Award: OCE-1851558

Bluefin tunas are the largest and widest ranging tunas. All three bluefin species (Atlantic, Pacific and Southern) live in rich feeding areas of the cool high-latitude oceans, but each migrates 1000s of miles each year to reproduce in a unique small tropical area that is warm and oligotrophic, meaning nutritionally very poor. For the endangered Southern Bluefin Tuna (SBT), its unique spawning region is the small gap between southern Indonesia and northwestern Australia, which is also the place (Indonesian Throughflow) where heated water from the western Pacific pours into the eastern Indian Ocean. In a program called BLOOFINZ (Bluefin Larvae in Oligotrophic Ocean Foodwebs, Investigations of Nutrients to Zooplankton), we visited this remote and poorly known region on a research cruise in January-March 2022, the peak time of SBT spawning. A major study  in 1987 had indicated that the tuna larvae (about ¼ inch in size) were starving and growing slowly. We wanted to learn how the system worked to produce the animal food (zooplankton) for larvae, including where nutrients (nitrogen fertilizer) came from for primary producers (phytoplankton), how phytoplankton were used among competing hungry consumers in the food web, whether larval conditions had changed over the past 35 years, and whether the region would be able to support tuna larvae in the future. We called our approach an end-to-end or physics-to-fish investigation, because the goal was not just to find a result for the larvae, good or bad, but to understand how the system operated to produce that outcome. This is important because larvae of other large and economically valuable fishes have similar feeding behaviors and face similar survival challenges in poor warm-water conditions. So a detailed study of one species in one place might apply more broadly.

We found some expected things, and others that would not have been predicted or known from past studies. Among the expected, primary production was done mainly by photosynthetic bacteria too small to be eaten directly by the types of zooplankton (copepod crustaceans related to, but much smaller, than shrimp) that bluefin larvae were known to prefer. We also found that the very warm surface waters (up to 86 degrees F) where larvae lived were too strongly stratified, because the warm water floated on the cold denser water below, to allow much mixing of nutrients from below. Most of the nitrogen nutrients came from special bacteria that fixed nitrogen from the air dissolved in water, the same way that some garden plants like peas and beans use bacteria in their roots to convert nitrogen gas from air to a usable fertilizer. One major accomplishment of the study was that the results from many experiments that were conducted produced a detailed map of the food web flows of carbon and nitrogen from their sources to their eventual fates. 

Among the unexpected results, we found that SBT larvae were feeding on a very different prey type than they did in the study 35 years ago and in almost every study done with different bluefin species since. Instead of almost 100% crustaceans, 60% of the larval diet was appendicularians, a small sea squirt-like animal that lives in mucus houses with fine filters able to capture bacteria-sized phytoplankton. The direct feeding connection from bacteria to appendicularians to tuna larvae is the most efficient way for the food web to operate without losses to middle steps. Because of the change in diet preference, the 2022 larvae were getting 4 times more food and growing faster than in the 1987 study. Appendicularians have been predicted to become more important as the oceans get poorer, and here we were possibly seeing the first evidence of that!

Using growth rings on the microscopic inner ears of the larvae to measure age (days) and nitrogen isotope indicators of diet from muscle tissues, we also found that the fastest growing larvae were those feeding the most on prey like appendicularians, a specialized (narrow) diet on prey closest to the food-web base. Additionally, the most successful larvae, and those most likely to survive to contribute to the fished stock, came from adult mother fish with similar characteristics of selecting a narrow specialized diet rather than broad mix of many prey types.

Overall, our study provided new insights into the factors that determine the characteristics of spawning habitats, larval feeding selectivity and maternal influences that can lead to success (fast growth, survival) of tuna larvae, and we documented a possible change in oligotrophic ocean food webs (selection for appendicularians) that will benefit tuna larvae. While this award only directly supported a small group of US investigators, it built and benefitted greatly from collaborations among colleagues from 11 US institutions and 11 non-US institutions. Nine students who worked on the project earned advanced degrees (2 MS, 7 PhD).

 

Last Modified: 02/17/2026
Modified by: Michael R Landry