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Diurnal fluctuations in total integrated echo abundance and in vertical density profiles were examined usingdata from the Norwegian combined acoustic and bottom-trawl survey for demersal fish during winter in the BarentsSea. The total echo abundance was about 40%–50% higher at day than at night. An unknown amount of fish was lostclose to the seabed in the acoustic dead zone, but the systematic changes in the near-bottom vertical density profilesdid not indicate that migration in and out of the dead zone was the major reason for the large diurnal differences inecho abundance. A more plausible explanation could be that diurnal changes in fish behaviour affect the mean acoustictarget strength. Based on the present study, we recommend that the time series of acoustic surveys should be reanalysed,taking the diurnal bias into account. Any comparison of the fish densities indicated by trawl and acousticsurveys will suffer if this bias is not corrected. We believe that model development utilizing this type of information iscrucial for future ecosystem-based monitoring

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