Sky-lit stage: lunar and solar rhythms shape the acoustic behaviour of marine mammals and fish

Rhythmic lunar and solar cycles are well recognised to influence the biology, behaviour, physiology, and ecological adaptations of many terrestrial organisms, yet their effects on marine organisms remain poorly understood. In this study, we investigated the influence of lunar and solar rhythms, together with total cloud cover as a light-blocking factor, on the acoustic behaviour of nine vocally active cetacean species, one pinniped species, and one fish species off Antarctica, the sub-Antarctic region, and the west coast of South Africa. These marine organisms exhibited species-, season-, and site-specific behavioural responses to cloud cover, lunar and solar rhythms. Total cloud cover, lunar phase or solar altitude (or, on a few occasions, moonlight intensity/lunar altitude) were the most important predictors of the observed acoustic behaviour. Observed behavioural changes related to light availability were also associated with breeding and migration phenology, communication, foraging and predator-avoidance strategies of the studied animals. On the whole, light availability drives the acoustic ecology and chronobiology of marine mammals and fish, likely as adaptations to their dynamic marine environments. Shifts in acoustic and diving behaviour relative to light availability have conservation implications, as sound production may reflect time spent near the sea surface and increase animals’ vertical risk of ship-strike. These predictable chronobiological patterns provide an empirical foundation for dynamic marine spatial management, suggesting that seasonal vessel slowdowns during dark-sky cycles could significantly reduce anthropogenic ship-strike risk in high-traffic zones like the South African coast.