Gunnison River Insects

By Merne Judson
The new "Prez" asked me to write an article for the newsletter, and during our conversation a hatch chart for the Gunnison River was mentioned. This reminded me of an article I had read over 10 years ago written by Rick Hafele. The article was about a study done by Dr. Ken Stewart, one of the leading Aquatic Entomologists in the world. Dr. Stewart, along with Randall Fullmer, did a paper called "The Food Habits of Stoneflies (Plecoptera) in the Gunnison River." In the process of doing a study a full assortment of aquatic life was collected. I had the pleasure of spending five days with Ken one summer and learned even more from him concerning aquatic insects.

You may wonder why an angler would want to know about the contents of a Stonefly's stomach. The study required collecting stonefly nymphs at different seasons, then comparing their stomach contents with the number and types of other insects and plants in the stream. As a result, a detailed list that peaked my interest.

The study was conducted on the upper Gunnison River at the Lost Canyon Resort, down river from Almont, Colorado. Stoneflies for the study and other aquatic insects were collected with kick nets. By scraping the surface of the rock, samples of the perphyton (the thin layer of diatoms growing on rock bttoms) and other aquatic insects were also collected. The first samples were taken in December, and the remaining samples collected monthly, from May through October of the following year. Sampling was done twice a day, once at mid afternoon and once midway between sunset and midnight. I feel this studies greatest value to a fly fisherman is the detailed number of each aquatic insect species present at the study site from May through October.

A total of 38 different taxons (scientific classifications) were collected during the study. The most abundant taxa and how their counts changed through the study period are summarized in the attached table. The numbers listed are only for nymphs, no adult insects were collected in the study. By looking for the months when the lowest counts of nymphs appear in the river, you can deduce these periods as "times of emergence". The months with the highest number of nymphs collected indicate that the eggs of that taxon had recently hatched.

Chironimids dominated the fauna in all six months of the study, showing their importance to the fish as a constant food source. Typical of most Western stream, the Baetis was second in most months. This is an important consideration to the angler. Caddis, while not dominant, was common in most months. The order Trichoptera (caddis flies) had the greatest number of "common taxa then any other orders." A large number of Oligochaetes (aquatic worm) were also present in the summer months. Although, not readily available to fish because they live under bottom debris, during high water and after storms they become a major source of food to the fish.

Let's not forget the stonefly, as this was the original intent in the study. A summary of their food habits follows.

Isoperla fulva (Little yellow Stone) - The diet of this small stonefly consists mainly of chironimid larvae. The importance of the chironimid increased as the nymph matured. Different types of algae were also important in the diet of the young nymph. Mature nymphs were collected in May and June with emergence in July, August, and September.

Classenia sabulosa - This is a common but unusual Western stonefly closely related to the Golden Stone (both belonging to the family - Perlidae) and are similar in appearance. A unique characteristic of this species is that most adults lack fully developed wings. Adults emerge in August and September, when they can be seen crawling along banks unable to fly. These nymphs require two years to reach maturity. During their first year they ate mostly chironimid larvae. During their second year they ate caddis larvae, mayfly nymphs, and chironimids.

Herperoperla pacifica (Golden Stone) - This stonefly ranks high in importance to anglers in Western rivers. Emergence typically occurs in June or July. The nymph requires two to three years to mature. Like Classenia sabulosa, these nymphs are predators. During the first year nymphs fed mostly on chironimid larvae. In the second year they shifted their eating habits to large caddis larvae, but chironimid larvae were still important, particularly prior to emergence.

Chloroperlidae (Little-green Stone) - This family contained a number of genera difficult to identify in their nymph stage, so they were not separated for the study. Most of the species emerged in late June through July and August. Only a single year is required for this nymph to mature. Young nymphs fed mainly on algae and detritus. During the last two to three months of development, however, chironimid larvae became their dominant food.

This study clearly points out the importance of the chironimid as a constant food source for both the fish and stonefly. Without midges, fishing might be less frustrating, but it would also effect and lessen the numbers of stoneflies. The end result would equate to fewer fish.

My hope is this. That this information, along with your own knowledge of this area will help you assemble your own "Hatch Chart" for the Gunnison River and surrounding area. My hatch chart uses this, and other information I've collected by keeping logbooks of my fishing experiences over the last 20 years.

I'll show you mine, if you'll show me yours.