Near-field Distance 1.0

Introduction<br />Fisheries acoustics is the use of underwater sound to detect, enumerate, and measure the distribution of fish and other living marine and freshwater resources and describe their habitat. This website is focused on the process of actively transmitting a sound pulse (ping) and analyzing the echoes generated by organisms in the aquatic environment. In fisheries, we are interested in information on abundance of fish and invertebrate species with which to ultimately derive population estimates and set harvest rates. In ecology, we are interested in spatial and temporal patterns in distribution and abundance to study such questions as habitat choice, predator-prey interactions, and food web dynamics. Acoustics have great potential in both fields. Passive listening to sound produced by aquatic organisms, sometimes referred to as bioacoustics, can also be informative but will not be dealt with here (see reviews by Hawkins 1993 or Myrberg 1997).<br /><br />As all sampling methods, hydroacoustics has advantages and limitations. Its main advantage is the ability to measure the distribution of organisms over large spatial scales both day and night without disturbing the animals. Sampling intensity is high, and the resulting echogram gives a two-dimensional picture of the distribution of fish in the water column. These two dimensions can be depth and distance as in mobile surveys, or time and distance as in stationary applications. It is possible to use the data to estimate absolute abundance with relatively low variance even for patchy populations. But hydroacoustics also has limitations and is not the solution to all sampling problems. The technique has difficulty differentiating between species, and limited ability to measure fish close to the surface and close to the bottom. Therefore, hydroacoustics is mostly used in conjunction with traditional fishing gear and biases associated with gear selectivity is retained in the acoustics estimates. Density estimates using hydroacoustics require knowledge of fish target strength (TS) and TS varies due to a variety of factors. Finally, the technique is complex and requires some understanding of the physics of sound propagation in water. There is a risk of arriving at the wrong conclusions without adequate training and experience. Even so, the use of acoustic data in fisheries management has increased over the past decades and is likely to continue to increase as the cost of scientific echo sounders declines. Many fisheries students may encounter and use hydroacoustics during their career. It is therefore important to acquire some familiarity with the technique. <br /><br />This website borrows heavily from two sources, a standard operating procedure for Great Lakes Acoustics by Parker Stetter, Rudstam, Sullivan and Warner (2008), a manuscript for a chapter on fisheries acoustics for the American Fisheries Society book Fisheries Techniques, 3rd edition by Rudstam, Jech, Parker Stetter, Horne, Sullivan, and Mason. Those works in turn build on acoustics techniques outlined in chapters of the AFS Fisheries Techniques books (Thorne 1983, Brandt 1996) as well as on other books, chapters and reviews on the subject (e.g. Johannesson and Mitson 1983, Gunderson 1993, Aglen 1994, Misund 1997, Mason et al. 2001). The recent book by Simmonds and MacLennan (2005, also MacLennan and Simmonds (1992)) is a comprehensive and up to date treatise of fisheries acoustics. For the reader interested in details on the physics of underwater sound propagation, we recommend Urick (1983), Medwin and Clay (1998), Lurton (2002) or Medwin (2005).

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