Montana Field Guide

This is the famous Salmonfly, and is the largest species of all mayflies, stoneflies or caddisflies hatching on any western river, up to 3 inches long. In late-May to early July, adults are hatching and providing trout fisherman with exciting dry-fly fishing on the surface or underwater with large artificial nymphs. Adults are fairly weak fliers and are very conspicuous by their size and erratic flight over the river making them easy pickings for feeding trout or insectivrous birds. The Giant Salmonfly range in Montana only overlaps with one other potentially confusing species; the American Salmonfly, Pteronarcys dorsata which has been reported to occur in the Smith and Missouri River basins. For an in depth report on the assessment of this species in Montana go online: http://mtnhp.org/reports/ZOO_MT_Salmonfly_2010.pdf

In late-May to early July, nymphs are crawling out of the water to the adjacent riparian vegetations and the adults are hatching out. June is usually prime emergence time, but can emerge much earlier (late-April) in warmer streams (Rockwell and Newell 2009).

Salmonflies are easy to identify mostly by their tremendous size; these stoneflies (Order Plecoptera: Family Pteronarcyidae) often measure nearly three inches in length. As adults, they have a bright orange or red band behind the head and the underside of abdomen with broad wings containing dark veins

This species is not known to make migration flights, but generally flies in an upstream direction.

Salmonfly nymphs live on the bottom, crawling around on cobbles and feeding on large organic materials (leaves) in the spaces between rocks for three to sometimes four years. They require well-oxygenated water, so they thrive in swift, bouldery, riffly stretches of the river; narrow canyon reaches such as Yankee Jim Canyon of the Yellowstone, Alberton Gorge of the Clark Fork River, Big Hole Canyon between Glen and Wise River or Bear Trap Canyon of the Madison are ideal habitat.

Pteronarcys nymphs generally are considered shredders as a functional feeding group, and feed on large particular organic and leaf materials that have collected in debris dams or behind boulders or logjams.

Pteronarcys nymphs grow through many instars (12 - 20). The length of the life cycle is 3 to 4 years depending on elevation and degree days (water, temperature and air)(Baumann et al. 1977). Mature nymphs migrate to shore in the spring and climb out of the water (mostly at night) before the final molt. Adults live for a few days to few weeks among shoreline vegetation or rocks and they feed little, if at all. Males attract mates by drumming rocks or wood, and mate while on the wing. Females lay the eggs on or above the water. The eggs separate and sink to the bottom to begin the next generation. Multiple cohorts may exist in any given river allowing a salmonfly hatch to occur every year, but some cohorts are stronger than others.

The biggest threats to the salmonfly are attributable to the loss of habitat through siltation, sediment filling interstitial spaces in the cobbles or increasing water temperatures beyond the species thermal limits.