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

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 sourer and a fixed receiver separated by 12km in relatively shallow (83m) 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 lint, 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.