Salmonflies, Montana and water quality

The famed, annual salmon fly hatch on our Big Hole River is wrapped up for another year. If you don’t know what I’m talking about, well, it’s quite an event. Naturally speaking, the salmon fly hatch is that time of year when this giant stonefly species, Pteronarcys californica, which is three-plus inches long, crawls out of the water, hatches into an adult, mates and then returns to the water’s surface to lay its eggs, thus continuing the cycle. The hatch only occurs on a handful of Montana’s rivers, making it very special, both in nature and to the fly fisherman.

The entire life cycle of the giant salmonfly takes three to four years. The nymph hatches in the water from an egg. During its relatively long life as an aquatic creature, the nymph lives in between and beneath the large cobbles and boulders of only a few streams in Montana. They feed on detritus – pieces of plant and organic matter decomposing in the stream and live the majority of their lives under the water. Then, beginning in late May and continuing into July, depending on rivers, flows and seasonal water temperatures, the hatch happens.

The nymphs are ready to hatch and begin making their ways from the middle of river’s bottom to the shores. Once at the shoreline, the nymphs crawl out of the water and climb up on rocks and trees, fence posts and bridges – anything stationary and out of the water – to dry and break out of their hard exoskeletons. If you’ve ever been lucky enough to witness this event, it is like nature’s version of a science fiction thriller, “attack of the giant salmonflies.”

The adults, which are just as large, emerge from the skeletons and are characterized by a bright, salmon-colored orange on their necks and underbellies, giving them their name. Once hatched, they spend a few days mating and then return to the river’s surface to lay their eggs and eventually die, a lot of the time from being eaten by fish or birds.

From the time the nymphs make their way to the shorelines to when the adults return to the river to lay eggs takes a couple of weeks. This period creates a literal feeding frenzy for fish and birds alike. It also creates a fishing frenzy for fly anglers and outfitters, as the “hatch” of wader-clad fishermen and boats almost rivals that of the bugs. This latter frenzy is an absolute blast to partake in and also provides a big economic boost to the areas lucky enough to have a hatch.

Why aren’t there salmonflies on every river?

Giant salmonflies can only live in the cleanest and coldest of moving water and require cobble-bottomed streambed homes in order to survive. And because the insects have such a long life (again, three to four years) the water needs to be clean and cold all of the time, not just for a little bit here and there. Take the requirements of clean and cold and couple it to the cobble-bottomed habitat, and there are very few rivers in Montana that produce a salmonfly hatch significant enough to draw a crowd.

The Big Hole, Rock Creek, the Blackfoot, the Yellowstone just outside of the park and the Madison are the only rivers in Montana that have hatches worthy enough to garner destination status from anglers. The rural economies of these areas partially depend upon this annual event, from the guides and outfitters to the lodging, food and beverage providers.  This is one more important reason for us to pay attention to our impacts as humans on our waterways. Climate change will take its slow, methodical toll. But removal of riparian vegetation through development, or sediment loading from erosion or improper road placement and maintenance also damage the giant stoneflies’ habitat.

 There are many “old timers” who will already tell you that “the hatch ain’t what it used to be.” That characteristic of remembrance does tend to come with a certain age, but there is likely to be some scientific truth to it as well. All we can do about it is learn more and do what we can to preserve the water quality in our rivers so that generations of Montanans to come can see the salmonfly hatch for themselves. Fish on!

Photo cutline: They’re big, but they don’t bite! Giant salmonflies, which are a distinct species of stonefly (Plecoptera) are a fly fisherman’s dream. On the rivers where they hatch (see exoskeleton on the bottom), they bring feeding fish to the surface, and droves of anglers to try their luck. (Photo credit to Montana Fish, Wildlife and Parks, Robert Newell photographer.)