8198

• UCREFRP Catalog Number: 8198
• Author: Tyus, H. M., C. W. Brown and J. F. Saunders
• Title: Movements of Young Colorado Pikeminnow and Razorback Sucker in Response to Water Flow and Light Level
• USFW Year: 2000
• USFW - Doc Type: Journal of Freshwater Ecology
• Copyright Material: YES

a rising river level brings water into backwaters and expands the habitat,
fish movement with the flow (i.e., "downstream") will carry the larvae
farther into the backwater, which would be sheltered from the high velocity
environment of the river and, due to a more shallow condition, perhaps
free from larger aquatic predators. High turbidity associated with the
riverine environment may also reduce terrestrial predation. In the extreme
case, increasing river stage can eliminate a backwater by flooding, but
moving away from the main channel would be a satisfactory response to
avoid being swept downstream in the main channel. Falling river level
drains water from the backwater and reduces the size of the habitat. A
shrinking habitat is likely to have high water temperatures and low
dissolved oxygen in summer, and isolated backwaters may eventually be
desiccated. Moving downstream would take the small fish out of the
backwater and away from deteriorating habitat conditions that could lead
to thermal limits, desiccation and death.
Movement patterns of the larger juveniles (CP4) were more
exploratory, which also has been noted in the riverine environment (Tyus
1991). Although entering the river channel presents a new set of risks and
challenges for small fish, juveniles are apparently large enough to make
such movements, sometimes on a routine diel basis (Tyus 1991).
Changing river levels have the same implications for habitat
alteration whether it is day or night, but larvae exhibit fundamentally
different patterns of activity during daylight hours. In particular, the
smallest larvae are more active during the day than at night when there is
no flow. As the larvae grow, they appear to increase their reliance on
visual cues. It is possible that a reliance on visual cues provides
information on habitat conditions that supersedes the basic response of
going downstream at night (when there are no visual cues). The pattern of
movement with flow at night is suggestive of flow entrainment due to a loss
of visual reference points (Pavlov et al. 1977, Manteifel et al. 1978).
However, water flow in the tank was so low that velocity was discernible
only in a small part of each backwater and flows could be avoided by
dropping to the lower levels of each chamber. In addition, all ages of the
fish explored their surroundings and were able to locate the small ports
connecting chambers in the experimental tanks.
In addition to providing a better understanding of larval behavior in
backwaters, results of this' study also offer insights regarding susceptibility
of larval fish to predation and the failure of stocking efforts . In
backwaters adjacent to the Colorado River today, there are many
introduced predators that attack larvae. Behavioral studies have
demonstrated a relative "naivete" when native Colorado River fish larvae
are in the presence of predators (Johnson et al. 1993), and a basic lack of
aggression of the young when compared to the same sizes of some
introduced fishes (Karp and Tyus 1990}. Our study showed that small
larvae move much more during the day than at night, and that would seem
to make the larvae more conspicuous to predators. One key to
understanding the apparently maladaptive behavior of the larvae is the
nature of predators in the present system. Virtually all of the predators
inhabiting backwaters today are nonnative species, and all of the
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