1.1 Development of Acoustic Methods in Fisheries
1.2 Contents of this Manual: Its aim and scope
An acoustic method for the detection of fish was first reported in the scientific literature in 1929. Continuous waves (CW) at a frequency of 200 kHz were directed across ponds containing goldfish. As the number of fish intercepted by the acoustic beam changed, so the amplitude of the signal recorded on an oscillograph varied. This work was published by Kimura (1929) in the Journal of the Imperial Fisheries Institute of Japan.
A most important development was the paper recording echo-sounder by Wood et al, (1935), which was used for surveys of cod (Sund, 1935) and is still widely employed. A flashing light echo-sounder made by the Marconi Company was also used at this time for detecting herring shoals but reported later (Balls, 1946).
Rapid advances in the application of echo-sounders to fishing occurred after World War II and they were also used for the estimation of relative fish abundance. For this purpose a technique of counting the numbers of millimetres of fish echo trace from the paper record was used, Cushing (1952).
For the next stage of development, attempts were made to count and size fish from their echoes (Mitson and Wood, 1962 and Cushing, 1964), but for various reasons these met with limited success. It was mainly the use of an electronic integrator to process the fish echoes that marked the beginning of quantitative assessment of fish stocks by acoustic methods as we know it today (Dragesund and Olsen, 1965).
The development of these methods for fish stock estimation has progressed steadily (Middtun and Nakken, 1971) during a period when major advances in electronics technology have taken place. As a result of these advances it is possible to process signals derived from fish echoes, fast and accurately, with equipment which is both reliable and stable. Attention is being increasingly directed toward aspects of fish behaviour such as their orientation, density and distribution, all factors which can bias the results of an acoustic survey. Thought is also being given to the planning of surveys and the statistical manipulation of data to give the best possible measurement of abundance.
In putting together this manual the authors have drawn on the experience and writings of many people in an attempt to provide a suitable amount of technical background and practical 'know-how'. An outline of the contents is given next.
Many FAO fisheries development projects included as one of their objectives the development of a national capability to independently accomplish programmes of fisheries resource studies, based on modern acoustic survey equipment and techniques. Transfer of expertise through direct training of project counterparts is an important activity towards this end and the preparation of this Manual represents an additional effort to prepare and train scientific and technical counterpart personnel for their future tasks.
Because there is no single traditional academic base for the mixture of knowledge and skills required by the profession, a manual such as this is thought to provide a useful additional resource for acoustic practitioners, especially at the earlier stages of experience. Emphasis has been placed on the practical aspects of the topics treated, assuming only a limited knowledge of the reader in mathematics and physics. An attempt has also been made to cover the essential ground in a coordinated manner with respect to the three previously issued FAO Manuals and technical papers on the subject, by Forbes and Nakken (1972); Burczynski Revision 1 (1982) and Bazigos (1981), keeping the overlap to a necessary minimum to maintain continuity. Hence, sections may not be fully comprehensive because they are intended to lead to some further study on each topic, using the recommended literature in Appendix 3.
The contents of the present Manual have been arranged to introduce the reader to some elementary theory related to the physical background and outlines the principles of the necessary equipment before deriving the working formulae. It then deals with the acoustic properties of fish in some detail before considering the general concepts behind acoustic estimates of biomass.
There is a section on acoustic and electronic calibration, after which the planning of work and shipboard procedures are presented in a practical manner. Probably the largest chapter (9) is that dealing with possible sources of error in the estimates and how these may be combated. The final chapter (10) is concerned with analysis of data collected during fish abundance surveys.
