trace metals - chromium, nickel, copper (Cu), zinc (Zn), cadmium (Cd), mercury (Hg) and lead - in the muscle of 443 albacore tunas, collected over two seasons and/or years in the western Indian Ocean (WIO: Reunion Island and Seychelles) and in the south-eastern Atlantic Ocean (SEAO: South Africa). The main factor that explained metal concentration variability was the geographic origin of fish, rather than the size and the sex of individuals, or the season/year of sampling. The elements Cu, Zn, Cd and Hg indicated a segregation of the geographic groups most clearly. For similar sized-individuals, tunas from SEAO had significantly higher concentrations in Cu, Zn and Cd, but lower Hg concentrations than those from WIO. Information inferred from the analysis of trophic markers (δC, δN) and selected persistent organic pollutants, as well as information on stomach contents, corroborated the geographical differences obtained by trace metals. It also highlighted the influence of trophic ecology on metal bioaccumulation. Finally, this study evidenced the potential of metals and chemical contaminants in general as tracers, by segregating groups of individuals using different food webs or habitats, to better understand spatial connectivity at the population scale. Limited flows of individuals between the SEAO and the WIO are suggested. Albacore as predatory fish also provided some information on environmental and food web chemical contamination in the different study areas.

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The mineral composition of target and non-target pelagic fish caught by purse-seiners and longliners in the western-central Indian Ocean was determined. From the ten essential elements analysed, selenium and zinc showed the highest concentrations in swordfish and blue marlin while Indian mackerel appeared as a good source of copper, iron and chrome. All catch had levels of lead and cadmium, two toxic elements, below the maximum sanitary limits. Although some concerns were raised regarding mercury concentrations in the largest species (wahoo, swordfish and blue marlin), molar ratios of mercury and selenium indicate that all oceanic pelagic fish from the western-central Indian Ocean are safe for human consumption. This study also gives insights on the relationships between the levels of essential and toxic elements in fish muscle and the size, trophic position and diet sources of the studied pelagic species.

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In this study we investigated the trophic ecology of albacore tuna in the western Indian Ocean and adjacent Atlantic waters based on stomach content analysis using a reconstituted length and weight of prey approach. From 686 non-empty stomachs collected between 2001 and 2015 across three biogeographic provinces, we describe the diet composition of albacore tuna, analyse its feeding habits and investigate the structure and diversity of mid-trophic-level communities. Epipelagic fish were found to be the principal prey by number and reconstituted weight; cephalopods were the second important prey group. Small organisms prevailed in the diet of albacore tuna, with predation on juvenile fish commonplace. Albacore tuna exhibits a flexible, opportunistic feeding strategy, from ram filter feeding on abundant schooling prey to visual predation on large individuals. Prey species richness varied highly across the region. Oligotrophic conditions within the subtropical gyre of the Indian Ocean generated the most diverse mid-trophic-level communities, with less diverse communities occurring in productive areas. Albacore tuna occupies a similar trophic niche throughout the global ocean, foraging on the same prey families and even species. This study indicates overall temporal stability of the Indian Ocean and south-east Atlantic ecosystems where principal prey species remain unchanged over decades.