How global warming makes fish more vulnerable to predators

By Mark Waghorn via SWNS

Global warming is changing the shoaling of fish, making them more vulnerable to predators, according to new research.

Vast schools, some numbering in hundreds of millions, make it less likely any one of them will be eaten.

They are among the most impressive sights in nature - packed so dense they can block out the light of the sun.

Now a study has found the iconic characteristic is being disrupted by ocean acidification.

Whales, dolphins, seabirds and other marine animals are finding it easier to pick them off.

Lead author Professor Ivan Nagelkerken, of the University of Adelaide, said: "Fish show gregarious behavior and cluster in shoals which helps them to acquire food and for protection against predators.

"Many gregarious tropical species are shifting poleward undercurrent ocean warming and interacting in new ways with fish in more temperate areas."

Rising concentrations of CO2 in the atmosphere is driving up sea surface temperatures.

Due to prevailing wind patterns and other natural phenomena, the ocean upwells nutrient-rich and more acidic or corrosive deep waters.

Lab experiments showed climate change is upsetting the way fish interact in groups.

Angus Mitchell, a Ph.D. student in the lab, explained: "We found tropical and temperate fish species tend to move to the right when coordinating together in a shoal, especially when spooked by a predator."

"But this bias significantly diminished under ocean acidification.

"Mixed shoals of tropical and temperate species became less cohesive under future climate conditions and showed slower escape responses from potential threats."

Earlier this year a report warned smaller portions of fish and chips will soon be on the menu because of global warming.

Co-author Prof David Booth, of the University of Technology, Sydney, said: "Our findings highlight the direct effect of climate stressors on fish behavior and the interplay with the indirect effects of new species interactions."

Previous research has identified warm water fish from the Mediterranean such as anchovy, red mullet, sea bass and John Dory shoaling northwards into the English Channel.

Added Professor Nagelkerken: "Strong shoal cohesion and coordinated movement affect the survival of a species - whether to acquire food or evade predators."

"If the ability for fish to work together is detrimentally affected it could determine the survival of particular species in the oceans of the future.

"Tropical species may initially fare poorly when moving into new temperate areas."

The findings in Global Change Biology follow a study in February that found cod and haddock in Scottish waters are shrinking.

Cold-blooded animals like fish develop faster at warmer temperatures but reach a smaller size, said the University of Aberdeen team.

Fishing is worth around £1.4 billion to the UK economy and employs over 24,000 people.