SACLANT Undersea Res. Ctr., La Spezia, Italy
Simulations of Weston's measurements of the temporal variability of anomalously large attenuation due to fish, suggest the possibility of tomographic inversion of pelagic fish parameters over large spatial dimensions (nominally 50 km radius) in shallow water. These measurements reveal resonances which are a function of fish bladder size and depth. Computations suggest that the latter may be inferred from modal attenuation measurements [Diachok, European Conference on Underwater Acoustics (1994)], hypothetically permitting unambiguous estimation of bladder size. In the horizontal dimension pelagic fish concentrate into ``clusters'' of schools at scales, which are driven by the ``patchiness'' of plankton, which in turn is hypothetically controlled by atmospheric and bottom conditions, and by oceanic mesoscale phenomena such as eddies, fronts, and upwelling events. A fixed, large-scale tomographic array consisting of several widely spaced transmitters and receivers, designed to invert ocean parameters at low frequencies and fish parameters over a broad range of higher frequencies, together with quasisynoptic measurements of plankton parameters (probably from ships), would permit investigation of fish adaptation(s) to the temporally evolving mesoscale environment. Criteria for the design of such a bioacoustic observatory, and its advantages and disadvantages compared to shipborne methods and fixed long-range active monitoring, will be discussed.