I urge all SEKI backcountry users to read through this High Sierra Hikers Assoc. alert.

Comments Due this Tuesday (Dec 17) by 10:59 pm Pacific Time


Tell the Park Service how you feel about its proposal to use chemical poisons to remove fish from lakes and streams in Sequoia and King Canyon National Parks

Background
Sequoia and Kings Canyon National Parks (SEKI) recently released a Draft Environmental Impact Statement (DEIS) that proposes to "restore" native species in high-elevation aquatic ecosystems within these parks. The plan focuses on eradication of nonnative trout from numerous backcountry lakes for the primary purpose of improving conditions for mountain yellow-legged frogs, which have suffered significant declines in recent decades, are preyed upon by trout, and are at growing risk of extinction due to several environmental stressors.

The plan calls for fish to be removed from 32 backcountry lakes, 50 ponds, 5 marshes, and 41 miles of streams. SEKI's "preferred alternative" proposes to use a combination of both physical methods (i.e., electrofishing, gillnetting, and manual destruction of fish nests or "redds") and poisoning wilderness waters with chemical pesticides.

The plan calls for poisoning 6 lakes, 26 ponds, 4 marshes, and 27 miles of stream---in some of SEKI's most remote and pristine areas---using pesticides containing rotenone, which kills all gill-breathing organisms by blocking the uptake of oxygen. Lakes on the "hit list" for chemical poisoning include: Amphitheater Lake (near Observation Peak), Slide Lake (near the Monarch Divide), lakes and ponds in lower Sixty Lakes Basin, Moose Lake (near Tablelands and Tokopah Valley), and two unnamed lakes in the headwaters of the Kern River (near Lake South America). The 27 miles of streams proposed to be poisoned include streams that connect various lakes and ponds, as well as streams below lakes that would be treated using only physical methods.

The High Sierra Hikers Association supports reasonable efforts to protect and restore the disappearing mountain yellow-legged frog, but we strongly object to the use of chemical poisons in SEKI's wilderness, for numerous reasons. First, the use of poisons is unnecessary; fish could be removed using manual methods. Second, many areas of the park would be closed during chemical poisoning, significantly affecting wilderness visitors. Third, wilderness waters would be unsafe to drink for weeks (or even months) after poisoning. And fourth, fish poisons such as rotenone are non-selective, and are known to kill other non-target animals.

The National Park Service (NPS) euphemistically labels rotenone and other fish-killing chemicals as "piscicides," implying that such poisons are selectively toxic to fish; however, the reality is that they are toxic to any gill-breathing organisms, including amphibians, aquatic insects, zooplankton, and other invertebrates. Consequently, the NPS is in effect arguing that the aquatic ecosystems to be "restored" for the benefit of frogs must first be largely destroyed.

The DEIS freely acknowledges that rotenone will have severe impacts on aquatic invertebrates and other non-target organisms, but opines that these impacts are outweighed by potential benefits to frogs and other organisms that are preyed upon by fish.

The NPS attempts to rationalize the use of chemical poisons by arguing that attainment of its restoration goals cannot be achieved using physical methods alone. It argues that 100% eradication of fish from the lakes it wants to poison is not possible because the lakes are "too large and deep" for physical methods. Yet, the NPS acknowledges that it has already entirely removed fish from 11 lakes within SEKI using only physical methods, and two of those lakes are larger in size than most (four of the six) lakes now proposed for chemical poisoning.

As one example, the NPS proposes removing fish from Wanda Lake---at 228 acres the largest lake in SEKI in terms of surface area---using only physical methods. How is it possible that NPS can achieve fish eradication in this lake using gill nets, but not in Amphitheater or Moose lakes? The likely answer is that the NPS does not want to employ chemical poisoning in full view of the public. Wanda Lake lies along the John Muir Trail, and the NPS would likely have to close the area to human use if this lake were to be poisoned.

The NPS also rationalizes chemical poisons in stream/wetland networks because they are "too complex" to eradicate fish using electrofishing and gill nets. Yet those same characteristics also make it extraordinarily difficult to achieve a complete fish kill using chemical methods. This means that many lakes, streams, and ponds would have to be repeatedly poisoned---multiple times over multiple years. And during those treatments the areas would be repeatedly closed to public use until the concentration of rotenone decreases to "safe" levels.

Because the breakdown of rotenone is temperature dependent (i.e., rotenone persists longer in cold water), it will be weeks or even months before wilderness visitors could safely drink the water after it is poisoned. Additionally, helicopters and/or packstock (i.e., horses and mules) will be used to transport supplies and crews of 8 to 15 people into these remote locations for these treatments, resulting in noise, commotion, damage to fragile ecosystems, and diminished opportunities for solitude.

The DEIS fails to present any compelling evidence that chemical poisoning of wilderness waters is essential for accomplishing the objective of establishing a network of refuge areas for frogs, and HSHA believes that these goals can be met using non-chemical methods, which are far less destructive to aquatic ecosystems, and far less intrusive for wilderness visitors. In short, despite the NPS's claims, the use of poisons is a matter of expediency, not necessity.

What you can do:

Send a letter or electronic comments to the Park Service saying that you strongly oppose the use of chemical poisons in the wilderness of Sequoia and Kings Canyon NPs. Here are some points that you might consider making in your letter, though please keep in mind that it is most effective if you use your own words to describe how the poisoning of wilderness waters would impact your use and enjoyment of SEKI's magnificent backcountry.

1) The DEIS fails to provide a compelling rationale for the necessity of chemical poisoning. Fish removal in most of the areas proposed for chemical treatments could be achieved through manual/physical means. In areas where physical removal is truly not possible, the Park Service should look for opportunities in other basins where fish could be removed by physical means, rather than resorting to chemical poisons.

2) As documented in the DEIS, chemical treatments have severe impacts on aquatic invertebrate communities even at the concentrations proposed for use by the Park Service. Point out that rare and/or endemic invertebrate species may be extirpated by chemical poisons, causing irrevocable changes to these ecosystems.

3) The impacts of physical fish removal on the enjoyment of park visitors would be far less than the massive intrusion of chemical poisoning operations.

4) Evidence from numerous other rotenone applications in the West indicates that frequently a single rotenone treatment does not result in a 100% fish kill, thus requiring multiple treatments over multiple years. Consequently, both the ecological and aesthetic impacts are more severe than the DEIS discloses.

We know this is a busy season of year, and that we haven't given you much time to respond. But your letter can be brief. The most important thing is to let the Park Service know that the public is strongly opposed to the use of highly destructive chemical poisons within wilderness areas---areas that are afforded the highest level of protection of any lands in the United States.

The deadline for electronic comments is:
10:59 PM Pacific Time, Tuesday December 17

Comments can be sent directly to the Park Service via its website at this link:

http://parkplanning.nps.gov/commentForm ... ntID=55701

The NPS's comment webpage for this project will disappear at midnight Mountain Time. You must hit the "send" button by 10:59 Pacific Time.

Comments can also be sent via regular mail (postmarked by Dec 17) to the following address:

Superintendent
Sequoia and Kings Canyon National Parks
ATTN: Restoration Plan/DEIS
47050 Generals Highway
Three Rivers, CA 93271

Fax: 559-565-4202



Please DO NOT forward this alert to the Park Service with your comments. (It's better to not comment at all than to forward our alert to the NPS.)

Get involved. Together, we can make a difference !!!


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This is interesting... and frankly quite bizzare. I was at Amphitheater Lake just last month... one of the most prisine areas there is out there. And no real trail going to it either. All these areas drain directly into steams and rivers that 1000s of people drink from in the backcountry... I am not sure how you reconcile telling someone they can't drink out the lake but can drink out of the stream it drains into. Nor where you get your water when you are at the lake in question either. Chemical poisons seems a bit extreme.

I recently heard about something similar involving killing invasive tree snakes in Guam, but in this case using Asprin (by implanting it in mice and feeding the snakes with it). Well, that's a chemical too but apparently one only toxic to the tree snakes. If these chemicals the NPS wants to use are toxic not only to fish but also to humans, or at least potentially a broad array of gill-breathing organisms then one has to question what else will be impacted. And certainly more species appear to be affected than just a single invasive species. There could be all kinds of unintented consequences... it certainly wouldn't be the first time NPS 'management' of wilderness areas blew up in their face (see forest fire suppression for a start).

This is why I hate seeing the NPS get more money... for basically anything. And I try to make sure they get as little of mine as possible. I use these lands as much as anyone, and if funds were used wisely I would certainly feel differently. But all too often it gets funnelled into questionable projects and causes like this one.

Whatever happened to just leaving the wilderness alone? These areas don't really need to be 'managed'. They did just fine for centuries before we came along.

Here is some info on the Guam tree snakes I mentioned: http://www.huffingtonpost.com/2013/02/2 ... 40733.html

Great. Apparently the Sierra wilderness is free of giardia so the NPS is going to make us all get Parkinson's instead.

Attention campers... bury your waste and wash your pots and pans 100 feet from any lakes and streams. And if you have any medications or chemicals to dispose of just go ahead and dump them in the big blue lake right over there.

Why not just dump the rotenone in the lakes in say, mid-November? Fish die, snow keeps everyone out for months, people and frogs come back in late spring. I never cared for the fish anyway.

Delicious looking frog!

Just promote that there is "good fishing" in the lakes targeted. A stampede of fishermen will do the job for you.

Moose lake has great fishing.

Not much use for frogs. It takes too many of them to make a decent meal.

Which eats more mosquitos - frogs or fishes?

I need this information before taking a position.

I think they should use the same successful program used to protect the Devil's Hole pupfish.

There are probably more skeeters now with fish there and frogs gone or in decline. The trout do eat some mosquitoes but will get a lot of their fill from other insects that spend most of their lives in the aquatic stage. Frogs eat mostly terrestrial insects including lots of mosquitoes. And any given alpine lake could support many times more frogs than trout. So frogs.

In grad school I spent some time reviewing scientific literature on the ecological consequences of fish introduction in alpine lakes. There are some very interesting (if you're a nerd) and surprising ecological changes that have taken place since the introduction of fish into those lakes.

There are some very interesting (if you're a nerd) and surprising ecological changes that have taken place since the introduction of fish into those lakes.

Any chance you'd like to give us fellow nerds a quick abstract?

I can't seem to find that particular zip drive, but I'll give it a shot. A couple things that really stuck out as I was researching:

One study done in the Sierra found that trout were likely having a substantial impact on birds in the alpine. Can't quit remember what species, but I believe it was some type of finch. In those lakes trout proliferate to such an extent that they are able to severely reduce or extirpate some aquatic insect species. They eat them while they're in the aquatic larval stage, so most never get the chance to hatch into the terrestrial flying form (the kind that birds prey on). With that food source gone, birds were absent from much of the areas around those lakes.

It has been documented that food webs in these high alpine lakes have been completely altered with the introduction of fish. Where it was once an insect (or other arthropod) species that was the apex predator, fish have usurped that role and eliminated the large, predatory invertebrates. The fish will eat the largest predators but the smaller invertebrate predators are too small for the fish to take notice. The changes reverberate down the food web in interesting ways, depending on how many trophic levels are present. It really provides a text book case of top down trophic regulation.

Trout introduction into previously fishless alpine lakes might be considered tragic and unfortunate in many ways, but there is no denying that it has provided ecologists with an endless array of perfectly designed field studies.

Also one other thing I remember- some of those invertebrate eggs can stay dormant in the mud at the bottom of the lake for many decades.


*Edit: used insect designation too loosely*

I have studied this issue extensively. It is sad the NPS is resorting to chemicals! I'm all for restoration efforts, but not at the expense of using toxic chemicals.

Here are the results an abstract by Roland Knapp on the subject of non-native trout and their impacts on native aquatic biota. If anyone is interested in the full abstract, PM me and I'd be happy to send it via email.

RESULTS: IMPACTS OF TROUT ON NATIVE AQUATIC SPECIES
Trout are highly-effective predators and their impacts on prey species are welldocumented
(e.g., Northcote 1988). This impact may be particularly severe in
oligotrophic lakes such as those found in the Sierra Nevada, since the relatively simple
food webs of such lakes are believed to make them especially sensitive to impacts from
introduced species (Li and Moyle 1981; McQueen, et al. 1986). In fact, based on an
extensive survey of lakes in the Sierra Nevada, Bradford et al. (1994a) concluded that "the
most profound human impacts on aquatic communities in the High Sierra appear to be
related to historical and on-going stocking of exotic fish species into High Sierra waters".
The following review documents the effect of introduced trout on native fishes,
amphibians, zooplankton, lake benthic invertebrates, stream benthic invertebrates, and
community structure in the Sierra Nevada.
Native fishes
The native fish fauna of the Sierra Nevada has been altered substantially by the
introduction of non-native trout, with impacts of introductions being particularly severe
for native trout. The range of the two golden trout subspecies was greatly reduced by the
1970's as a result of non-native trout introductions (USFS 1982). Extensive hybridization
with introduced rainbow trout and displacement by introduced brook trout precipitated
the listing of the Little Kern golden trout under the Endangered Species Act. Since its
listing, non-native trout have been eradicated from the entire Little Kern River and pure
populations of Little Kern golden trout are being re-established. During the 1950's and
1960's, introduced brown trout displaced the California golden trout from much of the
South Fork Kern River. Recovery of this subspecies required the removal of brown trout
from over 100 km of river and the construction of two fish barriers. The recent discovery
of brown trout above the lower barrier, however, has increased the likelihood of brown
trout reinvading the upper South Fork Kern River. Because of this threat, the U.S. Fish
and Wildlife Service is currently considering listing the California golden trout under the
Endangered Species Act.
The status of native rainbow trout on the west side of the Sierra Nevada is unclear.
Although rainbow trout populations probably still occur in most streams and rivers where
they occurred historically, extensive introgression with introduced hatchery rainbow trout
is likely. Although no data are currently available to support this possibility in the Sierra
Nevada, introgression has been documented between hatchery rainbow trout and the
native rainbow trout of the upper Sacramento Basin (Oncorhynchus mykiss stonei;
(Behnke 1992).
The habitat of the Lahontan cutthroat trout has been reduced by over 90%
throughout its native range by massive habitat alteration, water diversions, and
overfishing. In the remaining highly isolated populations, however, cutthroat trout are
subject to hybridization and competition with and predation by introduced trout (Gerstung
1988). Because of the severity of its decline, the Lahontan cutthroat trout was listed
under the Endangered Species Act in 1970. The recently released Lahontan cutthroat
trout recovery plan (Coffin and Cowan 1995) calls for the removal of non-native trout
from portions of the native range of Lahontan cutthroat trout as a critical recovery
strategy. Declines of non-trout fishes in the Sierra Nevada are widespread (Moyle and
Nichols 1973; Moyle and Nichols 1974; La Rivers 1994), but the few studies detailing
the causes of these declines suggest that they have been caused primarily by habitat
alteration and not trout introductions (e.g., Moyle and Nichols 1974).
Amphibians
Numerous native species of amphibians are found in the Sierra Nevada (see
Jennings 1995 for a detailed review). Several anuran species are reported to be declining
in abundance (Yosemite toad: Bufo canorus; California red-legged frog: Rana aurora
draytonii; foothill yellow-legged frog: R. boylii; and mountain yellow-legged frog: R.
muscosa: Moyle 1973; Hayes and Jennings 1986; Bradford 1991; Sherman and Morton
1993; Bradford et al. 1994b; Drost and Fellers 1994). Declines of the three Rana species
have been attributed in part to predation by introduced fishes, including trout (e.g., Hayes
and Jennings 1986; Bradford 1989; Bradford et al. 1993). The California red-legged
frog and the foothill yellow-legged frog are found in the western foothills of the Sierra
Nevada below 1500 m, and inhabit ponds and streams, respectively (Zweifel 1955). The
proposed negative effect of introduced fishes on the California red-legged frog and the
foothill yellow-legged frog is based largely on observations of a lack of overlap between
either of the species and introduced fishes (Hayes and Jennings 1986). These data,
however, are confounded by the fact that habitats containing introduced fishes are also
frequently inhabited by the bullfrog (Rana catesbeiana) (Hayes and Jennings 1986),
another introduced species proposed as a cause for the decline (Moyle 1973; Hayes and
Jennings 1986). In addition, former habitats of these species that now contain introduced
fishes have often also been altered by land management practices. As a result, the
importance of introduced fish relative to bullfrogs and habitat alterations as a factor
leading to the declines of the California red-legged frog and the foothill yellow-legged frog
remains unclear (Hayes and Jennings 1986).
The mountain yellow-legged frog is endemic to the Sierra Nevada and a few sites
in southern California. Historically, the mountain yellow-legged frog was widespread
throughout the Sierra Nevada at elevations above 1500 m (Zweifel 1955), having been
present in all major watersheds on the west and east sides of the Sierra Nevada. However,
based on a recent resurvey of historic localities in the central Sierra Nevada, Drost and
Fellers (1994) reported that the mountain yellow-legged frog was present in fewer than
15% of the sites where it was found in 1915.
Several attributes of this species make it particularly vulnerable to predation and
subsequent extirpation by non-native 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 tadpoles of other Sierran anurans that complete metamorphosis to
the terrestrial stage in a single summer, mountain yellow-legged frog tadpoles generally
require at least two years before metamorphosis to the terrestrial stage. This
overwintering requirement restricts breeding to bodies of water that are deep enough to
avoid oxygen depletion when ice-covered (>1.5 m; Mullally and Cunningham 1956;
Bradford 1983). The majority of these deeper lakes, however, now contain introduced
trout.
There is substantial evidence that introduced trout have severely reduced the
abundance of mountain yellow-legged frogs in the Sierra Nevada. As early as 1924,
Grinnell and Storer (1924) reported that mountain yellow-legged frog tadpoles and
introduced trout rarely co-occur in lakes and ponds in the Sierra Nevada. This
observation has been quantified repeatedly in different parts of the Sierra Nevada
(Bradford 1989; Bradford and Gordon 1992; Bradford et al. 1993; Drost and Fellers
1994). This lack of overlap is assumed to be the result of predation by trout on the
mountain yellow-legged frog, an assertion supported by Needham and Vestal (1938), who
observed trout preying on mountain yellow-legged frogs in a lake into which trout had
recently been introduced. Given that the presence of fish generally makes a pond or lake
unsuitable for mountain yellow-legged frogs, that lakes smaller than 1 ha are generally too
shallow to support mountain yellow-legged frogs (Matthews and Knapp 1995), and that
34-85% of formerly fishless lakes larger than 1 ha now contain introduced trout (see
Results: Current fish distribution), the amount of suitable habitat for mountain yellowlegged
frogs has likely been reduced by a similar amount.
In addition to the direct impact that non-native trout have on mountain yellowlegged
frogs via predation, Bradford et al. (1993) proposed that fish could also 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
sizes and a 10-fold reduction in connectivity between populations. Because amphibian
populations often fluctuate widely under natural conditions (Pechmann et al. 1991; Gulve
1994), and small populations are more likely to go extinct under stochastic population
fluctuations than are large populations (Wilcox 1980; Hanski 1989; Hanksi and Gilpin
1991), Bradford et al. (1993) proposed that the reduction in mountain yellow-legged frog
population size caused by trout introductions is likely to have increased the rate at which
individual populations are extirpated. In addition, they suggested that the increased
isolation of mountain yellow-legged frog populations would reduce the probability of
recolonization of formerly occupied sites. This reduction could result from the smaller
size of potential source populations, increased distance from source populations, and
predation by introduced trout on dispersing frogs (Bradford et al. 1993). Increased
isolation of remaining populations could also result in increased inbreeding with a resulting
decrease in genetic diversity within populations (Reh and Seitz 1990).
In a recent study, Blaustein et al. (1994) proposed that the transmission of
pathogens by introduced fishes may be another means by which trout introductions
indirectly impact amphibian species such as the mountain yellow-legged frog. Blaustein et
al. (1994) reported that the extremely high mortality of western toad (Bufo boreas) egg
masses in a lake in the Cascade Mountains in Oregon was caused by a Saprolegnia fungal
infection. This fungus is frequently found on trout raised in hatcheries, including on those
species commonly introduced into lakes in the Sierra Nevada (Seymour 1970; Richards
and Pickering 1978; Pohl-Branschield and Holtz 1985; Willoughby 1986). The recent
discovery of Saprolegnia fungus infecting eggs of the mountain yellow-legged frog in the
Sierra Nevada (Knapp 1993a) suggests that this proposed impact should be investigated
more fully in Sierran amphibians.
Several additional anuran and salamander species are found in the Sierra Nevada,
but direct impacts to these species from introduced trout are either unlikely because of a
lack of overlap in habitat use between the amphibian species and introduced trout, or are
likely but undocumented. All of the non-Rana anuran species in the Sierra Nevada
(western toad, Yosemite toad, Pacific chorus frog) are largely terrestrial and generally
breed in shallow ponds. Because these ponds are subject to desiccation in summer and
freezing in winter and are therefore unlikely to contain fish, direct effects of introduced
trout on these amphibian species are probably minimal. Most salamanders found in the
Sierra Nevada (Ensatina sp., Hydromantes sp., Batrachoseps sp.) live and breed in semiaquatic
sites such as springs and seeps, and are therefore also unlikely to be impacted by
introduced trout. However, the long-toed salamander (Ambystoma macrodactylum),
found in the central and northern Sierra Nevada, appears to be restricted largely to fishless
lakes (Bradford and Gordon 1992). Similar distributions have been described for the
long-toed salamander in other mountain ranges, and for other species of lake-dwelling
salamanders whose habitat contains introduced trout. For example, in lakes in North
Cascades National Park, densities of the long-toed salamander were reduced in the
presence of introduced trout (Liss and Larson 1991). The closely-related Ambystoma
gracile was also much less common in lakes containing introduced trout than in fishless
lakes. Burger (1950) reported the extinction of neotenic Ambystoma tigrinum nebulosum
in a mountain lake in Colorado after the introduction of trout. Therefore, ample evidence
exists that trout can impact lake-dwelling ambystomatid salamanders, and suggests that
the effect of introduced trout on long-toed salamander populations in the Sierra Nevada
should be investigated more thoroughly.
Although existing data suggests that the introduction of trout into Sierran lakes
has caused local extirpations of at least one amphibian species (mountain yellow-legged
frog), there are no published studies that have investigated the likelihood of amphibians
recolonizing habitats if fish are removed or disappear as a result of a termination in
stocking. Some recent survey data, however, suggests that mountain yellow-legged frogs
can readily recolonize lakes from nearby refugia. Zardus et al. (1977) presented biological
data on 137 lakes in Sequoia and Kings Canyon National Parks, including the presence or
absence of mountain yellow-legged frogs and introduced trout. They reported finding
trout but no frogs in three lakes in the Palisade Basin ("Barrett Lakes 1, 2, and 3").
Stocking was apparently discontinued in these lakes in the late 1970's or early 1980's.
When these lakes were revisited in 1993, Barrett Lake 3 still contained fish and no
mountain yellow-legged frogs, but Barrett Lakes 1 and 2 had reverted to a fishless
condition and contained large mountain yellow-legged frog populations (>100 adults;
Knapp 1993b). Several nearby ponds and lakes were probably never stocked with trout
(Jenkins et al. 1994), and mountain yellow-legged frogs in Barrett Lakes 1 and 2 probably
recolonized from these refugia. Second, in a study of the aquatic biota of several lakes in
Kings Canyon National Park, Taylor and Erman (1980) reported that all lakes in their
study contained trout, including "Lower Sixty" Lake. When this lake was revisited in
1990, it was fishless and contained a very large mountain yellow-legged frog population
(>500 adults; Knapp 1990). Although it is possible that mountain yellow-legged frogs
were present in "Lower Sixty" Lake during the Taylor and Erman (1980) study (since they
apparently did not survey the lake for mountain yellow-legged frogs during their research),
the scarcity of lakes in which trout and frogs coexist (Bradford 1989) makes it more likely
that mountain yellow-legged frogs recolonized this lake after the disappearance of
introduced trout. Several nearby lakes have never been stocked with trout, contain large
mountain yellow-legged frog populations (Zardus, et al. 1977; Knapp 1993a), and could
have served as sources for recolonization of "Lower Sixty" Lake. A third potential
example of recolonization by mountain yellow-legged frogs is apparently occurring in
Wolf Creek Lake, located north of Yosemite National Park. The California Department
of Fish and Game poisoned this lake in 1991-92 to remove the resident brook trout
population. No mountain yellow-legged frogs were seen in the vicinity of the lake before
or during the treatment. In 1994, however, DFG biologists reported seeing mountain
yellow-legged frog adults and tadpoles in a small pond immediately adjacent to the lake
(Knapp 1995b).
Zooplankton
The ability by fishes to dramatically alter lake zooplankton assemblages is widely
recognized (e.g., Carpenter et al. 1985, 1987). The introduction of fish to a lake 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 fish predation (Northcote
1988). Several studies have documented this effect of introduced trout on zooplankton
communities in lakes in the Sierra Nevada. Stoddard (1987) found that the presence or
absence of fish (primarily salmonids) was by far the most important predictor of the
distribution of zooplankton species among 75 alpine and subalpine lakes in the central
Sierra Nevada, with large-bodied species found in fishless lakes and small-bodied species
found in lakes with trout. Other studies on Sierran lakes have produced very similar
results (Richards et al. 1975; Morgan et al. 1978; Goldman et al. 1979; Melack et al.
1989; Bradford et al. 1994a). Effects of trout on zooplankton communities have also
been reported for lakes in the Rocky Mountains and Europe (Anderson 1971, 1972;
Northcote et al. 1978; Dawidowicz and Gliwicz 1983; Bahls 1990).
Fish introductions may result in the extirpation of vulnerable zooplankton species.
In Sierran lakes, large bodied Daphnia and Diaptomus species are commonly found in
fishless lakes but are rarely found in lakes with trout (Reimers 1958; Melack et al. 1989;
Bradford et al. 1994a). These results are in agreement with the results of a model by
Walters and Vincent (1973) that predicted that large-bodied zooplankton species would be
eliminated by trout predation even at low trout densities. Although these Daphnia and
Diaptomus species have apparently been extirpated from many lakes in the Sierra Nevada,
they are still relatively common in the range (e.g., Melack et al. 1989; Bradford et al.
1994a). In constrast, the phantom midge, Chaoborus americanus, may have been
extirpated from the Sierra Nevada by introduced trout (Stoddard 1987). C. americanus, is
common in high elevation lakes throughout western North America, but Stoddard (1987)
did not find C. americanus in any of his samples from Sierran lakes. C. americanus was
also absent from Sierran lakes sampled by Silverman and Erman (1979), Melack et al.
(1989) and Bradford et al. (1994a). The possibility that trout introductions are
responsible for the absence of Chaoborus in the Sierra Nevada is supported by studies
showing the complete elimination of Chaoborus from lakes by introduced trout (Northcote
et al. 1978).
Although trout introductions in the Sierra Nevada can apparently cause the
extirpation of vulnerable zooplankton species from lakes, it is not clear whether these
species reappear in lakes that revert to their original fishless condition. Some studies
show that vulnerable zooplankton species do not reappear (Reimers 1958; Anderson
1972, 1974; Leavitt, et al. 1994), while others show that they do (Walters and Vincent
1973; Bahls 1990). Many zooplankton taxa have resting stages (e.g., Thorp and Covich
1991), including those of one species recently shown to remain viable for over 300 years
(Hairston et al. 1995). If Sierran zooplankton also have long-lived resting stages, this
"egg bank" could allow recovery of the original zooplankton community after fish
disappearance. On the contrary, the introduction of fish may cause changes in lake food
webs that reduce the ability of some zooplankton species to recolonize (Leavitt et al.
1994). Therefore, further research is necessary to determine the effects of trout
introductions on Sierran lake food webs and zooplankton colonization dynamics.
Lake benthic macroinvertebrates
In addition to their effects on zooplankton communities, fish are also capable of
altering the structure of lake benthic macroinvertebrate communities. In the Sierra
Nevada, high elevation fishless lakes contain mayfly larvae (Ephemeroptera), caddisfly
larvae (Trichoptera), aquatic beetles (Coleoptera), and true bugs (Corixidae) that are
absent in lakes that contain introduced trout (Reimers 1958; Melack et al. 1989;
Bradford et al. 1994a). Similar results have also been documented in other mountain
ranges in the western United States (Walters and Vincent 1973; Bahls 1990). No data is
currently available to determine the rate at which benthic macroinvertebrates recolonize
lakes after trout disappearance.
Stream benthic macroinvertebrates
In contrast to the research effort that has been devoted to quantifying the impact of
introduced trout on native lake biota, few studies have examined their effect on native
stream biota. In the only study of trout impacts on Sierra Nevada stream benthic taxa that
I am aware of, Melack et al. (1989) found significant differences in the macroinvertebrate
assemblages of fish and fishless streams; these effects, however, were confined to a
minority of the taxa present. Studies outside the Sierra Nevada are equivocal on the
impacts of trout, with some studies showing no effect of trout on stream
macroinvertebrates (e.g., Allan 1982; Culp 1986), and others showing strong effects (e.g.,
Hemphill and Cooper 1984; Cooper 1988; Flecker and Townsend 1994). Cooper et al.
(1990) suggest that vulnerability of particular taxa to trout predation is likely a function of
a species exchange rate (i.e., immigration/emigration), with taxa with low exchange rates
being more vulnerable than those with high exchange rates. If true, then stream
communities may be more resistant than lake communities to changes caused by trout
predation because of the much greater magnitude of prey exchange in streams.
In addition to direct predation effects on stream macroinvertebrates, trout can also
have non-lethal effects. These effects include changes in diel behavior patterns (Douglas
et al. 1994), diets, and growth rates (Wiseman et al. 1993).
Community-wide effects
Although the effect of introduced trout on native aquatic biota is often presented
as an interaction between two trophic levels (e.g., trout preying on amphibians, trout
preying on zooplankton), large changes in one trophic level (e.g., as a result of trout
introductions) can have important cascading effects on all parts of the food web
(Carpenter and Kitchell 1993). Although multiple trophic level consequences of fish
introductions have not received much attention until recently, several potential
community-wide effects of trout introductions have been suggested for aquatic ecosystems
in the Sierra Nevada. Jennings et al. (1992) demonstrated that the garter snake,
Thamnophis elegans, depends heavily on frog tadpoles 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 likely to be one of the causal factors leading to the
decline of at least one Sierran amphibian (Bradford 1989; Bradford et al. 1993), trout
may also indirectly cause the decline of T. elegans. The loss of tadpoles from aquatic
communities may also have impacts on lower trophic levels, since tadpoles can
significantly reduce algal biomass (Dickman 1968) and alter lake nutrient cycling (Seale
1980).
Changes in the zooplankton community in lakes as a result of fish predation may
also have community-wide consequences. In subalpine Castle Lake (northern Sierra
Nevada), a decrease in the density of rainbow trout following the cessation of trout
stocking caused an increase in introduced zooplanktivorous fishes, a decrease in
zooplankton, a decrease in water transparency, and an increase in primary productivity
(Brett et al. 1994; Elser et al. 1995). In a study of alpine lakes in Canada, the loss of all
non-native trout following the termination of trout stocking resulted in the an increase in
grazing zooplankton and a decrease in phytoplankton abundance (Leavitt et al. 1994).
Similar results were found by Stenson et al. (1978) and Carpenter et al. (1985). Similar
trophic cascades have also been documented in streams (Power 1990; Flecker and
Townsend 1994).