Zoobenthos in dam tail waters can only inhabit that portion of the channel that has remained inundated for the past 3-5 weeks. Studies of colonization rates show that typically several weeks are required for benthic organisms to invade newly wetted habitat and approach an equilibrium of community composition (Thorp et al. 1985, Fuchs and Statzner 1990). Thus, the varial zone of regulated rivers is often devoid of benthic organisms, particularly when there has been frequent flow fluctuation. There may also be significant mortality of zoobenthic organisms because of stranding if flows have remained high in dam tail water for an extended period, permitting colonization,

Lake Biological Station. x ,' During 1960s and early 1970s, Mysis relicta were introduced into more that 100 lakes throughout the northwestern U.S. and British Columbia with the primary intention of providing a supplemental prey organism for salmonids, especially kokanee salmon {Oncorhynchus nerka) (Lasenby et al. 1986, Martinez and Bergerson 1989). Several lakes in the Flathead Lake catchment, northwest Montana, were stocked with M. relicta including Whitefish, Ashley and Swan Lakes. M. relicta are voracious predators on zooplankton, and previous studies have reported significant changes in zooplankton communities following other Mysis introductions (see reviews by Lasenby et al. 1986, Northcote in press). Kokanee salmon also rely on zooplankton as their principle food resource (Cooper and Goldman 1980, Leathe and Graham 1982, Rieman and Falter 1981, Vinyard et al. 1982). Previous introductions of M. relicta have been followed, in some cases, by declines in kokanee abundance. Heightened competition between kokanee salmon and M. relicta for declining zooplankton food resources has been implicated as a primary cause for reductions in kokanee abundance (Lasenby et al 1986, Spencer et al. 1991, Northcote in press). In some cases, other factors, including reduction in spawning habitat, have been implicated in kokanee declines (Fraley et al 1986, 1989, Northcote in press). M. relicta first appeared in Flathead Lake in 1981, presumably the result of downstream movement from previously stocked lakes (Leathe 1984). At that time, kokanee salmon represented the primary sport fish in the Flathead Lake. During the 1970s there were declines in kokanee spawner abundance in portions of the watershed affected by hydropower operations (Fraley et al 1986, 1989). Nevertheless, a sustained kokanee population and fishery existed in Flathead Lake through the mid-1980s, with adult kokanee populations of several hundred thousand fish, and natural kokanee recruitment of 9 million or more fry per year, coming largely from McDonald Creek in Glacier National Park (Beattie et al. 1988, Clancey and Fraley 1986). Then, within a few years after the appearance of M. relicta, the kokanee showed signs of decline, and by 1987 the fishery collapsed completely (Beattie et al. 1988). There has been considerable debate over the cause of the kokanee collapse in Flathead Lake, and at this time there is general agreement that several factors were involved. These include the introduction of M. relicta., predation by lake trout which expanded following the Mysis introduction, hydropower operations, and angler harvest.^ The present cooperative study between the Montana Department of Fish, Wildlife, and Parks and the Flathead Lake Biological Station was developed to provide information on one of these factors, namely the introduction of M. relicta and subsequent changes in the zooplankton community in Flathead Lake. The study also includes information on the distribution and abundance of M. relicta in Flathead Lake. Materials and methods Mvsis relicta samplin...