Why Can't Nemo Find a home?

Bethany Whant

I’m sure you have heard of the famous clownfish ‘Nemo’. But what if you were told that we are triggering a change in our oceans that is expected to cause rapid declines in this much-loved movie star.

Photo: Millie Rose

Photo: Millie Rose

The culprit? A form of global change termed ocean acidification (OA) ……

Since the industrial revolution, billions of carbon dioxide (CO2) and other greenhouse gases have been pumped into the Earth's atmosphere. Around 40% of this CO2 is now stored in the oceans, making the water more acidic. The Intergovernmental Panel on Climate Change have identified this process as a growing concern, which has already impacted the chemistry of our oceans. Numerous studies have documented how acidic oceans can make it difficult for shellfish and organisms to form shells, reproduce and survive. But now scientists are finding that acidic oceans are influencing species behaviour making them more likely to end up as a tasty snack at mealtime!

Philip Munday, a professor and research fellow at the Coral Reef study centre at James Cook University in Australia found that when clownfish were exposed to more acidic environments they became bolder in their behaviour and more attracted to smells they would normally avoid, including those of predators and unfavourable habitats.

Photo: Rose Boardman

Photo: Rose Boardman

Further research has now discovered that heightened CO2 levels can also significantly change their ability to smell. Clownfish rely on their sense of smell to indicate optimum sites in which to grow therefore this a great concern. In a controlled experiment, juvenile clownfish had a choice of two flow chambers (one with and one without the scent of predators).

Those exposed to heightened CO2 levels spent 93% of their time in the chamber with the predator's scent whereas the fish that were not exposed to acidic water avoided the flow that carried the scent of predators. Scientists suspected that higher CO2 levels interfere with a neurotransmitter called GABA-A, which helps to moderate activity in the brain.

Although the exact reason why heightened CO2 changes a fish's behaviour can’t be fully explained, it is certain that many species will struggle to survive as oceans continue to acidify. Beyond this, we must also consider that OA is not occurring in isolation, with other environmental changes such as increasing temperatures. These challenges highlight that we still have much to learn. Yet, from the understanding, we do have it is clear that OA is a process we should be striving to avoid and rectify.

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Rising Tides is a series focussing on the current threats facing our ocean, how these are being tackled and what you can do to help