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002483 macrophytes. In contrast, Vaseys Paradise is a fast-flowing, cool, dolomitic-type spring, with abundant wetland and phreatophyte vegetation, especially native crimson monkeyflower, sedge, smartweed, and poison ivy, and non-native watercress. Monkeyflower, sedge, smartweed and watercress are persistent aquatic wetland or hydrophytes (Kearney and Peebles 1960), and KAS is virtually restricted to those species at Vaseys Paradise (Stevens et al. 1997b, Kanab Ambersnail Interagency Work Group 1997a,b). Introduction of watercress at Vaseys Paradise provided KAS with an alternate host plant. KAS densities are generally higher on watercress than on the native host plants during the growing season (Kanab Ambersnail Interagency Work Group 1997b). Although watercress is an annual species, its life cycle at Vaseys Paradise is unpredictable. In part, this irregularity is due to the constant warm flows of the spring (ca. 160C), which keep the microenvironment wann enough to prevent freezing during moderately cold winter months. Also, warm winters, such as 1995-1996, do not freeze watercress back, while cold winters (e.g. 1990) freeze and kill the plants. Warm spring flow and warm winters affect the watercress life cycle, and limit predictability of habitat conditions. Demographic analyses based on size class distribution indicate that KAS is essentially an annual species, with much of the population maturing and reproducing in mid-summer (July and August), and most snails over-wintering as small size classes (Kanab Ambersnail Interagency Work Group 1 997b ). Loose, gelatinous egg masses were observed on the undersides of moist to wet live stems, on the roots of water-cress, and on dead or decadent stems of crimson monkey- flower in mid-summer of all years of study. No data on egg development or emergence success are presently available. In warm winters, such as that of 1995-96, KAS may emerge from dormancy early, and produce a double generation within one-year (Kanab Ambersnail Interagency Work Group 1997a). KAS at Vaseys Paradise are parasitized by the trematode flatworm, Leucochloridium cvanocittae, with 1.0% to 9.5% of the mature snails expressing sporocysts in mid-summer from 1995 through 1997 (Stevens et al. 1997b, Kanab Ambersnail Interagency Work Group 1997 a,b). Potential vertebrate predators of KAS at Vaseys Paradise include deer mice (Peromvscus crinitus and.e. maniculatus), as well as rainbow trout (Oncorhvnchus mvkiss) in the stream mouth), resident common raven (Corvus corax) and canyon wren (Catherpes mexicanus), summer breeding Says and black phoebe (Sayornis savi and S. nigricans). and winter resident American dipper (Cinclus mexicanus). Natural winter mortality may reduce the KAS population by nearly an order of magnitude: the lowest KAS populations observed from 1995 through 1997 occurred during emergence in March, indicating winter mortality rates of 43.5% to 84.7%. March floods may result in a lower total take of KAS, but the proportional take is probably approximately equivalent in any month from January through July. Additional factors to consider regarding differences in take between months are (l) that a BHBF when watercress is abundant and in the middle of its growth phase may result in increased proportional take, and (2) a BHBF from mid-May through July is likely to result in take of reproductively active snails, potentially affecting annual reproductive output. Therefore, although BHBF's later in the growing season may take an equal proportion of KAS, later high flows may exert relatively greater impacts on KAS reproduction.