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<br />.'. <br /> <br />-8- <br /> <br />Al!elGrowth Analyses <br /> <br />Fish scale samples were collected in April, May, and October 1991 to document the age structure of <br />the trout population and evaluate the growth rate of the trout (fable 3A, Appendix). The age <br />information was used to construct an age/length frequency distribution. This information was used to <br />divide the brown and rainbow trout population estimates into year classes and compile life tables <br />(fable 4A, Appendix). <br /> <br />Fish length/weight data collected during April and October electro fishing operations was used to <br />calculate trout body condition factors. Condition factor data for 1991 was compared with similar <br />information from previous years (1986-1990) to assess relative changes in body condition factors <br />between species, stations, and years. Body condition factor is a relative index of trout well-being, <br />i.e., a measure of plumpness, fatness, or robustness. This information, together with additional <br />length/weight data collected on recaptured trout with VI tags, provided information on trout growth <br />rates from spring to fall 1991. <br /> <br />RESULTS AND DISCUSSION <br /> <br />Population Density and Biomass - 1986 throul!h 1991 <br /> <br />Prior to the construction and closing of the outlet works on McPhee Dam in early 1984, no trout <br />population had existed since the early 1900s in the Dolores River reach currently under study. This <br />section of river was often dried up during the irrigation season due to upstream irrigation diversions. <br />Consistent with its management plan to create a high quality stream trout fishery, the CDOW began <br />to stock fingerling trout annually in this section of the river in 1984. Despite quite high peak spring <br />discharge levels and extended periods of lower water levels from 1984 through 1988 (see Table 1), <br />the trout population was becoming established at a low level by October 1986 (fables 2 and 3). <br /> <br />The trout population continued to expand at all stations from 1986 through 1988. During this time, <br />period summer flows rarely dropped below 60-70 cfs. Total annual discharge consistently exceeded <br />250,000 AF/year (fable 1). Under that flow regime, the trout population expanded slowly in density <br />and biomass. These parameters began to level off between October 1988 and September 1989 (fables <br />2 and 3) as reservoir releases decreased from those experienced in previous years (fable 1). The first <br />decline in trout density and biomass occurred at the most downstream station between October 1988 <br />and September 1989. <br /> <br />Two consecutive water years (October 1987 through September 1989) of below average run-off, <br />coupled with the prospect of a third year of sub-normal snowpack, led to 20 cfs flow releases from <br />McPhee Reservoir in early-March 1990. This release pattern remained in effect for more than 100 <br />days through a period of unusually hot weather in late-May and early-June 1990. Unusually low flow <br />releases, together with unusually high air temperatures, proved to be a deadly combination of events <br />for the Dolores River trout population. <br /> <br />Population density and biomass estimates for September 1989 will be considered the "benchmark" <br />year from a statistical significance standpoint. This was the last year prior to the steep decreases in <br />flow implemented in the spring of 1990. The data in Tables 2 through 7 indicate that most trout <br />population statistics were beginning to level off or even decline slightly between October 1988 and <br />September 1989, concurrent with the second year of the below normal run-off. ' <br />