The yellowfin tuna ( Thunnus albacares: YFT) is a widely distributed, migratory species that supports valuable commercial fisheries. Landings of YFT are seasonally and spatially variable, reflecting changes in their availability and accessibility to different fleets and metiers which, in turn, has implications for sustainable management. Understanding the dynamics of YFT behaviour and how it is affected by biological and ecological factors is therefore of consequence to fisheries management design. Archival and pop-up satellite tags (PSAT) were used in the South Atlantic Ocean around St Helena between 2015 and 2020 to collect information on the movements, foraging and locomotory behaviour of YFT. The study aimed to (1) identify vertical behaviour of YFT within St Helena’s EEZ; (2) assess the timing and depth of potential feeding events and (3) to use the information to inform on the catchability of YFT to the local pole and line fishing fleet. Results indicate that the YFT daytime behaviour shifted between shallow with high incidence of fast starts in surface waters in summer months (December to April), to deep with high incidence of strikes at depth in colder months (May to November). Catchability of YFT was significantly reduced between May and November as YFT spent more time at depths below 100 m during the day, which coincides with a reduction in the quantity of YFT caught by the inshore fleet.

The yellowfin tuna (Thunnus albacares) is a widely distributed, migratory species that supports valuable commercial fisheries throughout their range. Management of migratory species requires knowledge of movement, mixing and key life history parameters such as growth rate, natural and fisheries mortality. Current management is based on the assumptions that the species is highly migratory and populations are well mixed, but these assumptions have been questioned by recent studies. Since November 2015, yellowfin tuna have been tagged with conventional, archival and pop-up satellite tags (PSAT) in the South Atlantic Ocean around St Helena, with the goal of better understanding their movement patterns and ecology in this region. Conventional tags were attached to 4049 yellowfin tuna (size range 24-158 cm fork length, FL), PSAT tags were deployed on 15 yellowfin in inshore St Helena waters (size range 95-138 cm FL) and 7 yellowfin (size range 125-140 cm FL) at Cardno Seamount, and archival tags were deployed on 48 yellowfin tuna in inshore St Helena waters (size range 69-111 cm FL). Most yellowfin tuna remained within 70 km of their release location, suggesting a degree of retention to the region. Although displacement of yellowfin was generally low, the furthest distance travelled between release and recapture location was 2755 km, with other tuna also displaying large-scale movements. Tagging revealed connections between inshore regions and seamounts, as well as links between St Helena waters and key fishing regions and putative spawning grounds in the Gulf of Guinea.

Yellowfin tuna are the mainstay of the traditional tuna fisheries in St Helena waters, but there is limited knowledge of their ecology and feeding behaviour in the area. In this study yellowfin tuna stomach contents were used to assess spatio-temporal changes in feeding strategy and consider the role of tuna in the local ecosystem. Comparisons of the feeding spectra of yellowfin tuna between inshore regions of St Helena and oceanic seamounts demonstrated that in both areas the species was largely piscivorous. In inshore waters yellowfin consumed more neritic fauna, including significant numbers of crab megalopa, whereas around seamounts the diet included a greater diversity of epi- and mesopelagic fish and squids. The most important fish prey species in inshore waters was the St Helena butterflyfish Chaetodon sanctahelenae, and around seamounts was the pufferfish Lagocephalus lagocephalus. Results indicate that the diet spectrum of yellowfin tuna in St Helena waters is relatively similar to those of conspecifics living in waters with relatively low productivity, with strategies indicative of food-poor ecosystems. The availability of coastal fauna may make areas around islands and seamounts more attractive for feeding aggregations of yellowfin tuna, compared to the open ocean. The relatively unselective feeding of yellowfin tuna means that stomachs can provide valuable data on the species diversity, particularly in remote areas with limited opportunities for dedicated research expeditions.