As oceans warm, tropical fish are moving south. New friendships may be helping them survive

When you think about climate change in our oceans, you may picture coral bleaching, melting sea ice, or extreme weather events. But beneath the ocean’s surface, another quiet shift is underway. Australia’s tropical fish are heading south into cooler waters.

These fish are not just visiting. They are settling into the milder “temperate” reefs that used to be too cold for them. As they do, they encounter new environments, new challenges and new neighbours.

In our new research we studied the behaviour of these new migrants. We found some tropical fish are not just surviving in their new homes, they’re thriving. And, surprisingly, much of that success comes down to who they’re hanging out with.

A slow-motion invasion

Tropical fish travel poleward via ocean currents.

On Australia’s east coast, the fish typically hitch a ride on the strengthening East Australian Current as it pushes warm water and the tropical species further south.

Some species are showing up hundreds of kilometres beyond their usual home range. Many tropical fish arrive on temperate reefs during summer, and used to die over winter when the water grew colder. Now, as winter water temperatures increase, some tropical fish survive year-round in temperate reefs.

But life at the edge of your range is risky. These fish encounter colder water temperatures, unfamiliar predators and a reef full of competitors. So, how do they cope?

Risky business: but some fish can adapt

We studied five tropical fish species and two temperate species across a 2,000km stretch of Australia’s east coast, from the tropics to the cold temperate south. We observed how these fish fed, sheltered and reacted to threats, using underwater video cameras.

Analysis of the footage revealed tropical fish behaved differently in the colder waters. They spent more time hiding and less time feeding. They were also more wary of predators, displaying a cognitive shift in “lateralisation” — a preference to consistently turn left or right, which can help fish make faster escape decisions when threatened.

Such risk-averse behaviour is likely to help fish stay alive in unfamiliar reefs by avoiding predators. But it also reduces food intake and growth, unless these fish find new friends.

New school mates, better outcomes

Previous research has shown when tropical fish gather or “shoal” with temperate fish, they grow bigger and survive longer into winter than fish in tropical-only shoals.

We wanted to understand the mechanism for this phenomenon. Could tropical fish be learning from temperate shoal mates? And how might their behaviour change when shoaling with temperate fishes?

Using underwater videos, we found three tropical damselfish species spent more time feeding and less time sheltering when they formed mixed shoals with temperate fish. They also appeared bolder and were more successful at finding food.

We think these mixed shoals offer key advantages: safety in numbers, more eyes watching for predators, and perhaps most importantly, social learning. By shoaling with local temperate species such as the Australian Mado, tropical fish may learn where and when it’s safe to feed, and how to behave in these foreign temperate ecosystems.

This kind of behavioural “plasticity” is a powerful tool in a changing climate. Fish that can adjust their behaviours in ways that boost their fitness are more likely to survive as climatic conditions rapidly shift in our oceans.

Not all fish benefit

These interactions were not always beneficial. Two herbivorous tropical fish species, the convict tang and brown tang, did not show the same benefits, likely because their specialised diets made it harder to learn from omnivorous temperate species.

And for the temperate fish, the presence of tropical fish in shoals were often problematic. At the northern, warmer edge of their range, temperate fish fled more often and fed less when tropical fish were present. That’s worrying, because warming alone is already pushing many temperate species toward their biological limits. Adding new competitors might push them over the edge.

A changing reef community

All this comes amid dire news of the Earth’s oceans. Research published today shows 2023 set new records for the duration, extent and intensity of marine heatwaves.

Fish migration to temperate reefs is a glimpse of the future: even warmer waters, shifting species ranges and new species interactions.

Our results suggest these new species interactions and relationships, particularly mixed-species shoaling, can help tropical fish survive longer in temperate ecosystems. But they may also disrupt existing ecosystems and place extra stress on local temperate species.

In this way, climate-driven range shifts are more than just a temperature driven story. They’re stories about behaviour, relationships, and resilience.

Understanding how fish respond to their new neighbours and how those responses shape who stays and who goes, will be key to managing reefs in a rapidly warming ocean.