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<br />0'0'0'88.' <br />.. <br /> <br />, <br /> <br />South Fork of the Rio Grande River <br /> <br />Brown trout are the only resident wild trout species that currently <br />exists in the South Fork of the Rio Grande below the confluence with,Park <br />Creek. Minimum flow recommendations for brown trout spawning, incubation, <br />and hatching are 30 cfs, while 45 cfs is the minimum for fry, juvenile, and <br />adult life stages. 'Optimum flow recommendations range from 45-l25 cfs, <br />depending upon life stage (Table 3). <br /> <br />Examination of the flow records for the USGS gage on the South Fork of <br />the Rio Grande (SFRG) at South Fork, Colorado, for 1964-85 indicate that the <br />minimum flow of 30 cfs for spawning, incubation, and hatching has been <br />maintained in most years and in every year since 1979. Similarly, the <br />optimum flow regimes of 45-125 cfs are maintained in most months of most <br />years, except during the ,fall-early spring periods (October-March)., It is <br />not surprising that the brown trout density and biomass in the South Fork of <br />the Rio Grande River has remained as consistently high as it has. <br />Year-class strength is determined primarily by the amount of fry habitat <br />during June, the month of brown trout fry emergence on theSFRG (Nehring and <br />Anderson, 1984). Barring any unforeseen water development projects in the <br />basin that urastically alters the flow regime, the brown trout fishery in <br />the SFRG should remain a valuable asset and "drawing card" for the tourist <br />industry in the South Fork area. <br /> <br />St. Vrain River <br /> <br />Similar to the SFRG, brown trout are the only resident wild salmonid <br />species in the St. Vrain River within the study area. The minimum flow <br />recommendation for all life stages of brown trout is 20 cfs. Optimum flow <br />recommendations range from 40-80 cfs, depending upon life stage (Table 3). <br /> <br />Examination of th~ USGS gage records for the St. Vrain River at Lyons, <br />Colorado, indicate that the minimum flows drop below 20cfs with regularity <br />durlng,the winter-early spring period. Minimum flows have occasionally <br />uropped to catostrophicallylow levels, such as during ~he drought ,year of <br />1976-n when mean monthly discharge never exceeded lO cfs from D,ecember 1976 <br />through March 1977. ,We have observed very high levels of mortality on adult <br />brown trout between age 3 and 4 on the St. Vrain River ,from 1978 ,through <br />1981 (Nehring and Anderson 1985). Temperature records indicate that water <br />temperature at the study site should not be limiting to brown trout. Thus, <br />it is quite possible that flow-induced reductions in adult trout winter <br />habitat may be limiting the adult brown trout component of the trout <br />populations. Adult brown trout are the most cover-oriented trout species <br />(Butler and Hawthorne 1968) and are susceptible to dewatering of under-cut <br />banks and ,areas of over-hanging vegetation along the channel perimeter., <br />This trout population did not respond to a catch-and-release regulation <br />imposed on the fishery for approximately 5 years. Nehring and Anderson <br />(l985) hypothesized that a lack of adult brown trout habitat associated with <br />adequate cover (such as pools or overhead cover) was probably limiting this <br />trout population. <br /> <br />(....' ~G: :~. ':." .: <br />~~-. '*- Ii'" ...' ' ' <br /> <br />. .-' ,~, <br /> <br />Ii <br /> <br />rJ00023~3 <br />