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pi 1 t 1 I 1 1 P Ruedi 2012 Agreement Final EA Page 3-7 slight increase through the 1990's (Nehring, 2000). At one time, the river hosted significantly higher populations of rainbow and brook trout than are found in the river now. No studies have been done to prove or disprove the hypothesis, but indications are that brown trout predation on the young of other trout species is partially the cause for the population drops of other trout species (Nehring, 2000). Other factors such as undesirable river temperatures during the spring rainbow trout egg incubation also may have an impact on this species. Rainbow trout populations appeared to be rising through the 1970's, but then dropped in the early 1980's. A stocking program began in 1982 and their numbers increased until they hit record highs in the late 1980's, but dropped off significantly again in the early 1990's and have remained relatively steady since that time (Nehring, 2000). Rainbow trout stocking slowed in the late 1980's and early 1990's, and only occurred once (in 1998) since 1992. Even without stocking, the Fryingpan River supports some of the highest fish populations and highest number of large fish per unit area in Colorado. (Figure 3.3 and 3.3a) Brown trout populations have been fairly constant at about 1,500 fish/ha since 1992 and rainbow trout populations averaged about 300 fish/ha from 1992 to 1996 (Strange, 1998). The onset of a viable population of opossum shrimp (Mysis relicta) in Ruedi Reservoir in the mid-1980's, which subsequently began flushing through the outlet tubes of Ruedi Reservoir, has enhanced both the biomass and numbers of both brown and rainbow trout, especially for the first few miles just below Ruedi Dam (Nehring, 1991 & Nehring, 2000). The larger fish that result from this diet are probably particularly predatory on the young of other trout species, especially when flows decrease and less opossum shrimp are available (Nehring, 2000). The releases of opossum shrimp have clearly altered the diet of brown and rainbow trout in the reach immediately below the dam (Nehring, 1991). Figures 3.3 and 3.3a illustrate the trend in brown and rainbow trout populations in that reach of the Fryingpan River immediately downstream of Ruedi Dam. Estimates for the total number of brown and rainbow trout greater than 35 cm per hectare show similar trends. Whirling disease has been detected in the Fryingpan River since 1995, however effects have only recently been observed and only in rainbow trout populations. The disease is most prevalent in the lower reaches of the river, where sources for the disease have been identified in private ponds that spill into the Fryingpan (Nehring, 2000).