[kirrent]

Popularly it's been reported by mariners that the whales are asleep. It makes sense, they need to stay on the surface to breathe and there's no evolutionary reason not to sleep there. It's really not that simple though because whales are unihemispheric sleepers (one brain hemisphere sleeps at a time) who need to stay partially awake because all their breathing is voluntary. They maintain a degree of awareness to their environment because of this. It could be a factor though because it's possible that some whales lapse into a deeper sleep for periods between breaths (https://doi.org/10.1016/j.cub.2007.11.003) where they aren't responsive to approaching vessels.

When I was interested in whale collisions I was surprised to read this review (https://doi.org/10.3389/fmars.2020.00292) which didn't even consider sleeping as a large risk factor for collision. Instead, factors included:

- They're involved in distracting behaviours such as feeding, socialising, foraging, resting, etc.

- Acoustics are complex near the surface involving surface reflections and direct paths which can interfere.

- Ships may form an acoustic shadow in front of themselves. Not only the hull shadowing the propeller, but also other hull sounds.

- Sailing vessels, which are the source of a lot of reports (harder for them to miss it happened) are quiet.

- Even when they hear an approaching vessel, some species just move slowly to avoid them.

These collisions apparently used to be much rarer. Ironically, the increasing number of whale injuries and deaths are a result of recovering populations.