Metal-tainted trout operating on overload.

Some fish living in water contaminated with metals have efficient means of surviving the dirty environment. They both excrete the pollutants and store them in fat. However, new studies suggest a downside to this acclimation. The results also call into question previous laboratory findings that low-level metal contamination makes fish more tolerant of bigger doses.

Depending on the quality of water where brown trout (Salmo trutta) live, tissues involved in a fish's response to stress vary markedly in appearance, says David O. Norris of the University of Colorado at Boulder. Tissues of fish living in water contaminated with metals show signs of chronic stress, presumably due to the pollution. Moreover, these fish respond poorly to additional pollution and to the stress of confinement, he said in an interview this week.

The studies "have expanded the range at which [pollutants] have deleterious effects," says Cliff H. Summers of the University of South Dakota in Vermillion. "This work was particularly good."

Norris and his colleagues examined brown trout from three stretches of the Eagle River in Colorado to see if they show the hormonal changes typical of a chronically stressed fish. Under stress, fish release corticotropin-releasing hormone (CRH), which triggers the pituitary gland to produce more corticotropin. That hormone, in turn, causes the kidney to increase its release of cortisol, another hormone.

The fish came from one site that had no cadmium or zinc contamination from mining effluent and two sites contaminated to different extents. Fish from the polluted water had high concentrations of the metals in their tissues.

The fish that had lived in the contaminated water appeared to have thicker kidney tissues, more kidney cells, and more nerve cells responsive to CRH-all signs that the animals had suffered chronic stress, Norris says. He reported these results Dec. 28 at the American Society of Zoology meeting in Washington, D.C.

To find out whether the metals, rather than other factors, cause these changes, the team plans to expose uncontaminated fish to metals and then monitor their response to stress.

In another recent study, the scientists examined how brown trout from Eagle River and Clear Creek, also in Colorado, respond to the acute stress of being captured. They measured the concentration of cortisol in the fish's blood 10 minutes after capture and 1 to 3 hours later.

Compared to the fish in cleaner water, the contaminated animals' cortisol increased much more slowly in response to the confinement, he says. The team is now analyzing its data on the fish's other physiological responses to the stress of confinement.

In an earlier, unpublished study, Norris' colleague John D. Woodling of the Colorado Division of Wildlife in Denver examined how fish handle additional pollution. He put brown trout in cages for 96 hours in an area so poisoned by mining effluent that no fish lived there. Fish that had resided in water with mining effluent survived only 28 hours, while residents of cleaner waters survived almost 48 hours, Woodling says.

These studies hold a message for good-hearted anglers who toss their catch back after hooking them. Make sure to fish in clean water, because the stress of getting caught may be too much for contaminated fish, Norris warns.