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Fish is outside of the measurement volume, that is below 25 meters or along the edges.
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Empty Cage / No relevant signals
Fish taken out of cage, while equipment still operative. Inspector scales echograms by looking at the typical echostrenghts in the last 24 hours. Consequently, right after the cage has been emptied, the echogram is black as expected.
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However, the day after, the echogram in the bottom sensor looking up here is very noisy. This is because, the lack of physical objects in the water column leads to very little echo and a normalization scale which will enhance electrical + acoustic noise. It is likely that this will manifest itself in the upward looking beam. Simply because the coverage volume is smaller (due to surface cropping ~25m) and hence more likely to be empty than the top sensor looking down (~50m).
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Sensor is up from the water
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