Brook Trout Are Declining Sharply in Shenandoah's Streams, Mainly From Warming

By Jeb Wofford and Evan Childress, National Park Service. First published in Park Science magazine, Volume 38, Number 1, Summer 2024.

The presence of brook trout indicates clean, cold water. Credit: NPS / Tim Lambert.

Land use and pollution were once the brook trout's biggest problems. The latest research suggests that being a cold-water fish in a warming world may be its greatest challenge yet.

For almost 30 years, NPS biological technician David Demarest has surveyed the streams of Shenandoah National Park each summer, capturing and counting brook trout. The park covers just under 200,000 acres, with more than 70 brook trout streams. Recently he began noticing a change. The decline had been gradual and hard to see in some streams, he said, but in the last couple of years it had become obvious: at Hogcamp Branch, one of the best streams, the crew counted fewer than 100 trout at the monitoring site; in another stream, numbers had fallen from hundreds in the 1990s to single digits.

The authors worked with Demarest and U.S. Geological Survey scientists to measure the problem, and published the results in Transactions of the American Fisheries Society.

An early warning system

Brook trout are sensitive to pollution, non-native species, forest loss and excessive heat, so they warn of wider ecological trouble in Appalachian streams: a stream full of them is clean and cold. Many streams in the eastern United States have lost their brook trout to those stressors. In Virginia the fish now live mostly on public land protected from development — but even there, air pollution and climate change can threaten them.

How the fish are counted

Since the early 1990s, park staff and volunteers have captured trout by electrofishing — stunning them with electrical wands and scooping them up with nets — recording their numbers, ages and sizes, and releasing them unharmed. They also measure stream temperature and other physical features, and use maps to find each site's watershed size, elevation and geology.

Staff and volunteers electrofishing on the Rapidan River. Credit: National Park Service.

Using the data from 1996 to 2022, the team built mathematical models relating trout numbers to stream characteristics, which let them estimate population change for every stream in the park, including those not monitored.

What they found

• Widespread decline. Over those 27 years, brook trout fell by at least 50 percent in more than 70 percent of the park's streams, with similar trends for adults and juveniles.
• Local losses. In at least three streams, brook trout disappeared entirely during the study.
• Warmth. Elevation, stream size and underlying geology strongly shaped how fast and in which direction populations changed. Low-elevation streams ran warmer, and warmer water went with declines; air temperatures at the park have risen, and continued warming of streams will likely harm many sensitive populations further.
• Size. Small streams declined more sharply than large ones, perhaps because their limited habitat offers less refuge from warm water or low flow.

Larger streams showed fewer declines than smaller ones like this. Credit: National Park Service.

Recovery in surprising places

Shenandoah's complex geology brought some good news. Acid rain — precipitation made acidic by pollutants from coal-fired power plants and other sources — was greatly reduced by the Clean Air Act Amendments of 1990. Some streams were barely harmed, or recovered quickly, because their bedrock and soils neutralised the acid. In others, with less acid-neutralising bedrock, acidic compounds built up in the soil and keep leaching out, and recovery has been slow.

Surprisingly, it was mainly in those acid-sensitive streams that trout populations were increasing — likely a partial recovery thanks to cleaner air since the 1990s, as rain now causes smaller spikes in acidity. The improvement might also be masking the harm from warmer water.

Measuring a young brook trout. Credit: National Park Service.

Using the results

For anglers the message is plain: pick a Shenandoah stream at random, and it probably holds less than half the brook trout it did 30 years ago. For the park, the study points to the streams most resilient to change — generally larger, higher-elevation streams, and perhaps those recovering from acid rain. Staff can prioritise protecting them from other stressors, such as invasive aquatic species and forest pests (healthy forests shade streams and keep them cool), and the work can show where different fishing rules might better protect sensitive populations.

The future is uncertain. The recovery in some watersheds could be short-lived, or a sign of resilience; brook trout have bounced back from other environmental catastrophes before. But climate projections are not good, and the results point to a difficult future for the fish in Shenandoah — which makes continued research, to find the streams where the outlook is brighter, all the more important.

Sources

• Jeb Wofford (biologist, NPS Mid-Atlantic Inventory and Monitoring Network) and Evan Childress (chief of Natural and Cultural Resources, Shenandoah National Park), "Researchers See Startling Brook Trout Declines in Shenandoah Streams," Park Science 38(1), Summer 2024. https://www.nps.gov/articles/000/researchers-see-startling-brook-trout-declines-in-shenandoah-streams.htm
• The authors thank U.S. Geological Survey scientists Nathaniel Hitt and Benjamin Letcher, and the field staff and volunteers who monitored trout over the decades.
• Rewritten in hubnx's own words.