This article first appeared in the Charlotte Observer on Feb. 27, 2012.
A spot of good news is surfacing for North Carolina’s brook trout, and the anglers who hold their speckled brookies so dear.
Not so long ago, scientists forecast that much of what remained of eastern brook trout habitat would be severely affected by climate change. In fact, it was thought the only native trout in the Eastern United States might vanish from large parts of its southern range, leaving only a few populations concentrated mostly in western North Carolina.
But a new study in progress across seven Southeastern states has found reason to believe that many cold-water streams – those found at elevations where brook trout love to linger – may be less vulnerable to warming temperatures than previously forecast.
The study’s investigator, Andrew Dolloff, is team leader for the cold-water fisheries research unit of the Forest Service’s Southern Research Station in Blacksburg, Va. Brook trout are very sensitive to water temperatures, preferring to live in the cleanest cold water streams below a critical threshold of 69.8 degrees.
“They can survive above this threshold, but making a living is much, much more difficult for them,” Dolloff said.
Brook trout are also one of the most widely spread temperature-sensitive aquatic species in the East. This makes them good indicators for understanding the effects of climate change. “If a trout can’t live somewhere anymore, there’s going to be a whole bunch of other species that can’t live there either,” Dolloff said.
In 2006, Trout Unlimited released a report that showed a 20 percent reduction in the historic range of brook trout. Trout Unlimited is a not-for-profit organization which protects, conserves and works to restore cold-water streams and rivers in North America. The report also showed that brookies were greatly reduced in an additional 47 percent of their historic range. This led some to question what the future might hold, given predictions of a warming world.
An answer to this question came about, also in 2006, when scientists attempted to scale down findings from a continental-scale model of climate change. The model assumed a correlation between water and air temperatures of about 0.8 degrees. This meant that for every one-degree change in air temperature, they modeled a 0.8-degree change in water temperature.
Most climate-change models predict about a four degree increase in air temperature in the next 100 years, so the study predicted a corresponding 3.2 degree increase in cold-water stream temperatures over the same period. This resulted in a dire forecast that the brook trout’s remaining habitat would contract radically, leaving intact less than half of the habitat that remained.
“It caused a lot of consternation, and rightly so,” Dolloff said.
Dire forecast prompted questions
When Dolloff read the study, questions rolled through his mind. Was the influence of elevation upon water temperature really as straightforward as the authors had assumed, he wondered. What about the effect of springs upon cold-water streams at elevation? How might the presence of a forest canopy, or a nearby urban area, affect a stream’s coldness? What about streams on north-facing slopes, which don’t absorb as much sunshine during the day?
If any of these factors influenced stream temperatures, they might have a strong enough effect to alter the assumed 0.8-degree ratio used in the 2006 study. Dolloff initiated a pilot study in 2009 with the goal of answering his questions and refining the exact nature of the relationship between cold water streams and air temperatures. The pilot study used just 50 sites in Virginia, where Dolloff is associated with Virginia Tech, and where brook trout were present.
At each site, an instrument called a thermographer recorded air and water temperatures every 30 minutes for an entire year. While Dolloff expected to discover a small change from the 0.8 temperature ratio, he was stunned when the pilot study results halved this ratio. His data revealed that the temperature ratio ranged consistently between 0.3 to 0.5.
Such a big difference means that existing trout habitat will be more resilient to climate change than originally predicted. The pilot study results galvanized the Southern Research Station, which approved funding to increase the scale of Dolloff’s study to seven states: Maryland, West Virginia, Virginia, Tennessee, North Carolina, South Carolina, and Georgia. Today, the study includes 204 stream sites, and nearly half of these are nestled in Western North Carolina.
Air temp may not always affect water
Dolloff also discovered that in some sites, water and air temperatures are not coupled at all. “There have been areas where air temperature didn’t even matter; the water pretty much stayed within a certain range,” he said. In these cases, groundwater, usually a spring, keeps the stream temperature fairly constant year-round. “We love finding a groundwater influence,” Dolloff said, chuckling.
Though his expanded study still has two years to go, the first year of data from 2011 confirmed the pilot study results. This gave Dolloff the confidence to state that trout habitat is probably not as vulnerable to climate change as the 2006 study claimed. “I’m reasonably confident saying that there will probably be some range contractions in the future, but I don’t think we’ll completely lose brook trout,” Dolloff said. “This is really good news if you are a trout, or an angler.”
He concedes that other threats such as urbanization, habitat fragmentation from culverts and dams, and pollution still lurk in brook trout’s future, but says: “It likely won’t be climate change that does them in.”
Damon Hearne, the southeastern conservation coordinator for Trout Unlimited, said Dolloff’s study will help his organization to manage and restore streams in the Southeast.
“This new information doesn’t mean we can say, ‘Whew!’ and walk away from conserving trout,” Hearn warned. “It’s important to remember that climate change will not only manifest as a steady rise in air temperature. We’re also going to see a change in flooding and snowpack release regimes, and that will affect trout habitat too. Even if the air-water temperature ratios are smaller, there are still areas that will be lost, and fish on the edges of the range may still be pushed to higher elevations.”