Science Center Objects

All aquatic invertebrates drift downstream at some point in their life cycle. Invertebrates may drift to find more preferable habitats, to leave the water during their transition from aquatic larvae to terrestrial adults, or accidentally such as when swept off the river bed by a flood. Regardless, when they enter the drift, invertebrates become particularly susceptible to predation by several groups of drift-feeding fish. In Glen, Marble, and Grand Canyons, these fish include rainbow trout (Oncorhynchus mykiss) that sustain the Lees Ferry a Blue Ribbon trout fishery, and the native, federally-endangered humpback chub (Gila cypha). By researching and monitoring invertebrate drift, our group can better understand the health of these fish populations, and how they interact with one another.

Background & Importance

Two USGS researchers in a small boat collecting aquatic insects using a net. Glenn Canyon Dam is in the background.

Sampling aquatic invertebrate drift near Glen Canyon Dam, Arizona (March 4, 2016). (Credit: Dallana Garcia-Peña, USGS. Public domain.)

Aquatic insects are the principal prey for many species of fish, and monitoring their distributions helps scientists and managers better understand stream and river ecosystems. Because all invertebrate species drift downstream at some point in their lives, measuring the concentration of invertebrates in the drift is thus a useful tool for monitoring ecosystem health and food availability for fish. Especially on large rivers where traditional “benthic” methods of sampling the stream bed are impractical, collecting drift samples is an effective way of assessing aquatic invertebrate communities.

In the Colorado River in Glen, Marble, and Grand Canyon where we do most of our research, both the recreationally-important rainbow trout (Oncorhynchus mykiss) and federally-endangered humpback chub (Gila cypha) make their living primarily by feeding on drifting invertebrates. Our research in aquatic invertebrate drift in this ecosystem includes monthly monitoring of drift in the Lees Ferry Blue Ribbon trout fishery and periodic sampling throughout the Grand Canyon in river segments that are important to humpback chub. The data derived from these samples are used in tracking the status and trends of aquatic invertebrates throughout the Colorado River, and in bioenergetics models that are used to estimate how fish populations will be affected by changes in prey availability, water temperatures, and other environmental factors.

General Methods

We use a motorboat equipped with a large boom to sample drift in the Colorado River. A winch is mounted to this boom, with a net and 75-pound weight attached. The net is raised and lowered throughout the water column of the river, at approximately mid-channel, for approximately five minutes. The resulting sample is taken back to the lab, where any collected invertebrates are counted and identified.

Important Results

Two USGS researchers on a small boat in the Colorado River sampling aquatic insects with a net.

Sampling aquatic invertebrate drift on the Colorado River in Glen Canyon, Arizona (March 4, 2016).(Credit: Dallana Garcia-Peña, USGS. Public domain.)

Our research has revealed that the quantity of invertebrates in the drift is directly proportional to the quantity of invertebrates on the river bottom.  Additionally, we have found that the daily fluctuations in discharge associated with hydropower production from Glen Canyon Dam lead to daily fluctuations in drifting invertebrates, with generally higher drift during periods of higher discharge. Incorporating these drift data into bioenergetics models has shed light on how this affects fish populations.  For example, bioenergetics modeling demonstrates that the maximum size of rainbow trout in Lees Ferry is constrained by the overall small size of the invertebrate prey (average size of the invertebrate prey is around 4 millimeters), and by the low overall abundance of invertebrates in drift.