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  Figure 5. Measurements of resonance frequencies of 16-cm sardines in dispersed (f0) and school (F) modes during night (solid circle), sunrise (open triangles) and day (open circle). The diameters of the symbols are proportional to the magnitudes of the attenuation coefficients at f0 and F. Theoretical calculations of F are from Raveneau and Feuillade, 2015.
65 m during the day is consistent with Raveneau and Feuillade’s (2015) theoretical calculations of the reso- nance frequency of European pilchard schools.
Figure 5 also shows that biological attenuation was high- est during night when the fish were dispersed and lowest during the day when the fish were in schools. The transi- tion occurred during sunrise when some of the fish were dispersed and some were in schools. Why is attenuation due to fish in schools during the day much lower than attenuation due to dispersed fish at night? Theoretical calculations indicate that Ching and Weston’s (1971) speculation was essentially correct. The difference in biological attenuation between night and day is driven primarily by the difference in the separation between fish in dispersed and school modes and, to a lesser extent, by the difference in the effective radius of swim bladders (Diachok, 1999; Raveneau and Feuillade, 2015).
Possible Practical Applications of Bioacoustic Attenuation Spectroscopy The measurement approach, illustrated in Figure 4, is well-suited to test scientific hypotheses but is too cumber- some for practical applications. In particular, a vertical array that spans most of the water column is difficult and time consuming to deploy and recover. Furthermore, such an array is not needed for practical applications.
Diachok and Wales (2005) showed that the bioacoustic parameters of an aggregation of fish may be inferred from TL measurements with hydrophones at two depths. There are several technologically mature approaches that would permit BAS measurements with a small number of hydrophones. Consideration of the relative merits of these approaches is beyond the scope of this article.
Because TL is affected not only by bioattenuation but also by the geoacoustic properties of the bottom, the latter would have to be measured in areas of interest. The geo- acoustic properties of the bottom could be measured with direct methods (e.g., Turgut, 1990), inverted (derived) from TL data in the absence of fish or inverted from the application of the concurrent inversion method (Diachok and Wales, 2005) in the presence of fish. The usefulness of information derived from BAS measurements would probably have to be initially demonstrated by fisheries scientists charged with estimating fish abundance and could eventually be employed by fishers to reduce the vast bycatch (unwanted species) that fishers routinely catch and discard daily throughout our oceans.
The research reported here was supported by the Office of Naval Research Ocean Acoustics Program. I thank Arthur N. Popper for his painstaking reviews of the pre-
liminary drafts of this article.
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