Diurnal variation in acoustic densities: why do we see less in the dark?

No abstracts are to be cited without prior reference to the author.

In the Barents Sea, the echo abundance of demersal fish is about 50% higher at day than at night. Three possible reasons are: • Fish hide in the bottom dead zone at night. • Fish target strength changes due to pressure-related swim-bladder changes during vertical migration. • Fish target strength varies due to higher variation in the tilt-angle distribution at night. It is very often claimed that the dead zone is most important. We argue that variation in tilt-angle may also be important.