7025

• UCREFRP Catalog Number: 7025
• Author: Hawkins, J. A. and T. P. Nesler.
• Title: Nonnative Fishes in the Upper Colorado River Basin
• USFW Year: 1991.
• USFW - Doc Type: An Issue Paper.
• Copyright Material: YES

if'
~

12

predation. Colorado squawfish were considered potential prey for northern pike and
walleye, but none were found in 123 pike and 61 walleye stomach samples examined from
fish collected from the Green River, Utah (Tyus and Beard 1990). Grabowski and Hiebert.
(1989) also found no predation of Colorado squawfish based on stomach analysis of juvenile
channel catfish, black bullhead, green sunfish, and largemouth bass. But, they suggested that
the potential for predation still existed. Persons and Bulkley (1982) examined stomach.
contents of striped bass from Lake Powell during their spawning run up the Colorado River
and did not detect predation on Colorado squawfish. Colorado squawfish of suitable prey
size were present in the area sampled, but, of 321 striped bass stomach examined, none
contained native threatened or endangered fishes. Striped bass utilized the most abundant
prey source available, threadfin shad. They determined that adult striped bass would
probably not prey heavily on endemic fishes during spawning as long as threadfin shad are
abundant and as long as bass spawning occurs below Cataract Canyon rapids.

Inability to detect predation does not mean that it is not occurring. It may be due to
small sample size of predator stomachs, rarity of endangered fish, or inability to identify fish
remains (Tyus and Beard 1990). Meffe (1985) cautioned on the use of gut-content analysis
to infer population effects of predation. Stomach contents would need to be analyzed soon
after the initial colonization of the predator in order to observe actual predation effects on
the native population. Once predation has lowered the population, few natives would be
available for predation and would not be detected in gut analysis. In lab studies, prey were
identifiable for only 4 hours after ingestion. Even if predation rate is calculated, the impact
on the prey population would be difficult to predict because a low rate can have a large
impact on a species with low fecundity (Meffe 1985).

Osmundson (1987) confirmed predation as a significant mortality factor of
young-of-year (YOY) and yearling-sized Colorado squawfish stocked in riverside ponds along
the Colorado River, Colorado. Colorado squawfish were preyed upon by largemouth bass,
green sunfish, black crappie, and black bullhead Predation by channel catfish was suspected
but not confirmed Colorado squawfish were stocked into ponds with different predator
densities and even low numbers of predators had a significant impact on reducing numbers
of stocked Colorado squawfish. Laboratory studies identified that largemouth bass selected
Colorado squawfish for prey over green sunfish and red shiner. Selection was equal for
Colorado squawfish and fathead minnow, but, under pond conditions, abundant fathead
minnow did not decrease predation on Colorado squawfish. After stocking, largemouth bass
completely switched from other forage fish to Colorado squawfish. Small Colorado squawfish
were preferred over larger ones. Based on predator to prey size relationships, Colorado
squawfish over 250 mm TL were believed invulnerable to largemouth bass predation. In
aquaria studies of interactions between young Colorado squawfish and several nonnativeefishes, Karp and Tyus (l990a) did not observe predation, but attributed this to the lack of
a size advantage by the nonnatives.

Predation has been identified on many endangered fishes in the lower basin because
of widespread stocking programs there and the occurrence of numerous predators.