Long after the high is gone, illegal drugs linger in the environment. Cocaine and its breakdown products, for example, are found in rivers and lakes worldwide. And like children living with secondhand smoke, wildlife in polluted waters—including tiny crustaceans, fish, and even sharks—can’t help but take up these drugs.
Less is known about the impact. In laboratory studies, water fleas exposed to cocaine swim faster and crayfish will venture behind their hiding places, a risky behavior in nature. Now, a research team has conducted the first experiment with fish in the wild. The findings, published today in Current Biology, show salmon exposed to cocaine and its main byproduct roam more widely. “It is an important and very interesting study,” says Mark Servos, an ecotoxicologist at the University of Waterloo who was not involved.
To mimic the effect of living in polluted water, the researchers surgically implanted small devices into 2-year-old Atlantic salmon from a hatchery that slowly release chemicals at a dose equivalent to what fish would experience in water with relatively high amounts of drugs.
One group of 35 fish received implants containing cocaine. Another group got implants with benzoylecgonine, the main breakdown product of cocaine and often found in water bodies in greater amounts. The third group received control implants with no chemicals. All the fish were outfitted with small tags so the research team, led by behavioral ecologists Jack Brand and Michael Bertram of the Swedish University of Agricultural Sciences, could track their movements over 2 months in Lake Vättern.
In general, young hatchery salmon tend to explore more after they are released. As they learn about their new environment, they move around less and less. This is what the researchers observed over the 2 months—with a twist. The salmon with drug implants retained more of their adventuresome spirit. Compared with the controls, fish exposed to the metabolite benzoylecgonine swam as much as 1.9 times farther each week.
By the end of the experiment, the control fish had largely settled about 20 kilometers from the release site in the south of the lake. Cocaine-exposed fish were living somewhat farther afield, and those exposed to benzoylecgonine had dispersed roughly 32 kilometers from the release point.
The larger effect from the metabolite compared with cocaine itself is consistent with previous lab research, which found that benzoylecgonine persists longer in fish. Exactly how it alters behavior in fish in the wild isn’t known.
What these altered behaviors mean for salmon in the long term isn’t clear. It’s possible they affect how they hunt and are hunted, the researchers say, but that will take more study. Another question is whether the findings hold true for wild-born fish; hatchery-raised salmon are known to behave less cautiously, for example. What is certain is that drugs and other pollutants deserve a closer look. Servos says: “We need to carefully understand and manage all of the diverse chemicals society uses that can end up in our waterways.”