Fishermen tend to believe that moon affects fish catchability but, in fact, there are few studies investigating lunar cycles in fish behaviour. Having good amount of historical Atlantic salmon catch records at the mouths of three Finnish rivers at hand, we decided to investigate whether any moon effects on the timing of freshwater entry could be found. A motivation behind the hypothesis is that lunar cycles have been detected in seaward migration of salmonid juveniles and eels. Moreover, the three rivers (Torniojoki, Iijoki and Oulujoki) provided a unique opportunity to study moon effect in isolation from lunar cycles in tides and illumination. Such secondary effects would easily obscure inference of primary moon effects on salmon behaviour.
So, what was the outcome of the study? Well, no biologically significant moon effect was found: although the moon effect was “statistically significant” it was extremely small in comparison to other covariates. But hey, the applied statistical models were so elegant! And note the fancy little moon symbols in the figures. Made by R.
Kuparinen A., O’Hara R.B., Merilä J. (2009) Lunar periodicity and the timing of river entry in Atlantic salmon Salmo salar L. Journal of Fish Biology 74: 2401-2408