8162

• UCREFRP Catalog Number: 8162
• Author: Osmundson, D. B., R. J. Ryel, M. E. Tucker, B. D. Burdick, W. R. Elmblad and T. E. Chart.
• Title: Dispersal Patterns of Subadult and Adult Colorado Squawfish in the Upper Colorado River.
• USFW Year: 1998.
• USFW - Doc Type: \
• Copyright Material: YES

DISPERSAL PATTERNS OF COLORADO SQUAWFISH
rkm; and between Cameo and Rulison sites by
0.19/rkm.
Discussion
This study coincided with the recruitment of two
or three consecutive strong year-classes to the
adult Colorado squawfish population. This allowed
us to monitor the movements of many individual
fish and observe changes in adult distribution re-
sulting from these year-classes. Dispersal was gen-
erally in an upstream direction, but recruitment
also increased adult abundance riverwide. Mean
lengths increased in the lower reach as young fish
grew, and the influx of young adults moving to the
upper reach from the lower reach reduced average
sizes of fish in the upper reach.
Distance moved between captures was related
to fish size and reach of initial capture. Most fish
initially captured in the upper reach moved little
from initial capture sites and were predominately
larger adults. However, most fish initially captured
in the lower reach were smaller individuals and
were often recaptured farther than 10 km from
initial capture sites. In both reaches the majority
of movements were directed upstream. These
movement patterns suggest that young fish move
more than older fish, and less movement occurs
after arrival in the upper reach. Flannelmouth
suckers in the White River behaved in a similar
way with young individuals having moved more
than older ones (Chart and Bergersen 1992).
Movement to upstream strata by adults suggests
that upper reach habitats are preferred or have a
better mix of requisites for growth and survival.
Particularly revealing was the lack of fish move-
ments from the upper reach to the lower reach.
Riverwide catch rate patterns of larger prey spe-
cies combined with reach differences in Colorado
squawfish body condition support the hypothesis
that upstream displacements are a response to food
resource gradients.
Relatively small changes in location by larger
fish in the upper reach was consistent with the
hypothesis that adult Colorado squawfish select
and maintain fidelity to a home feeding range, as
suggested by Tyus (1990), McAda and Kaeding
(1991), and Ryden and Ahlm (1996). Adults of
this species appear not to be highly territorial (as
defined by Hixon 1980) given that they concentrate
in limited backwater habitats during spring runoff
(April-June), they congregate prior to and during
spawning in summer (Tyus 1990), and individuals
are occasionally located (via radiotelemetry) be-
953
side one another (USFWS, unpublished data) dur-
ing base flows of fall and winter.
High relative condition of Colorado squawfish
200-399 mm in the lower reach suggests that food
for young Colorado squawfish was not limited (at
least during spring), and may relate to the high
numbers of small-bodied, nonnative minnows in
backwaters there. However, relative condition de-
clined in the lower reach as Colorado squawfish
grew, suggesting that a diet consisting of small
minnows may be insufficient for larger individu-
als. If so, the lack of appropriate-sized prey may
be particularly acute in the lower reach where
warmer temperatures increase metabolic (demands.
Although little is known of food habits of larger
adult Colorado squawfish (and we were unsuc-
cessful in collecting such data), anecdotal evidence
indicates consumption of larger prey than was
found for size-classes examined in this study. A
687-mm Colorado squawfish captured in the San
Juan River regurgitated a flannelmouth sucker 235
mm long-34% of its own length (D. Propst, New
Mexico Department of Game and Fish, personal
communication). One 860-mm individual that we
captured in the lower Gunnison River regurgitated
a 310-mm white sucker X bluehead sucker hybrid
(36% of its length). An X-rayed, 599-mm adult
from the White River contained a 280-mm (47%
of its length) unidentified sucker (J. Hawkins, LFL,
personal communication). Scott and Crossman
(1973) reported the optimum food size of northern
pike was one-third to one-half the pike's length
and that growth and survival of large muskellunge
are often impaired if food of an adequate size is
not available, despite vast numbers of smaller fish-
es. Gillen et al. (1981) similarly reported an op-
timum prey size for tiger muskellunge E. lucius X
E. masquinongy as 40% of total predator length
when prey were soft-rayed and fusiform shaped.
Also, of three simulated mechanisms, lack of large
prey had the strongest effect on stunting of north-
ern pike (Diana 1987).
The most downstream thermograph site provid-
ed the most days with temperatures at or near 25°C,
the preferred temperature of yearling (Black and
Bulkley 1985a) and adult Colorado squawfish
(Bulkley et al. 1981), and the greatest number of
annual thermal units for growth. In addition, main-
channel temperatures there never became too
warm for these fish: daily averages never exceeded
26°C during 4 years of monitoring. Therefore,
growth potential should be highest in the lower
reach. However, warm water temperatures there
may be disadvantageous for adults: higher meta-