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

1942
JOHNSON ET AL.
and Minckley 1991; Olden et al. 2006), and over 40 of
these are established in the upper basin alone.
Preventing introductions has proven to be very
difficult, and it is now recognized that coping with
invasions will require the ability to predict range
expansions and the intensity of effects based on life
histories of native and invasive species and character-
istics of recipient systems (Moyle and Light 1996;
Ricciardi and Rasmussen 1998; Kolar and Lodge 2001;
Iguchi et al. 2004; Vander Zanden et al. 2004; Bestgen
et al. 2006). Case studies of the relative effects of
invasions by multiple species can provide valuable
insights that will assist managers in anticipating and
controlling new invaders.
Debate continues regarding the relative importance
of habitat loss versus nonnative fishes in rivers of the
region, but the two factors usually operate concurrently
and even synergistically and are thus confounded. The
Yampa River is unique in that it allows for an
assessment of the magnitude of nonnative fish effects,
as it has the most natural hydrograph of any river of its
size in the upper Colorado River basin (Boehm 2004).
The lack of main-stem dams below the headwaters and
the absence of large diversions have maintained the
natural spring peak in discharge, although summer
flow has been reduced by agricultural withdrawals
(Stewart et al. 2005). Despite the availability of
relatively high-quality habitat, populations of small-
bodied native fishes and juvenile life stages of larger
species have declined precipitously during the past
decade. Native speckled dace Rhinichthys osculus,
mottled sculpin Cottus bairdii, bluehead suckers
Catostomus discobolus, and flannelmouth suckers
Catostomus latipinnis were strikingly lower in abun-
dance or were extirpated in some reaches of the Yampa
River, and nonnative fish abundance was higher in
2002-2003 than in 1998-1999 (Anderson 2005).
Predation by nonnative fishes has been implicated in
the decline of small-bodied fishes and may also be
responsible for local declines in large-bodied species,
such as Colorado pikeminnow (Bestgen et al. 2007).
However, information on the relative predatory impact
of various introduced species was lacking, and control
of myriad species would be impractical. Further, some
of these species are popular sport fish, so their control
would require explanation and justification to the
public.
One widely held opinion was that northern pike Esox
lucius posed the gravest threat to native fishes of the
Yampa River. This assumption presumably arose
because northern pike are notoriously piscivorous,
large, and able to prey on a wide range of fish sizes
(Scott and Grossman 1998). Diet studies in the system
confirmed that northem pike preyed upon native fishes
(Tyus and Beard 1990). Channel catfish Ictalurus
punctatus were also considered a threat, both as
predators and as competitors (Tyus and Nikirk 1990;
Tyus and Saunders 2000). Removal programs for both
species were implemented in 1998 (Hawkins et al.
2005; Mueller 2005). The potential impact of small-
mouth bass Micropterus dolomieu on native fishes may
have been discounted, because historically they were
very rare. Additionally, many sources (Carlander 1977;
Scott and Grossman 1998; Tomelleri and Eberle 1990;
Sigler and Sigler 1996) have noted the predominance
of crayfishes in the diet of smallmouth bass; this notion
is well entrenched in sporifishing lore. An unprece-
dented increase in the abundance of smallmouth bass in
parts of the Yampa River during the past decade
prompted interest in determining their diet and the
predation pressure they exerted on native fishes in the
system relative to predation by channel catfish and
northem pike. A predominately local but avid angling
clientele developed as abundance of smallmouth bass
increased, necessitating objective information and a
thorough analysis of the effects of this nonnative
predator before management actions aimed at reducing
its abundance could be considered.
Assessing the potential predatory impacts on native
species by channel catfish, northem pike, and small-
mouth bass requires information on the diets of each
predator species. However, to determine the relative
intensity of piscivory on prey populations, information
on consumption rates and demographics of each
population are also required. Further, in some situa-
tions, predation may have already depleted prey
species of concern and predators will have switched
to alternate prey; in those cases, contemporary diet
information gives a misleading impression of preda-
tors' potential impacts on species of concern. Bioen-
ergetics models can integrate ecological, physiological,
and demographic information to compute consumptive
demand of predator populations, a relevant metric
when assessing relative impacts. A great strength of
simulation modeling is that alternative scenarios can be
evaluated to ask questions such as, "What if native
prey fish populations rebound and predators begin
feeding on them again?" or "How many of those prey
could the predators consume?"
In this study, our objectives were to (1) assemble
available data on diets and demographics of three
nonnative predators from the Yampa River and diet
information from a reference system and (2) employ
bioenergetics models to quantify the relative predatory
threat of each species. The information was then
provided to managers so that predatory fish control
efforts could be prioritized and applied in an
ecologically and fiscally efficient manner and so that