Monitoring the growth and mortality rates of pelagic fish with absorption spectroscopy measurements

Abstract

The experimental geometry for absorption spectroscopy measurements, which was employed during Modal Lion, can be adapted for monitoring the growth and mortality rate of pelagic fish. During the juvenile stage, in which sardines grow from 3.5 to 13.5 cm over a period of about 1 year and mortality rates are relatively high, absorption spectroscopy measurements (together with complimentary biological and oceanographic measurements) conducted for a few days approximately once per month would permit investigation of the environmental determinants of these parameters. Measurement sites would have to follow the migration route of juvenile sardines over a period of about 1 year. To minimize cost and simplify logistics, a broadband omnidirectional source (instead of a parametric source) would be suspended from a moored ship or buoy. A relatively low source level (∼170 dB) would be adequate. Simulations will be presented of the temporal evolution of the resonance frequency, absorption coefficient, and transmission loss due to sardines between 0.6 and 6.0 kHz. Simulations will illustrate the sensitivity of transmission loss to source and receiver depth, and the average depth and thickness of absorbing layers. In addition, calculations that illustrate the potential of estimating the depth and thickness of absorbing layers by varying the depth of an omnidirectional source will be presented. [This work was supported in part by the Office of Naval Research.]