In this post Justin Pomeranz and David Walters discuss their recent paper ‘Aquatic pollution increases use of terrestrial prey subsidies by stream fish

Iron precipitates visible in this highly impacted stream. Photo: Peter Leipzig-Scott.
Iron precipitates visible in this highly impacted stream. Photo: Peter Leipzig-Scott.

Have you ever walked next to a high elevation, Rocky Mountain stream? Picked your way through downed trees, pushing your way through thick willows, listening to the water pour and bounce over rocks and settle into plunge pools? Have you ever fished one of these streams? Gazing through the clear water, on the lookout for a big brown, rainbow, or better yet, one of our native species of cutthroat trout?

In these remote, scenic places, it’s easy to believe that you are in pristine conditions, far from the impacts of humanity and relatively untouched for millennia. Unfortunately, this is often not the case. Many of the streams and rivers in the Rocky Mountains of the United States are polluted with acidic runoff and trace metals. Some of the sources of this pollution are natural, caused by the action of wind, water, and air reacting with minerals and metals uplifted to the Earth’s crust during orogeny, or mountain building events. Other sources are from current and past mining activities, caused by humans looking to harvest and utilize those same minerals brought up during those chaotic and Earth-changing events.

Conceptual model of fish diet in a pristine stream. Size of arrow represents relative importance of two prey resources. Illustrations: Freshwaters Illustrated/USGS.
Conceptual model of fish diet in a pristine stream. Size of arrow represents relative importance of two prey resources. Illustrations: Freshwaters Illustrated/USGS.

The recent spill of 3 million gallons of highly toxic water in the Animas River, accidentally released by the US EPA near the town of Durango, Colorado, is a striking visual example of the threat that many of our waterways face. Despite the dramatic image of tang-colored water flowing through downtown Durango, clear water in many streams is not indicative of clean water. In the contiguous US, there are over 200,000 abandoned mines. In Colorado alone, there are over 5000 abandoned mine sites affecting >13,000 km of streams. Many of these streams don’t “look” polluted, and can be acidic, and contain toxic levels of trace metals.

Acid mine drainage, along with elevated levels of metals, can reduce insect abundances, and eliminate sensitive species such as mayflies, stoneflies, and caddisflies, the preferred meal for trout living in these streams. In highly impacted streams, the water can be the tang-color made famous in the front page photos from the recent Animas River spill, and the river bed can be coated in rust from iron particles precipitating out of the water column. In these conditions, you’d be hard pressed to find anything at all living in the stream. But what about the low to moderately impacted streams? What about the streams that have fish, but have reduced prey resources available to them? What are these fish eating?

In order to answer this question, we surveyed fish diets and the relative availabilities of prey resources in 16 streams, across a gradient of metal pollution in the Colorado Rocky Mountains. In short, we found that aquatic prey resources declined across the gradient, while terrestrial insect inputs to streams remained relatively constant. The fish diets reflected these relative availabilities, consuming more terrestrial insects as metal impacts increased – to a point. At the high end of our gradient, fish were eliminated, likely succumbing to the chronic effects of living in a toxic environment.

Conceptual model of fish diet in a metal impacted stream. Metals reduce availability of aquatic insects (no symbol). Size of arrows indicate relative importance of prey resources. Illustrations: Freshwaters Illustrated/USGS.
Conceptual model of fish diet in a metal impacted stream. Metals reduce availability of aquatic insects (no symbol). Size of arrows indicate relative importance of prey resources. Illustrations: Freshwaters Illustrated/USGS.

However, our findings have very important management implications, particularly for fisheries biologists trying to maintain a productive fishery in low to moderately impacted watersheds. A healthy, thick zone of riparian vegetation with a high volume of canopy overhanging the stream increases terrestrial insect inputs. By maintaining a healthy riparian zone, it may be possible to alleviate the stress of reduced food resources to fish in impacted watersheds. This is an important finding, as many streams, regardless if the metal and acid pollution is from natural or human activities, will never be fully “clean”.

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