April 21, 2009

Trout Impacts in California's Trinity Alps

When it comes to impacts of introduced trout on aquatic ecosystems, the Sierra Nevada is by far the best-studied such ecosystem in the world. However, an increasing number of studies are now being conducted elsewhere and results from this research make it clear that broad impacts are a common consequence of trout introductions into montane habitats. In recent years, a research group at the Forest Service Redwood Sciences Laboratory in Arcata has published several studies showing that introduced trout have severe impacts on a wide range of aquatic amphibians native to mountains in the Klamath bioregion, including the Cascades frog, Pacific treefrog, and long-toed salamander.

In a study just published in Freshwater Biology, Karen Pope and colleagues extend this past research by focusing on impacts to lake-dwelling invertebrates. Using a replicated whole-lake experiment conducted in the historically fishless Trinity Alps, they described changes over a three-year period in the emergence of aquatic insects from lakes in which (1) trout had never been introduced, (2) stocking of nonnative trout
continued during the study, (3) stocking was suspended, and (4) stocking was halted and trout populations were removed using gill nets. Trout removal caused rapid increases in aquatic insect biomass over the three-year study period. Insect emergence was low in both the stocked lakes and stocking-suspension lakes because trout densities remained relatively high in lakes assigned to these treatments. Therefore, trout introduced into these naturally fishless lakes caused significant reductions in the biomass of native invertebrates and these taxa recovered quickly following trout removal.

The focus of the study by Pope and colleagues on insect emergence helps to emphasize a key point which is that trout impacts on lake-dwelling invertebrate communities aren't likely to be restricted to the lakes themselves. Adult forms of aquatic insects emerge from lakes and become available to a wide range of terrestrial predators and scavengers, including reptiles, birds, bats, and ants. Therefore, the
reductions in insect emergence due to trout introductions could impact terrestrial species and the functioning of terrestrial ecosystems. Our increasing understanding of the connections between aquatic ecosystems and the adjacent terrestrial ecosystems is a fascinating area of study and provides a more complete picture of how pervasive the effects of trout introductions can be.

For additional information on the impacts of introduced trout on amphibians in the Klamath bioregion, check out the following papers:

Welsh, H. H., K. L. Pope, and D. Boiano. 2006. Sub-alpine amphibian distributions related to species palatability to non-native salmonids in the Klamath mountains of northern California. Diversity and Distributions 12:298-309 [link].

Pope, K. L. 2008. Assessing changes in amphibian population dynamics following experimental manipulations of introduced fish. Conservation Biology 22:1572-1581 [link].

Pope, K. L., J. M. Garwood, H. H. Welsh Jr, and S. P. Lawler. 2008. Evidence of indirect impacts of introduced trout on native amphibians via facilitation of a shared predator. Biological Conservation 141:1321-1331

Back to The Mountain Yellow-legged Frog Site.


  1. As this is funded by UCD, and the CDFG, i'd like to know why this isn't made public which lakes are erradicated prior to doing so? I think those that know great lakes in the backcountry (nice fish) have a right to know that where they may be going, might have been gillnetted, as it happened to me in 2006 in Yose. We do pay into the system for our fishing and hunting licenses, etc.

    If there is somewhere that it was posted that i didn't know about, i'm ok with that still want a pointer). But, as CDFG was involved, i would expect, as a public service, they would post such lakes as being gillnetted.

    I know you all have a job to do to restore to some mythical point in time, but i think nature is a matter of where you pick it to be. I'm not sure glacial time is the right pick. If frogs are gone in the sierras, or elsewhere, i'm not sure i'm missing anything from my experience.

    I do know that if endangered frogs are in an area, and if they go on the endangered list, i can't even get close enough to scare them, as i'll be in violation of the Endangered Species Act.



  2. Please know that i didn't mean to offend you on the last post. I've really know clue who decides what is the right thing to do in frog v. fish. It's not easy when you see a lake you loved to go to, suddenly vanishes off your stellar fishing radar, when you've been going there for decades and didnt know about it until a 2 day trek to see gill-nets.

    I think the science you all are doing is great. It gives more background for subsequent discussion around which basins are for frogs, and which might be held back for those who like the wilderness, off trail fishing experience.

    If i said it in a bad way, before, i'm sorry.


    PS: all i'm really looking for is disclosure at the trailhead/permitting stations as to the state of the lakes where i intend to go.

  3. Russ - Sorry to take a few days to publish your comment. I've been out of town. I have no control over how the agencies notify (or do not notify) the public regarding fish removal projects. I sympathize with your concern but you need to talk to the agency involved about the issue.

    As for lake restoration, those of us involved in removing fish from lakes aren't interested in returning lakes to their condition at the end of the Pleistocene. My argument has always been that to conserve native species we need to ensure an adequate distribution of fishless lakes across mountain landscapes. That would be arguing for returning some lakes to the condition they were in at approximately 1850.

    If you prefer to view Sierran lakes merely as trout ponds that is your choice. I prefer to view them as unique habitats that historically were occupied by species found nowhere else on Earth.

    Your draconian description of how the Endangered Species Act will be enforced is overblown to say the least. Such exaggerations don't provide much fodder for productive discussion.

  4. so, this is what concerned me as i looked around.

    This is from the NPS in the Pinnacles.

    "California Red-Legged Frogs are listed as a Threatened Species under the Endangered Species act. It is illegal to harass or harm them in any way. Approaching so closely that they jump is a form of harassment. Catching them not only disturbs them, but may also cause serious injury. If you hike to the Bear Gulch Reservoir and are fortunate enough to see these frogs, please enjoy them from a distance. With binoculars you can get a great up-close view while letting them go about their lives undisturbed. Please share any interesting frog-watching observations with us."

    This would be the same for MYLF, and push all lakes off limits to visitors. I'm not saying it'll get enforced, as, for me, the lake is already devoid of a reason to visit.

    Some find pleasure in amphibians, some find pleasure in a nice lake with fish. Not judging here, as your history and mine are different.


  5. I would argue that the situation with red-legged frogs in Pinnacles National Park is not a good model for what will likely happen in the Sierra Nevada when the southern and Sierra Nevada yellow-legged frogs get listed. Red-legged frogs are extremely rare in Pinnacles and are found at only a single location. That probably justifies the Park Service's concern over disturbance from people.

    Given that mountain yellow-legged frogs in the Sierra Nevada are still relatively widespread, being found at several hundred localities, a similar level of concern about human disturbance would not be justified.

  6. I thought some MYLF's were already put on the endangered list in southern california? Doesn't that harrassment clause apply to them as well? It seemed pretty clear that one wasn't to approach a frog and scare it into the water.

    I'm just asking, as you might know the answer.


  7. I'm no expert on Endangered Species Act (ESA) implementation but from my experience "harassment" under the ESA is a relative term. In areas where frogs are still common I'm guessing that the term would be applied much less rigidly than in places where frogs are extremely rare.

  8. Hello Roland,

    I guess I say this to you only to try to illustrate a point. Your reference to Russ's "draconian decription"--and productive discussion-- and his, I must say rather on-point reply (most notably as to what will happen to me if I cause a frog to jump in the water) is just the mentality that causes fisherman like myself to stop and say, "whoa, wait a minute". It's that type of thinking that for lack of a better word that is difficult for so many of us to relate to. It just is. I say this to you in the most reasonable of tones ok? Not an ounce of hostility. I respect your expertise, and what you are attempting to accomplish can in no way be judged as a bad thing. I'm a fishermen yes. Catch and release. I LOVE the high Sierra. I do NOT want to see the frogs or any species extinct. I've seen them up there planty of times. You, as a biologist, study and love all the creatures out there in their natural habitat. I do too. I always have in my layman way obsrved nature all my life in the Sierra and high Nevada desert, but I also like to fish... the thing with a lot of people like me and I would imagine Russ as well is we're ok with lakes being set aside for preservation of frogs and whatever other habitat. But how many? Is the ultimate goal the removal of all non-native species everywhere? I really enjoy fishing. I just do. I just hope this all ends in a compromise.

    I am curious though, and perhaps you have made reference to it elsewhere...what is your definition of an "adequate distribution of fishless lakes"? Has this basically been determined or is it "to be determined" (so to speak) at a future date. Thanks Roland for your time....................Jeff

  9. Hi Jeff. Thanks for your post. I understand the concerns that you, Russ, and others have regarding the future of angling opportunities in the High Sierra. I just wish that I had better answers. If the primary threat to mountain yellow-legged frogs was introduced trout it would be relatively easy to determine how many fishless lakes in any one area were required to ensure long-term persistence of frogs and other native species. Unfortunately, the important role of the amphibian chytrid fungus (Bd) in causing amphibian declines makes the answer to your question of "how many lakes" much more difficult to answer. Given the scientific uncertainties in how to ensure persistence of amphibians in the presence of Bd, all of us involved with this issue - California Department of Fish and Game, National Park Service, scientists, etc. - are learning as we go. The ultimate goal is not to remove all nonnative trout from Sierran waters. That is an impossible task and one that is not necessary to conserve native species. The solution will by necessity be a compromise between the ideal situation for native species (no trout) and the current situation. I am committed to helping find that compromise. Constructive involvement by the angling community in the process will be essential.

  10. You must be getting busy about this time of year so I’ll try to catch you here on this forum.
    Do you have list of effected lakes or watersheds in which you or others are planning on conducting research or have conducted research in. Furthermore, do you have a list of lakes or watersheds which you or others have prescribed management for amphibians or, have removed fish from in order to manage for amphibians or invertebrates?
    The reason I am asking is because I grew up in the Eastern Sierra (Lee Vining) close to some of your research areas, have an interest and education in natural resource science, and am curious and have been following your work for many years. I try to get out in our beloved high-country as much as humanly possible. I figure I get out and about just as much as anyone and am wondering what is going on with some of grandpas and great grandpas fishing holes. I’m noticing a decline in good fishing lakes along with a decline in fisher people (and frogs). This does not make sense. Do you have a comprehensive list of lakes or areas that you work in? Species specific population dynamics and that sort of stuff? Just trying to connect many dots myself!
    Matt Banta

  11. Hi Matt. The only areas where I have conducted long-term research on mountain lakes is in Humphreys Basin (Sierra National Forest). This work started in 1996 and involved trout removal from five lakes (Knob, Square, Marmot, Cony, and an unnamed lake). Three fish-containing lakes (Lower Desolation, Summit, and Mesa) served as controls against which I was able to describe changes in nutrients and fauna in the fish removal lakes. The fish populations in the control lakes have changed very little over the course of the study.

    In addition, trout were removed from Maul Lake (Hall Natural Area) in the early 1990s.

    The only other trout removals conducted on national forests in this area have been those conducted by the California Dept. of Fish and Game. For site-specific information on these removals, you should contact Curtis Milliron (cmilliro@dfg.ca.gov).

  12. That's interesting. When did your work conclude or is it on-going? The reason I ask is because I wonder if I stumbled upon some of your equipment on the north side of Mesa a couple of years ago. I was making my way back towards Muriel Lake from the Puppet Lake area. My son and I were wondering "what is this stuff?"

    I was wondering though because it has come up in discussion. How deep of a lake is required for the frogs? I assume the depth is needed for the winter months? I ask because there are SO many marshy areas all over the Sierras that are fishless, that one would normally associate with amphibians.

    I have also heard that there are lakes in the Desolation Wilderness where the trout and frogs co-exist quite well. But this may be rumor or overblown? I ask because I wonder if frogs in certain places have adapted, or it may be that these locations have much more ideal escape points from predation....Thanks for your time Dr. Knapp

  13. The Humphreys research is ongoing and that was my equipment stash at Mesa Lake that you ran across. The research conclusions to date can be read in a series of papers posted here: http://vesr.ucnrs.org/pages/knapp/publications/publications.html. As for the minimum water depth required by mountain yellow-legged frogs to overwinter, the general rule is at least ten feet. There are exceptions, of course, but ten feet of water ensures that sites don't freeze solid in the winter and dry up in the summer. Due to their shallow depths, the Sierra's fishless marshes generally do not constitute suitable winter habitat for mountain yellow-legged frogs. The idea that frogs and fish coexist in the Desolation Wilderness is more fiction than fact. There are almost no mountain yellow-legged frogs left in that entire wilderness area. In the few sites in the Sierra Nevada where mountain yellow-legged frogs reproduce successfully despite the presence of fish, two site characteristics seem to contribute to frog persistence: low trout density and the presence of shallow lagoons (avoided by trout but used by frogs).

  14. Nice to see this blog..

    Thank you very much...
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