Jeff Clements, Josefin Sundin, Timothy D Clark, Fredrik Jutfelt 

Ocean acidification – decreasing oceanic pH resulting from the uptake of excess atmospheric CO2 – has the potential to affect marine life in the future. Among the possible consequences, a series of studies on coral reef fishes suggested that the direct effects of acidification on fish behaviour may be extreme and have broad ecological ramifications. Recent studies documenting a lack of effect of experimental ocean acidification on fish behaviour, however, call this prediction into question. Here, we explore the consistency and robustness of scientific evidence over the past decade regarding direct effects of ocean acidification on fish behaviour by testing for a “decline effect”. Using a meta-analysis, we provide quantitative evidence that the research to date on this topic is characterized by a decline effect, where large initial effects have all but disappeared over a decade. The decline effect in this field cannot be explained by three likely biological explanations, including increasing proportions of studies examining (1) cold-water species, (2) non-olfactory associated behaviours, and (3) non-larval life stages. Furthermore, the vast majority of studies with large effect sizes in this field tend to be characterized by low sample sizes, yet are published in high impact journals and have a disproportionate influence on the field in terms of citations. We contend that ocean acidification has a negligible direct impact on fish behaviour, and we advocate for improved approaches to minimize the potential for a decline effect in future avenues of research.