Target strength of an individual fish at any aspect

Abstract

A set of empirical equations has been developed for use in determining the target strength or acoustic cross section of an individual fish at any insonified aspect as a function of fish size and insonifying frequency in the range 1?L/λ? 100, where L is fish length and λ is acoustic wavelength. The equations were developed by interpolating experimental data obtained by insonifying individual fish as they were rotated about one of their principal axes. It was found that acoustic cross section σ is proportional to slightly less than L2 for each aspect, indicating that σ is approximately proportional to insonified area. Since σ is almost proportional to L2, a modified set of empirical equations was developed with σ exactly proportional to L2, thus eliminating the dependence of σ on frequency. The resulting errors are relatively minor and in some situations the modified equations lead to considerable simplifications which make their use quite convenient.

Chief Joseph Kokanee Enhancement Project : Strobe Light Deterrent Efficacy Test and Fish Behavior Determination at Grond Coulee Dam Third Powerplant Forebay.

“…However, data are available in the form of target strength, which can be used to estimate the size of the fish detected (Love 1977). Using data supplied by the Lake Roosevelt Net Pen Program on the sizes of kokanee and rainbow trout released upstream of Grand Coulee Dam in Lake Roosevelt in 2001, we calculated that these fish would have target strengths within the range of -42 to -32 dB (Table 4.1).…”

Hydroacoustic Estimates of Fish Density Distributions in Cougar Reservoir, 2011

“…The average TS of the ensonified ping pong ball was -39.9 dB for Transducer 415; -39.5 dB for Transducer 418; and -39.8 dB for Transducer 428), and pre-and post-season averages were within 1 dB of each other. The final detection threshold for processing echoes was -56 dB referenced to 1 µPa at 1 m for on-axis fish, which is roughly equivalent to a 35-mm-long fish according to the any-aspect equation of Love (1977). Fine woody debris suspended in the water column, particularly during the pool refill period (April and May) had TSs between -60 and -56 dB, and influenced our choice of a detection threshold because we did not want to inflate counts with non-fish targets.…”

Kootenay Lake Fertilization Experiment; Years 11 and 12, Technical Report 2002-2003.

“…Fork lengths of trawl caught fish were converted to the same acoustic scale using Love's (1977) empirical relation and compared to acoustic size distributions in order to verify the most appropriate size cut-off to separate age 0+ from age 1-3+ kokanee. Since it was not possible to distinguish between age 1, 2, and 3 fish using acoustic data, the proportions of these age groups could only be based on trawl catches.…”