Trout seeking cooler waters can be poisoned by metals

When faced with a choice between warmer stream temperatures and low metal exposures or cooler temperatures but increased metal levels in water, trout head toward cooler, more contaminated waters, according to an article in the April 2009 issue of Environmental Toxicology and Chemistry.

Trout generally avoid waters containing metals, which have a range of effects on their abundance and health, as noted in the article, “Trout Density and Health in a Stream with Variable Water Temperatures and Trace Element Concentrations: Does a Cold-Water Source Attract Trout to Increased Metal Exposure?” by David D. Harper, Aida M. Farag, Christer Hogstrand, and Elizabeth MacConnell.

But the researchers found different trout behavior when they studied the Prickly Pear Creek watershed in Montana, where 125 years of hard-rock mining have left increased levels of metals such as zinc, arsenic, and lead in the creek. Although water quality there has improved since reclamation activities began in 1988, zinc levels in the water still exceed U.S. Environmental Protection Agency guidelines.

The authors examined trout behavior from June to August 2001 at three sites along Prickly Pear Creek: the first (PP1) was the reference site, upstream from most hard-rock mining activities; the second (PP3) was about 8 km away and downstream of Prickly Pear’s confluence with the metal-contaminated Spring Creek; and the third (PP5) was 15 km downstream from the Spring Creek confluence, where metal concentrations are more diluted because of input from tributary streams.

The researchers found that average water temperatures at the third site, PP5, were the warmest and frequently exceeded 22ºC, with temperatures at PP1 next highest, never exceeding 21ºC. Water temperatures were coolest at PP3, where they did not exceed 18ºC. Metal concentrations in the water were highest at PP3, followed by PP5 and PP1, according to the study.

However, the researchers also found that salmonid biomass was greatest at PP3, where lower water temperatures during summer help improve habitat quality. Most trout samples at this site also had higher concentrations of metals than samples from the other two sites.

Trout generally avoid waters containing dangerous metals—such as those found at PP3—but they also thrive in cooler waters. “When fish are exposed to multiple stressors, they must respond to the most acute threat,” the researchers say, which for trout living in Prickly Pear Creek appears to be higher water temperatures.

“Our results suggest that the preference of trout for cool water temperatures may supersede behaviors to avoid heavy metals,” the authors conclude.

To read the entire article, Trout Density and Health in a Stream with Variable Water Temperatures and Trace Element Concentrations: Does a Cold-Water Source Attract Trout to Increased Metal Exposure? (Vol. 28(4):800-808), visit  <http://www.allenpress.com/pdf/entc/ENTC_28.4_800_808.pdf> http://www.allenpress.com/pdf/entc/ENTC_28.4_800_808.pdf