Wide-bandwidth acoustical characterization of anchovy and sardine from reverberation measurements in an echoic tank

Abstract

Abstract The total-scattering cross-sections (σt) of anchovy (Engraulis mordax) and sardine (Sardinops sagax caerulea) were measured acoustically over a wide bandwidth (0.5–202 kHz) from ensembles of reverberation time-series. Measurements were made sequentially in two cylindrical, galvanized-steel tanks containing filtered seawater (21±1°C). The lower-frequency measurements were made from two groups of fish (35 anchovy and 10 sardine) in a 1000-l tank, and those at higher-frequency (≥36 kHz) from 10 individual fish in a 100-l tank for each species. Thus, wide-bandwidth, total target strengths (TTS = 10 log10(σt/4π)) were estimated for multiple sizes of these important pelagic species of the California Current. The TTS frequency dependence (TTS(f)) is significantly different for these two species. For sardine, it first increases and then decreases over the frequency range, whereas for anchovy it increases monotonically with frequency. Moreover, at 38, 70, 120, and 200 kHz the variations of TTS with fish length and weight were markedly non-linear. Empirical estimates of TTS were statistically compared with theoretical predictions derived using the Kirchhoff ray-mode model. When surveying with echo-integration methods these measurements should be useful in the acoustical identification and classification of anchovy and sardine and for estimating their sizes.