Fish are becoming part of smaller shoals, which offer less protection to individuals, as ocean pH becomes more acidic
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Ocean acidification is making reef habitats less complex and causing fish to gather in smaller shoals that offer less social protection, a study from Adelaide University has revealed.
Fish often move and feed in groups, as well as react to danger collectively. ‘For small reef fish, a shoal is a survival strategy – more eyes spot predators sooner, more bodies mean any one fish is less likely to be the unlucky one,’ said lead author Dr Angus Mitchell from Adelaide University.
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‘Fish in bigger groups tend to be bolder, as they forage more efficiently, stay out in the open more, and spend less time hiding,’ said Dr Mitchell, whose study was published in the Journal of Animal Ecology.
Mitchell’s study found that the size of a fish shoal affects both their collective and individual behaviour. Crucially, the study found that these changes in behaviour were not attributable to the direct impacts of higher temperatures and lower pH.
‘Across all reef types, even during a heatwave, the fish behaved in much the same way. They kept feeding. They did not suddenly become more active,’ said Mitchell.
The authors of the study said that while studies looking into direct impacts of climate change on the environment are important, the broader context should also be considered when assessing climate change impacts.
‘In the real world, fish do not experience climate change in isolation; they experience it as members of communities, shaped by the habitat around them and the other individuals they live alongside,’ he said.
‘Our results suggest that even when individual fish seem to be coping fine behaviourally under climate stress, the social structures supporting their behavioural expression can quietly fall apart.’
How bad is ocean acidification?
Ocean acidification is often coined as climate change’s ‘evil twin’ due to the harm it can cause. The process arises because of excess carbon dioxide in the atmosphere dissolving into the world’s seas, making water more acidic. Ocean carbon dioxide concentrations are now higher than at any time during the past 800,000 years, with the current rate of increase likely unprecedented.
Every day, the world’s oceans absorb around 22 million tons of carbon dioxide from the atmosphere, with significant consequences for marine life and ecosystems alike. Some marine species may be able to adapt to more extreme changes, but many will likely suffer, and some will go entirely extinct.
Ocean acidification causes problems particularly for species including oysters, clams, lobsters, shrimps and coral reefs, as acidic conditions eat away at the minerals they use to build shells and skeletons. In the waters off Scotland and Norway, as well as the Caribbean and Great Barrier Reef, more acidic waters are weakening coral structures and reducing habitat for fish and the resilience of the entire reef system. Weaker coral structures also lessen the support that they can provide for storm protection.
In addition, ocean acidification also affects society. Many jobs and economies around the world rely on the fish and shellfish that live in the ocean. Decreasing harvests could hurt the least developed nations, leading to social disruption and even conflict.
