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Traditional single and paired release-recapture models are incapable of providing unbiased estimates of dam passage survival because their assumptions can never be met. Nevertheless, regulatory requirements mandate the estimation of this important performance measure for migratory fish species passing through hydroprojects. We present a new release-recapture model that uses a virtual release of inriver migrants known to have arrived at the dam, combined with a paired release below the dam, to estimate dam passage survival. Analytical comparisons of the sampling precision of the proposed virtual-pairedrelease model and an established route-specific model found that the new approach was always more precise for equal release numbers of tagged fish. In a field trial at Rocky Reach Dam, Washington, using smolts of sockeye salmon Oncorhynchus nerka, the new approach estimated dam passage survival with more than twice the precision of the alternative method. The proposed virtual-paired-release design has the potential to provide robust and precise estimates of dam passage survival under a variety of hydroproject scenarios.

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.

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