As I Would Learn Later, These Salmonflies Relay an Important Message About the Health Of

Riverine Giants

By Adam Clause

We have been seeing them all day. Denise finds the first one, stuck to a boulder at Meral’s Pool. More and more appear as we travel downriver. Virtually every riverside willow, skeleton-like and leafless in April, is adorned with one or two. A mass emergence is clearly in progress.

The objects in question are the shed larval husks, or exuvia, of what Rob and Nick ably identify as giant stoneflies, or salmonflies, in the family Pteronarcyidae. At least two inches long, these insects are the largest aquatic invertebrates in California (Merritt and Cummins 2008). Will we be fortunate enough to observe living adults, I wonder?

These salmonflies relay an important message about the health of the middle Tuolumne system. As larvae, salmonflies are members of the shredder macroinvertebrate guild, feeding on coarse particulate organic matter (CPOM). This includes fallen leaves and other plant litter. They are also extremely sensitive to environmental disturbance. On a scale of 0 to 10 (with 0 being the most sensitive) Pteronarcyid salmonflies are garnered the lowest possible tolerance value (Merritt and Cummins 2008). Given this biology, the abundance of salmonflies on the mainstem is rather surprising. The Tuolumne’s razor-thin, depauperate riparian zone limits the litter-fall that these insects depend on for food. The anthropogenic disturbance of summer flow ramping would also seem to be prohibitive to the species, due to its sensitivity. But perhaps, in our zeal, we have underestimated the wild heart of this river. As the salmonfly flush demonstrates, the mainstem remains quite pristine.

Back on the river, our party is standing onshore near the Clavey river confluence, sharing and discussing the exuvia among ourselves, when an unannounced visitor abruptly joins us. On my sleeve appears the desired adult salmonfly, seemingly having materializing out of thin air. It seems just as surprised (but perhaps not as delighted) as we are. Its delicate coloration and striking silvery wings, coupled with its imposing size (over two inches long), made this insect a major biological highlight. A winged messenger from the river, both gorgeous and instructive—can it get any better?

Salmonfly exuvia. Photo by Adam Clause. Salmonfly adult. Photo by Denise de Carion.

Citation: Merritt, R. W., K. W. Cummins and M. B. Berg. 2008. An introduction to the aquatic insects of North America, 4th ed. Kendall Hunt, Dubuque, Iowa.