Background/Aims: Southern bluefin tuna (Thunnus maccoyii, SBT) range broadly in high latitudes of the southern hemisphere but spawn only in a small tropical area off northwestern Australia. During the peak spawning season of Jan-Feb 2022, cruise RR2201 of the BLOOFINZ (Bluefin Larvae in Oligotrophic Ocean Foodwebs, Investigations of Nutrients to Zooplankton) program conducted experimental studies in the spawning habitat to assess nitrogen sources, controls on primary production, plankton structure, grazing pathways and food web fluxes that support SBT larvae.
Methods: Concurrent analyses of SBT larval abundance (net tows), feeding (stomach analyses, prey abundances) and growth (otolith analyss) in 2022 also allowed direct comparison to similar results from the last major study of the region in Jan-Feb 1987.
Results: Consistent with climate change projections, surface waters were 2°C warmer and more stratified in 2022, but rather than negatively impacting habitat quality, feeding incidence, prey consumption and larval growth rates were all significantly higher than in 1987. The major change was a larval dietary shift from copepods in 1987 to appendicularians in 2022, which improved transfer efficiency from the microbially dominated food web to zooplankton prey. Based on these studies, we contrast two very different climate change scenarios for SBT recruitment.
Conclusion: While not necessarily indicative of long-term outcomes for SBT, the large differences in these results illustrate that climate-change impacts can involve complex trophic interactions that are difficult to predict from general warming and stratification trends in oligotrophic systems.