8096

• UCREFRP Catalog Number: 8096
• Author: American Fisheries Society.
• Title: Proceedings of the 19th Annual Meeting, Colorado - Wyoming Chapter, American Fisheries Society.
• USFW Year: 1984.
• USFW - Doc Type: March 7-8, 1984.
• Copyright Material: NO

Tc analyze for the relations
recruitment levels the subsequent
and age and growth data collected
of eggs available for deposition"
eggs available for deposition and
assumptions:
'zip between parent spawner density and YOY
fall we used the population density estimates
each year to determine the "potential number
each fall. Determination of the "number of
hatching" was made using the following
. 1) A 1:1 sex ratio for adult spawning population
2) Fecundity is positively correlated with length
3) All brown trout > 20 cm total length were sexually mature
4) Hatching success was 100% on all "eggs available for deposition"
Concerning assumption 1, we acknowledge that the literature abounds with exam-
ples of trout populations where the sex ratio is not l:l (Van Velson 1974;
Davis and Carlson 1983); however, the same literature indicates that whatever
the "real" sex ratio, it is quite constant from one year to the next and thus
should not bias the relationship between the number of spawners available and
the number of eggs deposited between years. Assumption 2, that fecundity is
positively correlated with body length is well documented in the literature
(Taube 1976). The third assumption (all brown trout > 20 cm in length) was
validated during field electroshocking. Brown trout fecundity relationships
used in this study were taken from Taube (ibid), in his paper on sexual matur-
ity and fecundity of brown trout in the Platte River, Michigan. Our fourth
assumption is the most tenuous; however, we maintain that whatever the percent
hatch, it is probably quite constant between years and therefore should not bias
the relationship between eggs deposited versus eggs hatched between years. The
fall-w~nter-early s~ring flows are very stable between years, generally 0.57-
1.13 m /s (20-40 ft /s), the river is virtually silt free due to the lack of
agricultural activity in the valley, and spawning gravel is abundant throughout
the study area. Thus for ease of mathematical computation we assumed a hatch
of 100%.
RESULTS AND DISCUSSION
The number of eggs available for deposition each fall (generated from
population estimates and the length-frequency distribution) was regressed
against the number of YOY brown trout collected the subsequent fall. Linear
regression analysis for seven sets of data (1976/77-1982/83) revealed a very
po~r correlation coefficient (r = +0.2437) and coefficient of determination
(r = 0.0594). [-Jhile the correlation coefficient was positive indicating a
positive relationship between parent spawner density in the fall and YOY brown
trout collected the subsequent fall, it was not strong enough to explain the
_ large variations we were observing in recruitment levels between years. Other
investigations have found the same thing in stream situations for both brook
and brown trout (Latta 1965). We concluded some other factor must be involved.
Analysis of stream flow regimes in 13 streams across the state of Colorado
~i~ehring 1979) using the IFG4 incremental methodology and interfacing the
output with the IFG3 model indicated that fry and juvenile brown trout habitat
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