Vessel comparison on the seabed echo: Influence of vessel attitude

Abstract

During an inter-vessel comparison of the NOAA ships Oscar Dyson and Miller Freeman in the Bering Sea in July 2006, significant vessel-differences in acoustic backscatter from walleye pollock (Theragra chalcogramma) were observed. However, very similar vessel-differences were observed in the seabed echo as well. Therefore, it was concluded that poorly understood differences in echosounder calibration or performance were likely the cause of the observed discrepancy in acoustic pollock backscatter from the two vessels. The seabed echo results were crucial to avoiding a faulty interpretation of differential vessel avoidance by fish in the water column. However, a careful examination of the seabed echo revealed that it was to some extent influenced by the pitch and the roll of the vessels. The most important pitch/roll variable during the inter-vessel comparison was the average roll (i.e., the list of the vessels). The seabed echo recorded by the NOAA ship Miller Freeman was more influenced by vessel list than was the seabed echo recorded by the NOAA ship Oscar Dyson. Since the sea was relatively calm during the experiment (wave height less than 2 m most of the time), the list effect was significant but small enough that the seabed echo could still be successfully used to help interpret echosounder output from both vessels. However, we believe that the effect could have been more severe under rougher sea conditions due to differential vessel motion, and – in fact – under very calm conditions as well, since the seabed echo may be extremely dependent on the incident angle of the acoustic beam, and a small deviance from zero incident angle could hence result in a too weak seabed echo. Therefore, the seabed echo must be interpreted with caution in all circumstances.


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