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There are two types of mortality that result from downstream passage of anadromous salmonid smolts through the Federal Columbia River Power System, instantaneous and delayed or latent mortality. As the name implies, instantaneous mortality is immediate in nature and measuring it is rather straightforward. On the other hand, delayed mortality occurs at some point after passage through dams and reservoirs and is more complex to measure. Fish Ecology Division researchers use two basic methodologies to examine delayed passage mortality of smolts. The first employs a closed artificial seawater recirculation system located at Bonneville Dam. Small groups of smolts that have experienced and survived different routes of passage through dams are held in tanks within this system for up to 6 months or longer to determine if delayed mortality is related to passage history. The second method relies upon adult returns from large groups of PIT-tagged smolts released above and below a series of dams and reservoirs to measure the overall delayed mortality associated with this process after adjusting for any direct or immediate mortality. Both methods provide data that are useful for estimating and reducing or eliminating latent effects of dam passage on outmigrating salmon smolts.
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