7194

• UCREFRP Catalog Number: 7194
• Author: Nesler, T. P.
• Title: Aquatic Nongame Research - 1985-86 Squawfish-Humpback Studies.
• USFW Year: 1986.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

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demonstrated the occurrence of Colorado squawfish larvae larger than 10 mm
TL throughout the lower 18 miles of the Yampa River over five years of
seine sampling in July and August, and documented a greater relative
abundance of squawfish larvae in 1983-84. Yet, none of the larger larvae
were collected in drift-net sampling. Furthermore; no Colorado squawfish
have been collected as fall yay or Age I juveniles in the lower Yampa.
It is hypothesized that Colorado squawfish larvae larger than 10 mm
utilize a more active or controlled downstream movement (versus passive
drifting). Perhaps even migration occurs. This is based on the premise
that Colorado squawfish reproductive strategy incorporates spawning in a
select river reach in Yampa Canyon and subsequent larval transport into
Green River nursery backwaters.
From another perspective, decreasing late-summer flow in the Yampa and
lack of zero-velocity habitat, including backwaters, may force these fish
to move downstream into the Green River. Concentrations of fall yay
observed in the Green River probably result from some combination of
drifting larvae deposited in the backwater habitats as well as post-larval
fish seeking this preferred habitat during active downstream movements.
As indicated in Archer and Tyus (1984) and Jones and Tyus (1985), the
magnitude of river discharge in the Green River in a given year determines
the amount of available zero-velocity habitat available~ and probably
determines the downstream distribution and survival of larval/post-larval
Colorado squawfish.
The narrow range of mean lengths~ ma:<imum-minimum length intervals,
and predominance of proto- and mesolarval Colorado squawfish in the drift
samples over the si;< week sampling period in 1985 suggests these fish
probably originated from the same locality. This lends further credence
to (but does not prove) the e:{istence of a primary spawning area upstream
versus generalized spawning throughout the lower Yampa.

piel periodic;j.\:L -- Results from 1983-84 8.nd 1985 indicate ::..ome
variability in diel periodicity in drifting Colorado squawfish larvae.
This is not interpreted to mean that a diel drift pattern does not e:<ist
for this species, but consideration of current knowledge of Colorado
squawfish spawning ecology in the Yampa - Green River system and the
drift-net sampling design suggest the latter has not been particularly
suited for documenting this pattern with regard to the environmental
variables known to influence larval drift. It has not been established
what the effect of temporal variability in turbidity has upon larval drift
in the Yampa River, or even what stimulus initiates the phenomenon.
Of concern here is the location of the spawning area versus the
sampling site. If the primary concentration of adult spawners actually
occurs around RM 16.5 - 18.2 on the Yampa River (Archer and Tyus 1984),
then the larval fish produced from this area represent the majority of
drifting Colorado squawfish in the lower Yampa. These fish are
subsequently sampled with drift nets at the Box Elder site at RM 2.0. In
essence, any diel periodicity displayed by the larvae as they drift out of
the spawning grounds may be variously diluted or disguised as fish
disperse downstream over the 14-16 river miles to Box Elder. The
estimated post--hatching age of larval Colorado squawfish suggests that the
river trip between the two sites may take 3-15 days. These factors may
account for the lack of a statistically significant diel drift pattern in
the 1983-84 results at Box Elder. Given the endangered status of this