Adding trout to mountain lakes disrupts food supply for birds

Lower Cathedral Lake in Yosemite National Park. Image credit, Frank K.It should come as little surprise that adding fish to naturally fishless mountain lakes can cause unintended negative ecological consequences.

Interestingly, though, findings from a new study from California indicate that stocking lakes with exotic trout reduces the abundance of certain birds from the adjacent terrestrial ecosystem during specific times of the year. Scientists believe that this occurs because the trout cut off of a linkage between the terrestrial and aquatic food webs in which insects emerging from the lake subsidize avian diets.

Mountainous areas often boast a high density of lakes formed from retreating glaciers around 12,000 years ago. While these lakes have naturally been fishless, people have stocked many of these waters with exotic species such as trout to allow recreational fishing. 

Peter Epanchin and fellow researchers compared the abundance of gray-crowned rosy finches at Sierra-Nevada lakes that had been stocked with trout and those that remained in their natural fishless state. At the stocked lakes, the finches were 83% less abundant during an important three-week period of the year when the birds feed on mayflies emerging from the waters.

This likely occurs because the introduced trout eat most of the mayfly young before they can emerge as adults. The researchers found that at fish-containing lakes, mayfly abundance was 98% lower than lakes without fish.

On the surface this may not seem like such a big deal - the decrease in abundance of the bird at the stocked lakes seems to only last for the 3 week period when the mayflies emerge. However, the study authors believe that this could have negative effects on rosy finch populations. Insects serve as an important food source for adults passerines feeding their young. The authors write,

"In each year of our study, the mayfly emergence coincided with either the provisioning of nestlings or with the fledgling period when chicks are still provisioned by adults. Rosy-Finches may well depend on the mayfly subsidy to feed and successfully raise their young during the short, single-brood breeding season and to improve their own body condition."

The authors suggest that the decrease in mayfly abundance could have other negative effect on the rosy finch populations by potentially reducing reproductive output and causing birds to expend greater energy to find insects at other lakes. The authors conclude,

"Given that nonnative fish have been introduced globally to naturally-fishless water bodies, we expect similar responses in other mountain ecosystems around the world...In the Sierra Nevada and elsewhere, fish removal efforts have been implemented with the specific goal of recovering native amphibian populations. While important, we suggest that the goal of fish removal efforts both here and in similar systems, be expanded to include the restoration of linkages between aquatic and terrestrial food webs."

--Reviewed by Rob Goldstein

Epanchin, P., Knapp, R., & Lawler, S. (2009). Nonnative trout impact an alpine-nesting bird by altering aquatic insect subsidies Ecology DOI: 10.1890/09-1974