Pacific bluefin tuna are majestic, torpedo-fast, super-sized apex marine predators. They're also delicious, according to sushi lovers around the world, which has led to a high-priced tuna fishery and a worrying decline in their numbers. Unlike most other fishes, tuna are warm-blooded and, just like humans, have a preferred temperature range at which the cost of keeping warm is lowest. Tuna occupy a vast interconnected habitat in the northern Pacific and migrate long distances to find patches of prey that are also on the move. Some marine predators survive by following rigid migration routes, a strategy that works well when conditions and prey are predictable. However, with climate change looming large, more flexible migrations might have benefits, by allowing predators to target prey as they become available. Surprisingly little is known about how tuna balance the needs for finding prey and staying warm, which is critical for protecting their vulnerable populations against overfishing and environmental change.
But how could one possibly track a wild torpedo-fast fish in the open ocean and measure its success at finding prey? Gemma Carroll at the University of California Santa Cruz, USA, with an international team of collaborators led by Barbara Block at Stanford University, USA, found a way. They caught tuna in California and surgically implanted high-tech bio-loggers into their bellies that recorded the fishes’ location, depth and water temperature, before releasing the fish to continue with their migration. But best of all, the devices also measured the fishes’ internal body temperature and the heat produced when digesting a meal, which revealed how much prey each fish consumed and where. The team repeated the procedure on hundreds of tuna over a staggering 15-year period, yielding a wealth of novel findings.
During this long-term experiment, ocean temperatures varied greatly between seasons and years. And yet, bluefin tuna managed to spend most of the time in waters that were within their thermal comfort zone. In warmer years, tuna shifted their migration pole-ward to colder waters; in cooler years, they headed south. This strategy even worked during the heatwave of 2015 when some tuna deviated from their usual routes by more than 1500 km and kept their cool by foraging in northern waters. This tremendous flexibility in migration patterns allows tuna to dampen the negative impacts of a warming and increasingly variable climate. But whether their prey follow suit is a different question.
Ocean heatwaves can disrupt entire food chains, first impacting primary producers, such as tiny algae, and then propagating the effects up to the apex predators. During the heatwave of 2015, tuna found less prey than they had in cooler years, but the northward shift in their migration route helped their foraging success. Therefore, tuna avoided the more devastating effects of the heatwave experienced by sea lions, grey whales and some seabirds, which are a more inflexible group of marine predators. To find scarce prey, tuna were even willing to leave their thermal comfort zone, spending more time hunting in northern waters that were colder than their usual preference. Being cold and miserable surely beats starvation.
Commercial fishery for Pacific bluefin tuna is seeing increasing catches, but that is not due to a recovery in their stocks, as their populations remain at an all-time low. Rather, it appears that changes in the temperature and prey landscape are pushing tuna into the fisher's nets, yielding record catches that, in the long run, are unsustainable. Exceptionally flexible migration patterns allow bluefin tuna to keep their cool during heatwaves, but may not protect them entirely from climate change at large. This is bad news for everyone: tuna, fishers and sushi lovers alike.