8097

• UCREFRP Catalog Number: 8097
• Author: American Fisheries Society.
• Title: Proceedings of the 18th Annual Meeting, Colorado - Wyoming Chapter, American Fisheries Society.
• USFW Year: 1983.
• USFW - Doc Type: March 2-3, 1983.
• Copyright Material: NO

Colorado Division of Wildlife stocked Dillon Reservoir with 300,000 to
1,000,000 fingerling (< 100 rom) rainbow trout per year. These finger-
lings provided the brown trout with an artificial food source, as predation
on stocked fingerlings was quite heavy (Stuber 1979). Stocking of finger-
ling rainbow trout was discontinued in 1979 due to very poor returns-to-
creel. After the cessation of fingerling rainbow stocking, it was felt
that kokanee salmon became the primary forage fish for only large (< 450 rom)
brown trout. Therefore, in the 1980 spawning migration, very few brown
trout grew enough to replace the larger brown trout that died after the
1979 spawning migration.
There appeared to be a decrease in growth and condition for fish between
2-4 years of age and an increase in older fish. A possible explanation
for this trend may be that brown trout become sexually mature at 2-4 years of
age (< 400 rom) in Dillon Reservoir. During sexual maturation, more energy
appeared to be used for gonad development than for growth. Ball and Jones
(1960) found a reduction in growth following sexual maturation in brown
trout from Llyn Tegrid, Scotland. Growth and condition appeared to increase
as trout diets changed almost exclusively to fish.

Evaluation of the Spawning Migration

During the past few years, the number of brown trout in the Blue River
spawning migration has declined. In 1977, 10,000 brown trout were estimated
to have migrated up the Blue River (pers. corom. G. Bennett, Fish Biologist,
Colorado Division of Wildlife). In 1978, the estimated total was 3000 brown
trout (pers. corom. G. Bennett). In 1979 and 1980 the estimated totals
were less than 1000 fish per year. Four possibilities were suggested for
the decline.
First, there may have been egg destruction caused by superimposition of
nests constructed by later-spawning kokanee salmon over nests of earlier-
spawning brown trout. Although this phenomenon was not observed, kokanee
salmon nests and brown trout nests were observed adjacent to each other.
McNeil (1968) observed this kind of nest destruction during anadromous
salmon migrations.
Second, the removal of brown trout eggs, through spawn-taking opera-
tions, by the Colorado Division of Wildlife, may have been a factor. To
date, none of the brown trout hatched from eggs removed during Blue River
spawning migrations has been returned to Dillon Reservoir. Removal of eggs
may have lessened density-dependent mortality while increasing the effects of
density-independent mortality (i.e., anchor ice, adverse weather, or extreme-
ly high or low water levels in the Blue River).
Third, severe reservoir drawdowns of 1977 and 1978 probably had a large
impact on the number of brown trout in the spawning populations. Food
reserves of brown trout following spawning may have been low; the declining
water levels probably reduced benthic invertebrate food items and further
stressed the brown trout.
Lastly, harvest of mature brown trout may have had an adverse effect
on the brown trout spawning migration. The majority (65%) of the brown trout
harvested from Dillon Reservoir ranged 305-405 rom TL, which corresponds
to the length range of first-time spawners. As many as 1820 brown trout in

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