Monitoring transitions between dispersed and school modes of sardines and anchovies with Bioacoustic Absorption Spectroscopy (BAS) measurements

Abstract

BAS measurements at a biologically intense site in the Santa Barbara Channel permitted monitoring the temporal variation of absorption lines associated with fish in dispersed and school modes. Transmission Loss (TL) was measured at frequencies between 0.25 and 5 kHz between a fixed source and fixed receiver array separated by 3.7 km for 20 hours. Echo-sounder measurements: the average depth of fish at night was about 13 m. Trawls: 16 cm sardines and 10.5 cm anchovies dominated. The frequency dependence of bio-attenuation was derived through comparison of TL measurements and calculations. The latter were based on a normal mode code recently extended for broadband calculations (Porter, 2019). Bio-attenuation at 1.1 kHz, the resonance frequency of dispersed sardines, was 35 dB at 2200 l, 15 dB at 0100 l and 35 dB at 0400 l. When bio-attenuation at 1.1 kHz was minimum, bio-attenuation 0.4 kHz, attributed to schools of sardines, was maximum. The absorption line at 0.4 kHz is consistent with previously reported measurements and theoretical calculations of the resonance frequency of schools of 16 cm sardines (Diachok, 1999). Bio-attenuation at 1.8 and 0.6 kHz, attributed to 10.5 cm anchovies in dispersed and school modes respectively, were also anti-correlated. [Research supported by ONR.]BAS measurements at a biologically intense site in the Santa Barbara Channel permitted monitoring the temporal variation of absorption lines associated with fish in dispersed and school modes. Transmission Loss (TL) was measured at frequencies between 0.25 and 5 kHz between a fixed source and fixed receiver array separated by 3.7 km for 20 hours. Echo-sounder measurements: the average depth of fish at night was about 13 m. Trawls: 16 cm sardines and 10.5 cm anchovies dominated. The frequency dependence of bio-attenuation was derived through comparison of TL measurements and calculations. The latter were based on a normal mode code recently extended for broadband calculations (Porter, 2019). Bio-attenuation at 1.1 kHz, the resonance frequency of dispersed sardines, was 35 dB at 2200 l, 15 dB at 0100 l and 35 dB at 0400 l. When bio-attenuation at 1.1 kHz was minimum, bio-attenuation 0.4 kHz, attributed to schools of sardines, was maximum. The absorption line at 0.4 kHz is consistent with previously reported...