Estimation of the number density of fish from resonance absorptivity and echo sounder data

Abstract

Diachok, O., Liorzou, B., and Scalabrin, C. 2001. Estimation of the number density of fish from resonance absorptivity and echo sounder data. -ICES Journal of Marine Science, This paper provides a comparison of number densities (number per m 2 in the horizontal plane) of pelagic fish, which were derived from acoustic absorptivity measurements and nearly coincident echo sounder and trawling data. Absorptivity measurements were made with a broadband source (between frequencies of 0.6 and 5.0 kHz) as a function of time between a fixed source and a fixed receiver separated by 12 km in relatively shallow (83 m) water. The sardine (Sardina pilchardus Walbaum) was the dominant species at this measurement site. The second most common species was the anchovy (Engraulis encrasicolus Linnaeus); there were approximately twice as many sardines as anchovies. The number densities of all other species were small. Strong absorption lines were evident at the resonance frequencies of dispersed sardines. The frequencies of these absorption lines changed in accordance with the vertical migration of sardines at twilight. A correction for the influence of anchovies on the absorption line, which was dominated by dispersed sardines at night, was applied to the absorption-based estimate. Initial echo sounder based estimates of number densities, which were based on a ''universal'' pressure independent equation of target strength, were corrected for pressure dependence, attenuation by near surface bubbles, and avoidance. The corrected absorption based number density of sardines (1.4 m 2 ) was in excellent agreement with the average corrected echo sounder based estimate (1.0 m 2 ). These results suggest the possibility of estimating number densities from broadband, tomographic, transmission-loss measurements over large areas.

Field measurements and modeling of attenuation from near-surface bubbles for frequencies 1–20kHz

“…One notable difference between model and data is the Ͼ2-dB attenuation at 4 kHz for the 50-m receiver depth. It is possible that bio-absorption due to swim bladder-bearing fish 11 may be in effect, although 4-kHz measurements made at the same time (16:00 UTC) on 10 Aug. do not exceed 2 dB. Also, with the order 10 −6 void fraction in effect for 15 Aug., there is a bubblemediated sound-speed change.…”

Bioacoustic Absorption Spectroscopy: Estimation of the Biomass of Fish With Swimbladders

“…The extrapolated values of r 0 and E 0 of 15.5-cm-long sardines were 0.80 em and 10 respectively. Previously reported calculations, which were based on direct measurements of extracted swimbladders, were within 20% of measurements (Diachok et al 2001). The relatively weak absorption line at 0.9 kHz is correlated with the presence of adult 14.5-cm-long anchovies at 6 meters (Holliday 1978).…”

“…Equation 4 was employed in previous derivations of n from measurements of the absorption coefficient at the resonance frequency (Diachok et al 2001).…”

“…Experimental measurements of absorption coefficients, which were derived from transmission loss measurements in the Gulf of Lion, are shown in Figure 1 (Diachok et al 2001). The most prominent absorption line at 1.3 kHz is consistent with calculations, which were based on equation 1; the measured values of d, which was 25 meters; trawling data, which indicated that the dominant fish during this experiment were 15.5-cm-long sardines; and values of r 0 and E 0 , which were extrapolated from Jech and Horne's (2001) x-ray measurements of the swimbladder dimensions of 24-cm-long pilchard sardines Sardinops ocellatus.…”

Resonance classification of mixed assemblages of fish with swimbladders using a modified commercial broadband acoustic echosounder at 1–6 kHz

“…The above work, which all involved backscattering of sound, has been complementary to that of Diachok and colleagues, who have used a wide range of frequencies to measure the attenuation of sound due to the presence of fish with swimbladders (Diachok et al 2001;Diachok 2002Diachok , 2005. In this geometry, the source and receiver are separated by a large distance.…”