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Study reveals new climate threat for fish

Climate change affects the ability of fish to form tight, defensive schools, leaving them more vulnerable to predators, Australian scientists say.

They’re also less able to dart quickly in the same direction, giving whatever is chasing them a better chance of a bigger meal.

University of Adelaide Professor Ivan Nagelkerken led a new study that used tanks to simulate two primary effects of climate change – warmer seas and ocean acidification.

He and his team then studied how those simulated conditions affect schooling behaviour – the main defensive mechanism for many species including tuna, sardines and anchovies.

The results aren’t good news.

Schools were less cohesive and less compact under future conditions, and showed slower escape responses from potential threats.

“A school that is more compact has better protection than a school that has fish with a bigger distance between each other,” Prof Nagelkerken said.

“What we found, under a future climate, is that schools of mixed species are much less compact. We also found these schools allow predators to approach to a closer distance before they would try to swim away.”

Ocean acidification also appears to affect a natural tendency for fleeing schools to move towards the right.

Typically schools will head right most of the time but will also throw in a handful of leftwards manoeuvres to keep whatever’s chasing them guessing.

“We found that under ocean acidification, the tendency to deviate to the right was much more diminished. That means the school, as a whole, functions differently compared to evolutionary times,” Prof Nagelkerken said.

The study indicates schooling fish will face increased mortality from predators as the climate continues to change.

“It doesn’t mean that all fish will die, but predators will likely be able to capture more fish,” said the professor from Adelaide University’s Environment Institute and Southern Seas Ecology Laboratories.

“When you combine that with the other impacts on the ocean, like destruction of habitat, overfishing, pollution, then less effective schooling behaviour is yet another thing fish will need to cope with in the bigger scheme of things.”

The study looked at both single-species schools, and mixed-species schools, with the latter of particular interest due to the southern migration of tropical fish into temperate waters now warm enough to sustain them.

The research has been published in the journal Global Change Biology.



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