Comparative feeding strategies of yellowfin tuna around St Helena and adjacent seamounts of the South Atlantic Ocean

Abstract

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.

Yellowfin Tuna Behavioural Ecology and Catchability in the South Atlantic: The Right Place at the Right Time (and Depth)

“…Previous studies show similar diel patterns which have been linked to YFT tracking the behaviour of prey inhabiting the deep scattering layer (Weng et al, 2009). The increasing role of crustaceans in the cold season (Laptikhovsky et al, 2020) might also explain the switch in behaviour to strict predominance of DVM as mesopelagic shrimps became available at night and in relatively deep lower epi-pelagic region (Burdett, 2016).…”

“…Although in the tropics, St Helena experiences considerable seasonal environmental variation (e.g., SST ∼18 to 26 • C) which result in discrete and highly seasonal spawning events, providing crucial feeding opportunities for YFT. An increased incidence of rDVM in the summer season is consistent with YFT seeking to exploit seasonally available productivity close to St Helena island, including the butterfly fish Chaetodon sanctaehelenae (Laptikhovsky et al, 2020). YFT likely target butterfly fish and similar juvenile neritic fish during the day as these species are inactive and rest on the seabed during the night, and so are unavailable to the tuna during this time (Ehrlich et al, 2009).…”

“…For the YFT in the present study, it is not possible to assess the exact threshold when a fast start occurred without video footage recording fast start behaviour and aligning with the time series of acceleration (Broell et al, 2013). YFT target fast-moving prey including small pelagic fish and squids (Laptikhovsky et al, 2020), with multiple prey consumed in a single feeding period.…”

“…The yellowfin tuna are known as relatively non-selective, opportunistic feeders and consume a broad range of available prey (Laptikhovsky et al, 2020). They can utilise species that are available only for a short period of time due to temporal increases in abundance like swimming crabs (Dissanayake et al, 2008).…”

“…Analysis of stomach contents from St Helena's YFT population is summarised in Laptikhovsky et al (2020). Although in the tropics, St Helena experiences considerable seasonal environmental variation (e.g., SST ∼18 to 26 • C) which result in discrete and highly seasonal spawning events, providing crucial feeding opportunities for YFT.…”