High resolution acoustic structure of fish

Eight freshwater fish species and mechanical models were insonified with an experi-mental focused array transducer at 220 kHz. This laboratory apparatus insonifies a fish or model at small intervals (approximately 1 cm) along its length to produce backscattering curves for an individual fish or model. Each fish species tested had a unique backscattering curve, suggesting that if sonars are developed with a resolution high enough to obtain backscattering curves then they may be able to distinguish individuals at least to the family taxonomic level. In general, the proportion of backscattered energy from the swimbladder was greater for larger swimbladders; they could, at maximum, account for 95 % of the total backscattered energy in some species (e.g., bloater (Coregonus hoyi) and rock bass (Ambloplites rupeslris)). In the species studied the proportion of backscattered energy from the head ranged from 2 to 26%; vertebral column, from 3 to 23%; and the viscera and musculature, from negligible to 83 % in the deepwater sculpin, which lacks a swimbladder. The data can be presented acoustically as a series of point scatterers along the fish, with a concen-trated scattering function occurring at the swimbladder, for numerical simulations of sonar measurements.