Diurnal variation in frequency response of gadoids in the Barents Sea

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Earlier studies have demonstrated substantial diurnal variation in acoustic abundance of fish. Both the level and the uncertainty of abundance estimates from surveys carried out around the clock are affected. It has been hypothesized that diurnal variation in acoustic densities is caused by a diurnal variation in tilt angle. As the directivity of fish is higher at higher frequencies a diurnal variation in tilt angle will affect the acoustic recordings more for higher than for lower frequencies. This hypothesis is tested through an analysis of the frequency response of acoustic recordings of cod and haddock in the Barents Sea from 18, 38, 70 and 120 kHz Simrad EK 60 echo sounders. The results demonstrate, as expected, a systematic diurnal change where the reduction in acoustic densities during nighttime is more prominent for higher frequencies. The result is expected since gadoids are visual feeders being active with a more directed swimming pattern at daytime than during night when they have a more relaxed swimming with more variable tilt angle. Applications of the results for improving the quality of survey abundance estimates are discussed.