9663

• UCREFRP Catalog Number: 9663
• Author: Johnson, B. M. and e. al.
• Title: Ranking Predatory Threats by Nonnative Fishes in the Yampa River, Colorado, via Bioenergetics Modeling
• USFW Year: 2008
• USFW - Doc Type: North American Journal of Fisheries Management
• Copyright Material: YES

1948
JOHNSON ET AL.
objectively rank-the threat posed by the three nonnative
predators based on relative consumptive demands.
Determining the predatory threat posed by a fish
population cannot be gauged simply by its abundance,
size structure, or characteristic dietary preferences.
Rather, we have shown how a number of factors
interact to determine a given species' capacity to harm
native fish populations; these factors include prey
choice, population size structure, abundance, and
physiological attributes and environmental suitability.
Bioenergetics models integrated all of these factors to
provide direct impact estimates in the form of the
consumptive demand for native fishes by each predator
population. These consumption estimates quantified
the current relative predatory threat posed by each
nonnative species, providing managers with hard
evidence with which to prioritize control efforts.
Despite their moderately high abundance, channel
catfish in the Yampa River contributed only about 1%
of total piscivory because their diet contained few fish.
Channel catfish were the most gape limited of the three
piscivores in our study; the gape width of a 400-mm
channel catfish was only about 77% of the gape width
of a 400-mm smallmouth bass (B.M.J., unpublished
data). Low piscivory is consistent with findings of
Tyus and Nikirk (1990), who reported that channel
catfish in the Yampa River rarely ate fish, even when
small-bodied fishes were abundant. In that study, only
large channel catfish (mean TL = 392 mm) were found
to have consumed any fish. Brooks et al. (2000) found
that piscivory by channel catfish was also low in the
San Juan River, Colorado-New Mexico-Utah, where
the prey assemblage was similar to that of the Yampa
River. Crayfish and insects were the predominant food
source of channel catfish in the Yampa River.
Competition for food between channel catfish and
native fishes has been put forward as a rationale for
channel catfish control efforts. Based on comparisons
of our channel catfish diet data with diet studies of
native fishes elsewhere (Vanicek and Kramer 1969;
Karp and Tyus 1990; Quist et al. 2006), considerable
diet overlap between channel catfish and native species
probably exists in the Yampa River. However, biomass
and production of invertebrate prey organisms in the
Yampa River are unknown; hence, resource limitation
and competition for food among native and nonnative
fishes cannot be inferred.
On aper-capita basis, northern pike consumed more
fish than the other predators in both the realized and
potential piscivory scenarios; this result is attributable
to the large body size of northern pike, their preference
for piscine prey, and the suitability of environmental
temperatures. However, because northern pike abun-
dance was relatively low, population-scale consump-
tion of fish by northern pike was similar to that by
smallmouth bass even though the smallmouth bass diet
contained a much smaller percentage of fish. Because
they were far less gape limited than smallmouth bass,
northern pike were able to maintain a relatively high
fraction of f sh in their diets after small-bodied fishes
declined; thus, northern pike were able to prey on
older, larger individuals that were still relatively
common in the system (Anderson 2005). Northern
pike are potent piscivores, capable of ingesting adults
of even large-bodied native species; therefore, continu-
ing efforts to reduce northern pike numbers in riverine
habitats where they prey on native species are
warranted.
We believe that the small contribution of fish to the
smallmouth bass diet and the incidence of cannibalism
in our data suggest that piscivory in the Yampa River
was limited by low availability of fish prey within the
gape limit of smallmouth bass. Spatial overlap between
small-bodied fishes and smallmouth bass (and northern
pike) is high all year long, and there are no
microhabitats that serve as refuges for native fishes,
Total piscivory by the smallmouth bass population was
similar to that by northern pike simply because
smallmouth bass were more abundant than northern
pike. smallmouth bass were highly piscivorous in the
GVR, where the invasion was more recent and where
small-bodied fishes were common (Bundy and Bestgen
2001). Based on potential piscivory scenarios, if
suitably sized fish prey were available in the Yampa
River, then smallmouth bass predation could-have been
10-fold higher than that of the other two predators
combined. Thus, the Yampa River smallmouth bass
population possessed a considerable level of latent
piscivory. Our analysis indicates that smallmouth bass
and northern pike each pose a serious threat to native
fishes but that smallmouth bass have the greatest
capacity to hamper native fish recovery in the Yampa
River by virtue of their high abundance. The
synergistic effects of the two predators may be
particularly devastating, because prey fish that manage
to outgrow the gape of smallmouth bass will remain
vulnerable to northern pike for years or a lifetime,
depending on the species.
Historic data on the fish assemblage in the Yampa
River before either northern pike or smallmouth bass
were abundant suggest that small-bodied fish density
was similar to present-day density in the GVR. Wick et
al. (1985) performed seining in shoreline and backwa-
ter habitats (n = 1,828 seine hauls) in the Yampa River
during 1981 and 1982. Mean density of mostly native,
small-bodied fishes for the 2 years combined was
34,000 fish/ha at RKM 165-191 and 45,000 fish/ha
over our entire study area (RKM 80-191). In the GVR,