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SCIENCESPOT - CONFRONTING COPPER
While California's salmon no longer run a gauntlet of hungry grizzly bears, they face more subtle dangers. According to recent research by NOAA Fisheries ecotoxicologist Nathaniel Scholz and his colleagues, salmon, particularly those in urban streams, are affected by chemicals that could compromise their ability to dodge obstacles, evade predators, capture prey, find mates, and locate their natal rivers at spawning time.
Although Scholz and his team have documented behavioral effects from diazinon, their work in the last few years has focused on copper. The element has been long been known to disrupt the olfactory systems of salmonids and other fish, but no one had linked this to actual fish behavior. Dissolved copper from worn brake pads, roofing materials, treated wood, algicides, and fungicides is pervasive in urban runoff. Post-storm copper levels in northern California streams have ranged from 3.4 to 64.5 parts per billion (ppb), with an average of 15.8. Salmon avoid point sources of copper but are vulnerable to the diffuse copper in these storm pulses.
These fish have supersensitive olfactory systems, comparable to those of dogs. "The neurons in a fish's nose are in direct contact with the water, so the nose would be the first part in contact with the contaminants," says Oregon State University graduate student Jason Sandahl, who collaborated with Scholz on an article appearing in Environmental Science & Technology this spring. Earlier work demonstrated that exposure to dissolved copper levels at or above 25 ppb could kill salmon olfactory nerve cells, which could take weeks to regenerate. At lower levels, down to 5 ppb, the neurons don't die but become temporarily unresponsive.
Such doses aren't lethal to salmon but may dramatically reduce their chances of survival In the 1930s Karl von Frisch, best known for his bee communication studies, discovered that European minnows release an alarm pheromone-von Frisch called it "Schreckstoff"-when attacked by a predator. It's a mechanical process, triggered by broken skin. Other fish of the same species take evasive action when they detect the chemical. Most widespread in the large order of fish that includes carp, catfish, and piranhas, chemical alarm signals were confirmed in rainbow and brook trout in the 1990s, although the nature of the mechanism in salmonids remains unclear.
Scholz and Sandahl, working with young coho salmon, found that the fish respond to alarm pheromones from a piece of coho skin smaller than a grain of rice in 100 liters of water. When skin solution is introduced to a clean tank, the salmon, which had been swimming actively, drops to the bottom and hovers in place. But after three hours' exposure to copper at a concentration of ten ppb, there's no reaction-a good way to become someone's dinner.
The nose may not be the only sensory system at risk. Teleosts (bony fishes) rely on their lateral line-clusters of mechanosensory neurons strung along their sides-to detect vibrations in the water. This ability can be critical for shoaling, orientation to stream flow, predator and obstacle avoidance, and prey capture. "The toxicity of copper to the lateral line is more similar to the nose than to the classical pathway of toxicity mediated by the gills," says Scholz. In zebra fish-a common lab surrogate- dissolved copper destroys the lateral line's specialized neurons, as it does olfactory neurons.
With funding from the EPA's Science to Achieve Results program, Scholz and a graduate student are now investigating how water chemistry interacts with copper to affect olfactory function. Studies of olfactorybased homing ability and comparisons of hatchery-reared and wild fish are planned. Copper, of course, is only one ingredient in the chemical brew of urban stormwater, along with other metals, pesticides, and pharmaceuticals. "Our focus has been on known compounds and potential interactions," Scholz explains.
Meanwhile, the research raises regulatory issues. "Of all the chemicals we have looked at, this effect was clearly happening at levels well below the current copper standards for water quality," says OSU toxicologist Jeff Jenkins, another co-author of the recent article. "It raises the question of whether the current standards are as protective as we thought."
CONTACT: Nathaniel Scholz, Nathaniel.Scholz@noaa.gov. JE
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