Otolith fluctuating asymmetry: a misconception of its biological relevance?

For decades, disruption of the bilateral symmetry of body structures has been related to underperformance and, hence, to fitness. In fish, this concept coupled with the claimed evidence for increased fluctuating asymmetry (FA) of pairs of hard structures, such as otoliths under conditions of stress, has led to the use of otolith FA (OFA) as a proxy for individual fitness and population success. Interpreting those significant differences in OFA in relation to stress-inducing environments and using them to identify suboptimal conditions for survival is especially appealing in the context of, for example, assessing climate change using historical otolith collections. Despite several works that give evidence to support the usefulness of OFA approach, we report on a number of unpublished and published datasets from a wide range of both, wild populations and experimental designs that show no correlation between OFA and any of the usual indicators of fitness or between OFA and expected stress gradients. Our results suggest that there may be a strong bias in the published literature towards positive relationships, despite the enormous importance of negative results for understanding the significance of OFA. These results might shed light on the interpretability of OFA in both laboratory and field studies.