Salmonflies: A Madison Keystone

Updated: May 23

Pteronarcys californica

The greatest event of the natural world on the Madison River annually is the emergence of the king of all aquatic insects: the salmonfly. But do we appreciate how important they really are, both to the ecological and economic communities? Which begs another question, are we paying attention?

Salmonflies (often exceeding 6 cm in length) are incredibly popular among flyfishers. Spending 2-5 years as larvae living among gravels in the riverbed, they are cued by water temperature in early summer to emerge in spectacular hatches that draw anglers from around the world. Hatches generally last for around a week at any given location, rising and lowering in density.

The most thorough research on salmonflies on the Madison was a Master's thesis by Heidi Anderson at Montana State University in 2018 titled (Environmental Drivers of Salmonfly Ecology in Southwest Montana). In addition to her own research, she was able to utilize citizen science data from a fellow who somehow managed to survey Varney for first emergence date of salmonflies every year since 1977. Pretty incredible. Another study on salmonflies in Colorado last year (2020) is also fascinating ("Quantifying the habitat preferences of the stonefly Pteronarcys californica in Colorado").

Salmonfly emergence on the Madison, as we anglers already know and Heidi documented in detail, are locally synchronized but throughout the entire reach of the Madison River, the hatch is more broadly, asynchronized. For example, the maximum difference in median emergence dates among surveyed sites in 2016 and 2017 averaged 22.5 days across the two years (21 days in 2016 and 24 days in 2017). River wide emergence is considered asynchronous when emergence timing does not overlap among sites on the same river. Hence, when the hatch is on, you may see fishing reports on the upper Madison referencing that the hatch has "moved" upstream to a certain area. And, that's an important distinction from many other rivers where salmonflies are present. For example, on the nearby Gallatin River, just over the Madison Range, Heidi documented that salmonflies are far more synchronized in their hatch throughout the river. Meaning, emergence dates at the bottom of the river are closer to the emergence dates at the upper end of the river, than observed on the upper Madison. This is further exemplified in that emergence of salmonflies on the upper Madison have been documented to last on average 2.25X longer than the Gallatin. This is one of the biggest reasons the upper Madison is such an iconic location for anglers worldwide chasing salmonflies. They're around for quite a while.

This asynchronicity is hugely important to the larger ecosystem as this variable resource pulse can extend the duration of resource availability, and thereby stabilize and boost consumers’ (like birds, snakes, spiders, etc.) seasonal energy intake. Fat cheeseburger bugs served up for a longer period of time.

In addition to being large, iconic, indicative of clean water, and charismatic, drawing people from around the world to the region to experience, the salmonfly indirectly contributes to local economies through eager anglers traveling to the region en masse for the opportunity to cast a dry fly as long as their fingers. The salmonfly also plays a critical ecological role including processing detritus, dominating aquatic subsidies to terrestrial ecosystems, and providing an important carbon resource pulse for over twenty species of aquatic and terrestrial consumers.