312 USDA Forest Service Proceedings RMRS-P-15-VOL-5. 2000

In: Cole, David N.; McCool, Stephen F.; Borrie, William T.; O’Loughlin,

Jennifer, comps. 2000. Wilderness science in a time of change conference—

Volume 5: Wilderness ecosystems, threats, and management; 1999 May

23–27; Missoula, MT. Proceedings RMRS-P-15-VOL-5. Ogden, UT: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Research

Station.

R. A. Knapp is a Research Biologist, Sierra Nevada Aquatic Research

Laboratory, University of California, Star Route 1, Box 198, Mammoth

Lakes, CA 93546 U.S.A. K. R. Matthews is a Research Scientist, USDA

Pacific Southwest Research Station, Box 245, Berkeley, CA 94701 U.S.A.

Effects of Nonnative Fishes on Wilderness

Lake Ecosystems in the Sierra Nevada and

Recommendations for Reducing Impacts

R. A. Knapp

K. R. Matthews

Abstract—Wilderness areas of the Sierra Nevada, California contain thousands of lakes and ponds, nearly all of which were historically fishless. After more than a century of fish stocking, introduced

trout are now present in up to 80% of larger lakes. These nonnative

fishes have had profound impacts on native fishes, amphibians, and

invertebrates. Several of these native species are either already

listed under the Endangered Species Act, or are likely to be petitioned for listing in the near future. Reducing impacts to aquatic

ecosystems within wilderness areas should be a high priority, and

will require that some lakes be restored to their historic fishless

condition.

One of the primary purposes of the National Wilderness

Preservation System is to protect natural ecosystems. As

human-caused modification of lands outside of wilderness

intensifies, the protection of wilderness ecosystems will be

increasingly important and challenging. These protected

areas already are being affected by anthropogenic impacts

both internal and external to wilderness areas (Cole and

Landres 1996). Attempts to minimize these impacts have

typically focused on protection of terrestrial ecosystems,

using tactics such as regulation of visitor use and allowing

the return of natural fire regimes. In contrast, little attention has been focused on impacts to aquatic ecosystems in

wilderness, habitats that have also been substantially

altered.

In the western United States, where wilderness areas

typically encompass high-elevation montane ecosystems, the

most ubiquitous impact to aquatic ecosystems is the introduction of nonnative fish species (Bahls 1992). Many of the lakes

in these areas were historically fishless, but have been stocked

with several different trout species to create a recreational

fishery. When the National Wilderness Preservation System

was created in 1964, language was included in the governing

legislation to ensure that fish stocking could continue in these

areas (Kloepfer and others 1994). Today, fish stocking in the

western U.S. continues in many national forest wilderness

areas, as well as at least one national park.

Despite the potential impacts caused by fish introductions

into wilderness lakes, Bahls (1992) concluded, after interviews with state fishery managers, that the practice of

stocking trout into mountain lakes was generally conducted

with “little concern for protection of native fish species in

lakes or downstream systems, no evident concern for maintaining representative pristine lakes, and no consideration

of the effects of trout stocking on indigenous fauna, aquatic

ecosystems, and lakeshore recreational impacts”. In addition, Bahls (1992) found that although stocking effort is

intensive, research is minimal. As a result, changes in the

distribution of fish caused by fish stocking and the effects of

fish introductions on aquatic ecosystems remain relatively

poorly understood. This lack of information has generally

precluded comprehensive efforts to reduce these impacts.

The effects of nonnative fish introductions on aquatic

ecosystems in wilderness areas of the Sierra Nevada, California are relatively well-studied, compared with aquatic

ecosystems in wilderness areas in other parts of the western

U.S. Although the results of these studies have important

implications for the management of wilderness fisheries

throughout the western U.S., this body of research has only

rarely been reviewed. The goals of this paper are to 1) review

the changes in fish distribution resulting from over a century of fish stocking in wilderness lakes of the Sierra Nevada, 2) review the impacts of these fish introductions on

lake ecosystems, and 3) provide recommendations aimed at

reducing these impacts. By making this information more

accessible to scientists, federal wilderness managers, and

state fisheries managers, we hope that this paper will help

to focus much needed attention on how to better balance the

interest of providing recreational fisheries in wilderness

with the need to maintain or restore natural ecosystems.

Study Area _____________________

The Sierra Nevada of California is largely federally owned,

with the majority of its five million ha (12 million acres) lying

within national parks, national monuments and national

forests (Palmer 1988). Eighty-four percent of the national

park acreage and 24% of the national forest acreage is

officially designated as wilderness (Palmer 1988). The area

above 1,800 m (6,000 ft) contains thousands of lakes and

ponds, and most of these habitats are located within designated wilderness. More than 99% of these lakes and ponds

were historically fishless (Moyle and others 1996). Instead of

fish, these water bodies were inhabited by a unique assemblage of amphibians, zooplankton and benthic invertebrates.
USDA Forest Service Proceedings RMRS-P-15-VOL-5. 2000 313

Starting in the mid-1800’s, trout were introduced into

formerly fishless lakes to provide recreational fishing (Moyle

and others 1996). Although some of these introductions were

interbasin transfers of trout native to the Sierra Nevada

(Little Kern golden trout (O. mykiss whitei), California

golden trout (O. mykiss aguabonita), Lahontan cutthroat

trout (O. clarki henshawi), Paiute cutthroat trout (O. clarki

seleniris) and coastal rainbow trout (O. mykiss irideus)),

many were introductions of trout species not native to

California. These included brook trout (Salvelinus fontinalis),

lake trout (Salvelinus namaycush), and Atlantic salmon

(Salmo salar) from eastern North America, kokanee salmon

(Oncorhynchus nerka) from northwestern North America,

and brown trout (Salmo trutta) from Europe (Moyle and

others 1996). Early trout planting efforts were aimed primarily at establishing trout in formerly fishless waters, and

were carried out largely by sporting groups and the U.S.

military. In the early 1900’s, the California Fish and Game

Commission (the precursor to the current California Department of Fish and Game) began coordinating the fish planting effort, and by the 1940’s fish stocking was conducted

almost entirely by the California Department of Fish and

Game (CDFG). Today, the CDFG is responsible for nearly all

authorized trout stocking throughout the Sierra Nevada,

although the emphasis has changed from introducing trout

into fishless lakes and streams to stocking waters to augment existing nonnative trout populations.

Sequoia, Kings Canyon, and Yosemite National Parks

began phasing out trout stocking in 1969 as a result of

recommendations made in the Leopold Report (Leopold

1963). Limited stocking in these parks was continued until

1991, when all stocking was halted. Trout continue to be

stocked into lakes within national forest wilderness areas,

and stocking is accomplished using airplanes.

Distribution of Nonnative Fishes in

Sierra Nevada Lakes _____________

As a result of more than a century of fish stocking, the

majority of historically fishless wilderness lakes in the

Sierra Nevada now contain introduced trout. Bahls (1992)

estimated that 63% of California’s 4,000+ mountain lakes

(natural lakes at elevations above 800 m, most of which are

found in the Sierra Nevada) now contain nonnative fish

populations and 52% are currently stocked. Of the estimated

37% of lakes that remain fishless, most are small (< 2 ha

surface area), shallow (< 3 m), and generally incapable of

supporting trout populations (Bahls 1992). Only 3% of larger

lakes (> 2 ha surface area, > 3 m deep) remain fishless.

Similar results were obtained by Jenkins et al. (1994), who

projected that one or more species of nonnative trout would

occur in 63% of high elevation lakes in the Sierra Nevada

(lakes at elevations > 2400 m and > 1 ha surface area).

Golden trout were projected to occur in 36% of lakes, rainbow

trout in 33%, brook trout in 16%, brown trout in 8%, and

cutthroat trout in 0.5% of lakes.

A greater proportion of lakes within national forest wilderness areas contain introduced trout populations than

lakes within national parks. Based on a survey of fish

populations in 2,000+ wilderness lakes in the Sierra Nevada, Matthews and Knapp (1999) reported that 80% of

lakes larger than 1 ha within the John Muir Wilderness

contained introduced trout versus 40% of lakes within the

adjacent Kings Canyon National Park. Similar proportions

of fish-containing lakes in national forest wilderness and

national parks in the Sierra Nevada were reported by Botti

(1977), Bradford and others (1993), Knapp (1996) and Wallis

(1952). The lower percentage of trout-containing lakes in

Sierra Nevada national parks than in national forests is

likely the result of both a lower historical stocking intensity

and the recent termination of all stocking in the national

parks. The termination of fish stocking in Sequoia-Kings

Canyon National Park and Yosemite National Park was

expected to cause 30% and 40% of previously stocked lakes,

respectively, to revert to a fishless condition (Botti 1977;

Zardus 1977).

Ecological Effects of Fish

Introductions Into Sierra

Nevada Lakes __________________

Native Fishes

Although fish were absent historically from nearly all

mid- to high-elevation lakes and ponds in the Sierra Nevada,

native trout species were present in streams in several

watersheds. These included the Little Kern golden trout,

California golden trout, Lahontan cutthroat trout, Paiute

cutthroat trout and coastal rainbow trout (Knapp 1996).

Distributions of some of these native trout populations were

altered when nonnative trout species were stocked into

fishless headwater lakes, and subsequently moved downstream to hybridize with or displace the native populations.

For example, the Little Kern golden trout (O. mykiss whitei)

is native only to the Little Kern River in the southern Sierra

Nevada. The introduction of nonnative brook trout into the

headwater lakes of this drainage and their dispersal downstream caused the near-extinction of the Little Kern golden

trout as a result of competitive displacement. Consequently,

the Little Kern golden trout was listed as “threatened” under

the Endangered Species Act (Stephens 1999).

Similarly, the California golden trout is native only to the

South Fork Kern River and Golden Trout Creek in the

southern Sierra Nevada. The stocking of hybridized golden

trout into the headwater lakes of the Golden Trout Creek

drainage resulted in extensive hybridization with the downstream native California golden trout population. Partly as

a result of this threat, the U.S. Fish and Wildlife Service is

currently considering listing the California golden trout

under the Endangered Species Act (Stephens 1999). Similar

impacts to native fishes resulting from the stocking of

headwater lakes with nonnative trout species are apparently common in wilderness areas throughout the western

U.S. (Adams 1999; Bahls 1992).

Amphibians

Populations of four of the nine anurans (frogs and toads)

native to the Sierra Nevada are reported to be declining

(Yosemite toad: Bufo canorus; California red-legged frog:

Rana aurora draytonii; foothill yellow-legged frog: R. boylii;
314 USDA Forest Service Proceedings RMRS-P-15-VOL-5. 2000

and mountain yellow-legged frog: R. muscosa; Jennings

1996). Only one of these species, the mountain yellow-legged

frog, was a common inhabitant of lakes historically and is

therefore the species most likely to be affected by the introduction of trout into these habitats. Despite its former wide

distribution throughout the Sierra Nevada (Zweifel 1955), a

recent resurvey of historic localities in the central Sierra

Nevada indicated that the mountain yellow-legged frog is

now present at fewer than 15% of the sites where it was

found in 1915 (Drost and Fellers 1996). Severe declines have

also been noted elsewhere in the Sierra Nevada (Bradford

and others 1994).

Increasing evidence from the Sierra Nevada indicates

that introduced trout are a primary factor in the decline of

the mountain yellow-legged frog. As early as 1915, Grinnell

and Storer (1924) reported that predation by introduced

trout on mountain yellow-legged frog larvae prevented the

co-occurrence of these two taxa in lakes and ponds. This

observation has now been quantified in several different

areas in the Sierra Nevada (Bradford 1989; Bradford and

others 1998). Although these studies have generally been

done using a relatively small number of sites (< 100), recent

research based on surveys at more than 1,700 sites in Kings

Canyon National Park (KCNP) and John Muir Wilderness

(JMW) provided similar results (Knapp and Matthews 2000).

These results can be summarized as follows:

1) The KCNP study area had fewer trout-containing lakes

than the adjacent JMW study area, and this difference in

trout distribution was associated with a seven-fold higher

percentage of lakes containing mountain yellow-legged frogs

in the KCNP study area than the JMW study area.

2) Drainages with a higher percentage of total water body

surface area containing trout had a lower percentage of total

water body surface area containing frogs.

3) After accounting for habitat differences between lakes

with and without trout, the probability of occurrence for

mountain yellow-legged frog larvae in individual water

bodies was three times higher and the abundance of larvae

was six times higher in fishless than in fish-containing

water bodies.

Together with the results of previous studies, there is now

compelling evidence that the mountain yellow-legged frog

has been extirpated from much of its historic habitat by the

introduction of trout into historically fishless lakes. The

results presented in Knapp and Matthews (2000) suggest

that these impacts have been particularly severe in national

forest wilderness areas, and that the severity of these

impacts could eventually require the listing of the mountain

yellow-legged frog under the Endangered Species Act.

In addition to the direct impact that nonnative trout have

on mountain yellow-legged frogs via predation, Bradford

and others (1993) suggested that fish could impact mountain

yellow-legged frogs indirectly by isolating remaining populations. They reported that fish introductions into lakes in

Sequoia and Kings Canyon National Parks have resulted in

a four-fold reduction in effective mountain yellow-legged

frog population size and a 10-fold reduction in connectivity

between populations. Because amphibian populations often

fluctuate widely under natural conditions (Hecnar and

M’Closkey 1996; Pechmann and others 1991), and small

populations are more likely to go extinct as a result of

stochastic population fluctuations than large populations

(Hanski 1994), Bradford and others (1993) proposed that the

reduction in mountain yellow-legged frog population size

caused by trout introductions probably increased the rate at

which individual populations go extinct. In addition, they

suggested that the increased isolation of mountain yellowlegged frog populations would reduce the probability of

recolonization of formerly occupied sites. This lower probability of recolonization could result from the smaller size of

potential source populations, increased distance from source

populations and predation by introduced trout on dispersing

frogs (Bradford and others 1993).

Several attributes of the mountain yellow-legged frog

make it particularly vulnerable to predation and subsequent extirpation by nonnative trout. First, adult mountain

yellow-legged frogs are highly aquatic and are found primarily in lakes (most of which now contain trout). Second, in

contrast to the larvae of other Sierran anurans that complete metamorphosis to the terrestrial stage in a single

summer, mountain yellow-legged frog larvae generally require at least two years to complete metamorphosis. This

overwintering requirement restricts successful breeding to

permanent water bodies (typically those deeper than 2 m;

Bradford 1983; Knapp and Matthews 2000; Mullally and

Cunningham 1956). The majority of these deeper lakes,

however, now contain introduced trout.

Zooplankton

One of the best studied and most consistent effects of

introduced fishes on lake ecosystems is the alteration of

zooplankton communities (Brooks and Dodson 1965; Zaret

1980). The introduction of zooplanktivorous fishes into

fishless lakes generally shifts the zooplankton community

from one dominated by large-bodied species to one dominated by smaller-bodied species, as a result of size-selective

predation. Several studies have documented this effect of

introduced trout on zooplankton communities in wilderness

lakes of the Sierra Nevada. Stoddard (1987) found that the

occurrence of introduced trout was the most important

predictor of zooplankton species composition in alpine and

subalpine lakes, with large-bodied species found in fishless

lakes and small-bodied species found in lakes with trout. A

recent study by Bradford and others (1998) reported comparable results. Similar effects of trout on zooplankton communities have also been reported for mountain lakes throughout western North America (Anderson 1980; Bahls 1990;

Carlisle and Hawkins 1998; Liss and others 1995).

Benthic Macroinvertebrates

The introduction of fish into fishless lakes also causes predictable effects on benthic macroinvertebrate communities

in which large conspicuous species are eliminated, while

burrowing or otherwise inconspicuous species are relatively

unaffected (Zaret 1980). In the Sierra Nevada, the benthic

invertebrate communities of high-elevation fishless lakes

are typically dominated by several conspicuous taxa of mayfly larvae (Ephemeroptera), caddisfly larvae (Trichoptera),

aquatic beetles (Coleoptera) and true bugs (Corixidae). These

taxa are rare or absent in lakes with introduced trout.
USDA Forest Service Proceedings RMRS-P-15-VOL-5. 2000 315

Instead, the benthic macroinvertebrate community of troutcontaining lakes is typically dominated by midge larvae

(Chironomidae), alderfly larvae (Sialis), aquatic mites (Acari) and fingernail clams (Pisidium) (Bradford and others

1998; Reimers 1958), all taxa that either burrow into lake

bottom sediments or are distasteful. Similar effects of trout

on benthic macroinvertebrate communities have been reported from mountain lakes throughout the western United

States (Bahls 1990; Carlisle and Hawkins 1998; Walters and

Vincent 1973). As noted by Liss and others (1995), however,

the effects of introduced trout may be less pronounced in

areas where lakes contain naturally occurring vertebrate

predators such as salamanders. In these situations, the long

evolutionary history between predatory salamanders and

their invertebrate prey may have resulted in adaptations by

the prey to reduce predation risk, and these adaptations may

also reduce their vulnerability to introduced trout. This

possibility merits additional study.

Food Web Effects

The effect of introduced trout on native aquatic taxa is

often presented as an interaction between two trophic levels

(trout preying on amphibians, trout preying on zooplankton). However, changes in one trophic level can have important indirect effects on all parts of the food web. Although

multiple trophic-level consequences of fish introductions

have not received much attention until recently, at least one

such effect has been suggested for aquatic ecosystems in the

Sierra Nevada. Jennings and others (1992) demonstrated

that the garter snake, Thamnophis elegans, depends heavily

on frog larvae as prey items, and they suggested that the

decline of amphibians in the Sierra Nevada may also result

in the decline of T. elegans. Because introduced trout are an

important factor in the decline of at least one Sierran

amphibian (Bradford 1989; Bradford and others 1993; Knapp

and Matthews 2000), trout may also indirectly cause the

decline of T. elegans.

Trout introductions may also cause trophic cascades, in

which changes caused by the introduction of a new top

predator (fish) propagate to cause substantial changes at the

primary producer level (Carpenter and others 1985). Trophic

cascades have now been reported from a diverse array of lake

types (Carpenter and Kitchell 1993), but studies of trophic

cascades from trout introductions into Sierra Nevada lakes

are just beginning. In a study of alpine lakes in Canada, the

introduction of nonnative trout resulted in a decrease in

large-bodied herbivorous zooplankton and an increase in

phytoplankton abundance (Leavitt and others 1994;

McNaught and others 1999). The elimination of amphibian

larvae following trout introductions may also influence

lower trophic levels, since amphibian larvae can have important effects on algal biomass (Dickman 1968) and lake

nutrient cycling (Seale 1980).

Conclusions and Management

Recommendations ______________

The management of nonnative fish populations in wilderness lakes of the western U.S. has been the focus of considerable controversy for at least two decades (Gottschalk 1976;

Hall and May 1977), with debate generally focusing on the

question of whether nonnative fishes impact wilderness

ecosystems. The preponderance of evidence collected during

the past two decades leaves little doubt that the introduction

of nonnative trout into historically fishless lakes causes a

series of predictable changes in the recipient ecosystems;

therefore, discussions over the management of nonnative

fishes in wilderness lakes should be shifted from whether

there are impacts to determining what level of impact is

acceptable and how to reduce current impacts to this level.

In the Sierra Nevada, introduced trout have caused dramatic changes in the distributions of several native trout

species, one amphibian, and several invertebrate species.

This current level of impact is clearly unacceptable if wilderness areas are to serve the purpose of maintaining natural

processes. We suggest that an acceptable level of impact

would be one that, at a minimum, allows for the long-term

persistence of all native taxa across their historic distribution within wilderness lands. In the Sierra Nevada, reducing

current impacts to this level will take significant resources

from the state and federal agencies with jurisdiction over

management of these ecosystems.

To reduce the impacts of introduced trout on wilderness

lake ecosystems in the Sierra Nevada, it will be critical to (i)

eliminate the stocking of lakes harboring self-sustaining

trout populations, and (ii) restore fishless habitat for the

native taxa most seriously effected by nonnative trout. The

current California Department of Fish and Game stocking

program for wilderness lakes is based on the untested assumption that stocking is required to maintain the target

fisheries. Similar assumptions are commonly made by fisheries managers throughout the western U.S., and appear to

result in the frequent stocking of self-sustaining trout populations (Bahls 1992). Available evidence for wilderness lakes

in the Sierra Nevada indicates that the majority of stocked

lakes have sufficient natural reproduction to maintain these

fisheries in the absence of stocking (Botti 1977; Matthews

and Knapp 1999; Zardus 1977). This unnecessary stocking

brings with it considerable risks to native aquatic species, as

a result of stocked fish hybridizing with or displacing native

fishes and of fish being stocked into the wrong water bodies

(fishless lakes).

Because most trout populations in Sierra Nevada wilderness lakes would be self-sustaining in the absence of stocking (Matthews and Knapp 1999), restoration of mountain

yellow-legged frog populations to even a fraction of their

historic habitat will require the active eradication of fish

populations from some lakes. Remaining mountain yellowlegged frog populations within national forest wilderness

areas are typically extremely isolated (Knapp and Matthews

2000), and are therefore unlikely to persist over the long

term (Bradford and others 1993). To expand the few remaining mountain yellow-legged frog populations and enhance

their likelihood of persistence, one of us (R. Knapp) is

currently using gill nets (Knapp and Matthews 1998) to

remove fish populations from lakes in the immediate vicinity of existing frog populations. The goal of this work is to

create clusters of interconnected fishless lakes and ponds

that would provide high quality habitat for mountain yellow-legged frogs and that could be naturally recolonized

from nearby source populations. Preliminary results indicate that frogs are rapidly recolonizing these lakes after fish
316 USDA Forest Service Proceedings RMRS-P-15-VOL-5. 2000

removal. The success of these pilot projects suggests that the

creation of clusters of fishless habitat across the historic

range of the mountain yellow-legged frog could reverse the

decline of this species and reduce the need to list it under the

Endangered Species Act. In addition to benefiting the mountain yellow-legged frog, these fishless habitat clusters would

also benefit fish-sensitive invertebrate species.

Implementation of these recommendations would represent a significant step toward reducing impacts to Sierra

Nevada wilderness lakes from nonnative fishes. However,

outside of the study area surveyed by Matthews and Knapp

(1999), information on self-sustainability of fish populations

and locations of fish-sensitive native species in the Sierra

Nevada remain rudimentary at best. Resolving the ongoing

controversy over the management of nonnative fisheries in

these wilderness lakes will take a considerable and sustained effort to survey aquatic habitats for nonnative fish

and native aquatic taxa, evaluate the self-sustainability of

fish populations, and design and implement restoration

measures for these sensitive species.

Acknowledgments ______________

During the writing of this paper, R. Knapp was supported

by Cooperative Agreement PSW-96-0007CA between the

USDA Pacific Southwest Research Station and the Marine

Science Institute, University of California, Santa Barbara.

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