Evaluation of side-aspect target strength and pulse width as potential hydroacoustic discriminators of fish species in rivers

Split-beam hydroacoustic data were collected on 98 tethered and 10 unrestrained Pacific salmon of known size to evaluate side-aspect target strength and pulse width as species discriminators in rivers. Pulse width was better able to discriminate chinook (Oncorhynchus tshawytscha) from sockeye salmon (Oncorhynchus nerka) than target strength, although its discriminatory power may have arisen from differences in behavior as well as size between species. Standard deviation of -12 dB pulse width was the best univariate size predictor and species discriminator among tethered fish but performed poorly among unrestrained fish at close ranges; mean pulse width at -12 dB performed well among both. Discriminatory power of all variables declined as the number of echoes per fish was reduced. Fish orientation, lateral movement, and spatial position of the fish in the beam affected hydroacoustic measurements. As fish moved more from side to side, and as fish orientation departed from full side aspect, target strength declined monotonically but mean pulse width increased and then declined. We show how some of the confounding effects of fish behavior on hydroacoustic measurements can be removed using corrections based on statistical models.