Experiments on the discrimination of fish and seabed echoes

During acoustic surveys for biomass estimation, fish aggregations near the seabed may not be correctly measured due to false detection of the bottom echo. The extent of this problem depends on the spatial and density features of the near-seabed aggregations. We conducted experiments using a tower structure deployed close to the bottom of a sea loch. The tower is 10 m high with a split-beam transducer at the top and a fish cage at the bottom. The effect of a bottom slope is simulated by tilting the transducer. In experiments with various densities and sizes of gadoids in the cage, echoes from the vicinity of the seabed were studied over hard and soft ground. In addition to the range, the split-beam echosounder gives two angular coordinates of the target direction. These may be combined into one measure of angular displacement, namely, the angle between the apparent target direction and the transducer axis. We call this the split-beam angle (SBA). We found that the SBA is not necessarily an accurate indication of the target direction. Echoes from fish aggregations and the seabed have different characteristics in this respect. When the seabed echo is detected with few interfering targets above, the SBA is an accurate indication of the seabed slope, and assuming the slope does not change over a short series of pings, the SBA is highly correlated. On the other hand, the SBA from multiple fish echoes is highly variable, as expected, and the ping-to-ping variation is essentially random. Furthermore, when the seabed echo is transmitted through a substantial density of fish, the interference can change the SBA, although the ping-to-ping correlation of the seabed SBA remains superior to that of fish aggregations. We also studied records from acoustic surveys on various research vessels to provide comparable results at full scale. When there is a low density of near-seabed fish, the correlation between the fore-aft SBA and the seabed gradient is optimal at the start of the first seabed echo; it declines at sub-bottom ranges. When there are dense aggregations of fish near the seabed, the automatic bottom-detection algorithm may be located on top of the aggregation, so that the echo integration misses a substantial quantity of fish. Examples from acoustic surveys in the North Sea are presented to illustrate this problem.