A broadband echosounder for resonance classification of swimbladder-bearing fish

A new echosounder that exploits the resonance frequency of swimbladder-bearing fish is described. The system is a custom configuration of a commercial device originally developed for studying the seafloor and adapted for studying fish. The frequencies span the range 1.7 kHz to 100 kHz, with some gaps, with the lower portion of that range corresponding to the resonance frequency of common adult fish. Through pulse-compression processing of the broadband signals, the range resolution is greatly improved to about 20 cm at the lowest frequencies and about 3 cm at the highest frequencies. As a result of the resonance classification at these low frequencies, significant ambiguities that are normally associated with interpretation of high frequency echosounder data are eliminated. Summarized in this paper are the design and calibration of the system, as well as results from using the system to study schools of fish in the ocean.