April 14, 2009

Trout Impacts on Yosemite Streams

Most research on the impacts of nonnative trout on aquatic habitats in the Sierra Nevada have focused on lakes, in part because these are the habitats favored by the declining mountain yellow-legged frog. This research has shown dramatic changes to vertebrate and invertebrate species composition caused by trout predation on the larger, more conspicuous taxa. Trout also alter nutrient cycling in these lake habitats as a consequence of the changes in species composition.

A study
by Dave Herbst and colleagues, just published in Freshwater Biology, shows very clearly that impacts of introduced trout on Sierran streams are similar to those documented for lakes. Herbst et al. compared the invertebrate communities, algal cover, and algal biomass in 21 paired streams in Yosemite National Park, one stream of each pair containing introduced trout and the other member of the pair remaining in a natural fishless condition. Densities of 10 out of 50 common invertebrate taxa were significantly reduced in the trout-containing compared to the troutless streams, and these taxa tended to be conspicuous forms whose native habitats are primarily at high elevation above the original range of trout.

Reductions in species richness in the trout-containing streams caused significant increases in algal cover and biomass compared to levels in troutless streams.
Increases in algae are likely a consequence of grazing invertebrates being reduced in the presence of trout and consequent reductions in herbivory.
These indirect effects (trout reduce the density of grazing insects, reduction in grazers causes an increase in algal biomass) are in agreement with numerous other studies conducted on the effects of trout on streams all around the world.

The findings of this study serve as yet another example of the considerable impacts of trout introductions on Sierra Nevada aquatic ecosystems. Given the almost complete absence of watersheds anywhere in the Sierra Nevada that haven't been stocked with trout, it is clear that we need to think about how to restore at least some entire watersheds to their historic fishless condition. That is no small undertaking.

Back to The Mountain Yellow-legged Frog Site.


  1. It seems as though the science describing the negative effects of trout stocking is unequivocal. Is the next step a public relations campaign to convince state fisheries biologists and anglers that it shouldn't be done? If so, what progress have these efforts made?

  2. Hi David. I've spent lots of time encouraging the California Dept. of Fish and Game (CDFG) to improve their fish stocking practices. Those efforts have met with considerable success in the Sierra Nevada and less success elsewhere in the state. The CDFG is now actively involved in removing fish from lakes in the Sierra Nevada to aid in the recovery of mountain yellow-legged frogs. The CDFG is also in the process of writing management plans that seek to reduce impacts of fish stocking on amphibians and other native species. Elsewhere in California the CDFG continued (until recently) to stock fish with little regard to native biodiversity. That changed when they were sued over violations of the California Environmental Quality Act. The court-mandated environmental document (EIS/EIR) is due in draft form to the public this summer. I'm hopeful that this document will put into place a series of measures that reduce the impacts of stocking on native fauna. Unfortunately, a halt to stocking will not be sufficient to remove trout from those streams and lakes where they have been introduced as most quickly establish self-sustaining populations. Those populations can only be eliminated using active removal methods (e.g., gill nets in lakes, electrofishing and piscicides in streams).

  3. Sounds like an intensive and arduous process. I'm glad you're persistent in your conservation efforts. I hope to soon read articles about yellow-legged frog recolonization rates following exotic fish removal.

  4. Actually, those articles already exist. Check out the following:

    Vredenburg, V. T. 2004. Reversing introduced species effects: Experimental removal of introduced fish leads to rapid recovery of a declining frog. Proceedings of the National Academy of Sciences, USA 101:7646-7650.

    Knapp, R. A., D. M. Boiano, and V. T. Vredenburg. 2007. Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa). Biological Conservation 135:11-20.

  5. Glad to hear it.

    I recently read an article describing how introduction of game fish increased Rana (Lithobates?) populations because the frogs were generally unpalatable to fish, which preferred to feed on a secondary predator of tadpoles, i.e. insect larvae. Why do you think this process isn't in play with yellow-legged frogs?

  6. I suspect you are referring to the work by Mike Adams et al. in Ecology Letters (2007). As you mentioned, bullfrogs and their tadpoles are unpalatable to fish. Unpalatability likely evolved due to the intense fish predation pressure in their native range, which is east of the Rocky Mountains, a region host to a wide diversity of fish species. In the west, fish faunas were relative depauperate and large areas (such as most of the Sierra Nevada in California) were entirely fishless. Mountain yellow-legged frogs and their tadpoles are highly palatable to fish. So, amphibian palatability drives the observed differences between the two systems.

  7. The northern boundary of Yosemite has many unstocked lakes, and the difference is striking. The trout also take food from the birds. At Many Island Lake, for example, a colony of swifts come out each morning to take the mayfly hatch, the lake swarms with tadpoles, and I've seen a giant waterbug over three inches long. Never in a stocked lake.