Absorption spectroscopy:: A new approach to estimation of biomass

Abstract

The results of an interdisciplinary experiment, Modal Lion, which was designed to isolate the effects of absorptivity due to fish with swim bladders from other effects on transmission loss, are described. The experiment was conducted at a 83 m deep site in the Gulf of Lion in September 1995. Absorption losses were measured between 0.6 and 5.0 kHz at a range of 12 km. Trawling operations indicated that the dominant fish in this region were 16.5 cm long sardines. Absorption lines were evident at 1.3 kHz (18 dB) when dispersed sardines were near the surface at night, and at 2.7 kHz (35 dB) when sardines were dispersed near the bottom at dawn just prior to school formation. In addition, an absorption line attributed to 6.5 cm long, dispersed juvenile sardines was evident at a frequency of 3.9 kHz (8 dB) at night. Measured resonance frequencies of 16.5 and 6.5 cm long dispersed sardines at night were in good agreement with theoretical computations based on swim bladder dimensions derived from trawling operations. Results suggest the possibility of spectral separation of juvenile from adult fish, and long term monitoring of the growth and mortality rates of juveniles with absorption spectroscopy measurements.