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There are many situations when it is important to know accurately the behavior of fish as a function of time and space in a fixed, three-dimensional volume. One example is the optimal design of techniques that minimize the mortality of fish approaching hydroelectric dams or the cooling intakes of a power plant. The behavior of fish in other fixed volumes, such as estuaries and open rivers, is also of interest in the case of many migrating fish stocks. Both active (echosounding) and passive systems based on acoustic-emitting tags implanted in fish have been used to collect behavioral data. Active acoustic systems, including those with electronically and mechanically steered beams, only insonify a small part of the total volume of interest at any given time. Tag systems, on the other hand, can be used to monitor the behavior of tagged fish over the entire volume. A number of advances in the implementation, deployment, and analysis of acoustic-tag systems have been made over the past few years. These improvements include techniques for positioning optimally the receiving hydrophones to minimize the location measurement errors, the development of acoustic-signal waveforms that provide both unique target identification and accurate location estimates, and the development of tracking algorithms that associate and track the multiple returns from an individual fish. These various techniques are described. Guidelines are presented for selecting the various parameters for the tag system, including the positions of the hydrophones. Specific examples that compare the predicted and actual performance of the tag systems are described.

During surveys to assess fish stocks, the acoustic scattering properties of the fish population are required to obtain stock size measurements. The recently developed dual beam technique of target strength measurement has shown great promise as a method for fish target strength measurement. An improved dual beam target strength measurement system is described, and we report the application to a marine population of walleye pollock (Theragra chalcogramma). In addition, we discuss field calibration using a ping-pong ball as a standard target. Results of target strength measurements of the fish and ping-pong ball are compared with predicted values based on simulation analyses of the technique operating under the noise conditions observed in the field. The agreement between field results and theoretical predictions were quite good. Key words: hydroacoustic, walleye, pollock, dual beam, target strength measurement, fish assessment

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