The small amount of evidence presented in section 7.2 indicates that the acoustic method of pelagic fish stock assessment has very good precision for fish numbers below about 10 m. Near the surface the precision is not so good, and we have also seen from Figure 5 that the echo-sounder underestimates the number compared with the trawl catches in the surface layers. These two observations could be explained by a hypothesis that surface fish are distributed by the boat towing the transducer. The data also indicate that the acoustic method appears to give reasonable population estimates, but it has not been possible to test their accuracy critically.
The target strength distributions have been shown to mirror the fish length distributions from the trawl catches, but a close calibration of target strength and fish length is not yet possible, and, with a multi-species population, this will be difficult to establish.
The advantage of the acoustic method is that it is quick and is not labour intensive, and within the limitations mentioned above, appears to give reasonable results. Its disadvantage is that it is not possible to identify the species present and so must be supplemented by another conventional fishing method. The Simrad equipment is being used successfully in ten Finnish lakes for pelagic fish stock assessment.
