Ethan Clark Taxman
ENVS 4100
April 2014
Simplified Expert Sciences Guide: Lakes At High Elevations & Trophic Interactions
Over the past month I actively corresponded with Tommy Detmer, a Ph. D.
candidate in the Ecology and Evolutionary Biology program here at CU Boulder. By
conducting an interview, attending his colloquium, and e-mailing back and forth, I
gained insight into his area of study, which focuses on trophic dynamics in high
elevation lakes—specifically at Rocky Mountain National Park. Tommy’s love for fishing
and the outdoors led him on a path where he now studies the same lakes that reel in
countless sports-fisherman.
Interestingly, many lakes located at high elevation in the Western United States
did not have native fish populations before European settlers arrived (Detmer). During
the turn of the century, in the early 1900’s, fish stocking efforts grew in order to
perpetuate sport fishing in the park (NPS). Fish stocking started as a process that
involved carrying fish in assorted vessels using mules to access lakes. Fish are now
even stocked by airplane—literally flying fish, dropping them into lakes from above.
According to Tommy, 95% of the lakes at high elevation were historically without fish,
but that number has a new role: now 95% of the lakes deeper than 3 meters and larger
than 2 hectares contain trout (Bahls 183). This striking reversal of percentages was
something that leaves Tommy questioning: to what extent do fish change
macroinvertebrate community structure? How do they alter the trophic dynamics of the
overall system?
Previous research analyzing the macroinvertebrate presence in lakes with and
without fish indicates that the macroinvertebrate system changes with the presence of
stocked fish. A four-year study of 16 lakes in the Klamath Mountains area of California
showed that fishless lakes had 33% greater benthic macroinvertebrate abundances
(Pope and Hannelly 211). In an effort to better understand the question, Tommy set out
on a four-year adventure to seek answers. Although the monitoring stopped just this
past year, and is not technically active at the moment, it indeed was an ongoing study
that involved active sampling in Rocky Mountain National Park. Under the advice of
William Lewis, Director of the Center for Limnology, and the Associate Director, James
McCutchan, Tommy’s study was underway. As mentioned, Tommy began in 2009 and
the duration of his experiment was a four-year period that ended in 2013. By utilizing
willing undergraduates, they sampled three to four times per year between the four
years during both good and bad weather conditions and times of day. To properly
conduct the study, both lakes with fish and lakes without fish were monitored. Some
physical and chemical parameters were gathered, but the main focus of the study was
the macroinvertebrate collection. To sample for benthic macroinvertebrates in these
lakes, the students and Tommy used bottom sediment cores, as well as a benthic sled,
which is basically a kick net that is dragged along the bottom by rowing the boat. In
addition to the benthic macroinvertebrate sampling, Tommy also conducted hook and
line surveys in the lakes to verify the presence of trout, as well as verifying with
historical stocking records. During and after the course of the sampling period,
extensive analysis was conducted to determine the significance of their study. Repeated
measures ANOVA analysis demonstrates that lakes without fish averaged almost
double the benthic macroinvertebrate biomass than lakes with fish (832 mg m-2 in
fishless lakes compared to 448 mg m-2 in stocked lakes—see graph 2).
Tommy notes that lakes with fish have fewer large taxa, which is a direct result of
predation by fish. His finding is similar to a comparison of three kinds of lakes in
Yosemite National Park that were originally fishless, had stocked fish remaining or were
reverted back to fishless. That study indicated that stocked-fish lakes had 16% fewer
taxa than lakes that had never been stocked (Knapp, Hawkins, Ladau, and McClory
843). In addition, the study of the never stocked fishless lakes in the Klamath Mountains
revealed about 40% higher taxon richness (Pope and Hannelly 201). The fish are
tempted by larger macroinvertebrates, and suppression occurs.
Years ago, when the National Park Service decided to stock non-native fish in
lakes that were previously untouched by fish, they may not have known the extent of
their actions. After all, stocking fish is seemingly harmless, especially when the goal is
to promote an industry that has been and remains extremely popular in Colorado. It is
interesting to think that the most “wild” lakes in the United States are actually totally
manipulated, because they are derived of populations of fish that were introduced by
humans, not by natural occurrence. In 1970, stocking efforts by the park were
reconsidered, and according to the National Park Service, in 1975 only native
Greenback cutthroat and the Colorado River cutthroat Trout were being stocked (NPS).
Graph 1 demonstrates the number of fish stocked over time. It is clear where the initial
policies on stocking and revised policies occur. Non-native fish are not being stocked
and active removal is underway. It is definitely a good thing that they reconsidered their
policy on fish stocking. Some of the reasoning behind this was to protect the natural
system, which was home only to the two previous trout species mentioned above, and
they were not in 95% of moderately sized lakes at high elevations historically. That
being said, as part of an effort to please multiple stakeholders, they were able to
demonstrate that they indeed cared for the integrity of the environment by removing
non-native trout species, and continuously stocking native trout species. By doing so,
they made fisherman happy, as well as nature enthusiasts who are unhappy with nonnative species. But, this does not address the issue of the macroinvertebrates. The Park
Service is obligated to protect native species, but even though two species of trout may
be native to this region, the thing is, they were not native to the lakes at high elevation,
because the geographic relief creates obstructions that prevent spawning, according to
Tommy. It puts the Park Service in a hard position, and they have tried their best to
manage the situation, but in my opinion, this is a decision that had unforeseen
consequences at the time of its highest popularity and we are realizing this later—a
common theme in environmental issues.
Trophic interactions in a system are important to monitor because they can be
directly correlated with ecosystem dynamics. Tommy mentioned in the conclusion of his
EBIO Colloquium that suppression of larger taxa of macroinvertebrates in these lakes
disrupts the bird communities. If the fish are selecting larger prey, this leaves less food
for the birds that rely on aquatic macroinvertebrates as part of their diet. During my
interview with him, he mentioned that there are many more birds around fishless lakes
than lakes with fish. He said he even saw an otter in a fishless lake, which is an
extremely rare sighting. Research supports Tommy’s observations. For example, a
study of the Gray-crowned Rosy-Finches (Leucosticte tephrocotis dawsoni) in the Sierra
Nevada Mountains in California showed that there were 98% fewer mayflies in troutstocked lakes than in the fishless lakes, which directly affected the food supply for these
birds (Epanchin, Knapp and Lawler 2411). The trout were able to consume the majority
of the mayfly nymphs as they emerged, leaving very few mayfly adults for the Rosy-
Finches to forage. The study determined that in that area, mayfly production was
reduced by 70% due to the introduction of trout, causing an 83% average decrease in
the number of Rosy-Finches feeding at lakes where trout had been introduced
compared to fishless lakes (Epanchin, Knapp and Lawler 2411). Notably, this research
indicated there were 5.9 times more Rosy-Finches at fishless lakes than at troutstocked lakes (Epanchin, Knapp and Lawler 2406).
To me, even though Tommy’s study involves monitoring underwater, where we
may not easily see the direct impacts non-native fish bring to the pristine system, this
study has merit. His research showed that the presence of fish do have an impact on
the trophic dynamics of the ecosystem. As well, interactions between consumers,
primary producers and water quality are important considerations. Tommy’s findings
demonstrate that human actions have consequences, and it is important to review all
possible outcomes before undergoing such actions.
2000000
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
1886
1891
1896
1902
1907
1912
1917
1922
1927
1932
1937
1942
1947
1952
1957
1962
1967
1972
1977
1982
1987
1992
1997
2002
2007
2012
Number of fish stocked in Rocky
Mountain National Park
Graph 1:
Year
Graph depicting the stocking activity—demonstrates the policy of reduction of fish
stocking in the 1970s, also demonstrates the boom at the turn of the century. Data from
Fish and Wildlife Service.
Graph 2:
Biomass (mg m-2)
Benthic invertebrate biomass per
unit area
1000
900
800
700
600
500
400
300
200
100
0
Lakes without fish
Lakes with fish
Graph depicting biomass of macroinvertebrates in lakes with and without fish. Analysis
found using repeated measures ANOVA (p<0.001%) by Tommy Detmer.
Works Cited:
Bahls, Peter. “The status of fish populations and management of high mountain lakes
in the western United States.” Northwest Science 66.3(1992) 183-193. Web.
30 April 2014.
Detmer, Thomas. “Lakes at high elevation: A model system for explaining variation in
cascading trophic interactions.” University of Colorado Boulder. 4 April 2014. Ecology
and Evolutionary Biology Department Colloquia Graduate Seminar.
Detmer, Thomas. “Presentation abstract and speaker biography for ‘One fish too…many
fish? Understanding the changes caused by fish introductions in lakes at high
elevations’.” Boulder County Nature Association. 16 March 2013. Song of the Alpine:
Climate Change and the Resilience of High Elevation Ecosystems 20th Annual Boulder
County Ecosymposium. Web. 5 April 2014.
Detmer, Thomas. Personal interview. 2 April 2014.
Epanchin, Peter, Roland Knapp, and Sharon Lawler . “Nonnative trout impact an alpinenesting bird by altering aquatic-insect subsidies.” Ecology 91.8(2010): 2406–2415.
Web. 26 April 2014.
"Fishing." Rocky Mountain National Park. United States National Park Service, n.d. Web.
5 April 2014.
Knapp, Roland., Charles Hawkins, Joshua Ladau, and Jodi McClory. “Fauna of Yosemite
National Park Lakes has low resistance but high resilience to fish introductions.”
Ecological Applications 15.3 (2005):835-847. Web. 26 April 2014.
Pope, Karen and Erin Hannelly. “Response of benthic macroinvertebrates to whole-lake
non-native fish treatments in mid-elevation lakes of the Trinity Alps, California.”
Hydrobiologia 714(2013):201–215 Web. 26 April 2014.