Ocean acidification refers to the decrease in the pH of the world’s oceans owing to the absorption of carbon dioxide from the atmosphere. One of its alleged effects is marked changes in the behaviour of fish that inhabit coral reefs.
For example, one 2010 study published in Proceedings of the National Academy of Sciences found that exposing larval fish to elevated CO2 disrupted their olfactory systems, leading them to become attracted to the smell of their predators. As a result, they experienced a 5–9 fold increase in mortality from predation.
Such studies have generated substantial media coverage, including headlines such as ‘Increasingly acidic oceans are causing fish to behave badly’ and ‘Losing Nemo – acid oceans prevent baby clownfish from finding home’. And they’ve even been presented at the White House.
Yet according to a recent meta-analysis, effect sizes in this literature have declined dramatically over time – suggesting that early studies (like the one mentioned above) overstated the impact of ocean acidification on fish behaviour.
Jeff Clements and colleagues reviewed 91 studies published between 2009 and 2019, and obtained an ‘effect size’ from each one. Here, the effect size was a measure of the difference in fish behaviour between the control group, and the treatment group that had been exposed to lower pH.
The authors main results are shown in the chart below. The left-hand panel shows all the effect sizes (91 in total), while the right-hand panel shows the average value for each year.