Our Drugs Make Fish Flounder
2012-11-17 09:07:07
Scientists have known for years that human medications, from
anti-inflammatories to the hormones in birth-control pills, are ending up in
waterways and affecting fish and other aquatic organisms. But researchers are
only beginning to compile the many effects that those drugs seem to be having.
And it isn't good news for the fish.
One such drug, fluoxetine, is the active ingredient in the antidepressant
Prozac. Like some other pharmaceuticals, fluoxetine is excreted in the urine of
people taking it, and reaches lakes and waterways through sewage-treatment
plants that are unequipped to remove it.
To investigate the effects of fluoxetine, researchers have turned to a common US
freshwater fish species called the fathead minnow (Pimephales promelas).
Normally, fathead minnows show a complex mating behavior, with males building
the nests that females visit to lay their eggs. Once the eggs are laid and
fertilized, the males tend to them by cleaning away any fungus or dead eggs.
But when fluoxetine is added to the water, all of this changes, said Rebecca
Klaper, an ecologist at the University of Wisconsin-Milwaukee's Great Lakes
Water Institute. Klaper presented her results this week at the 2012 meeting of
the North American division of the Society of Environmental Toxicology and
Chemistry in Long Beach, California.
Female fathead minnows seem to be unaffected by the chemical, but at
concentrations of fluoxetine that are roughly comparable to the highest levels
documented in fresh water, male minnows start to spend more time building their
nests. When the dose is increased tenfold, the males "become obsessive, to the
point they're ignoring the females", Klaper said.
When fluoxetine concentrations are increased yet again, fathead reproduction
completely halts. "The males start killing the females," she said. Klaper also
noted that if females are introduced a month after males are exposed to the
chemical, the males no longer show this aggressive behavior, but the females
still don't lay any eggs. "Something happens in that time," she said.
Easy prey
Reproductive behavior isn't the only thing that can be affected by trace
pharmaceuticals. At the same symposium, Dan Rearick, an aquatic toxicologist
from St Cloud State University in Minnesota, reported that a chemical found in
birth-control pills, 17-β-estradiol, reduced the ability of fathead minnow
larvae to elude predators.
After exposing the larvae to estradiol, Rearick then subjected them to sudden
vibrations, similar to those produced by approaching predators. Using high-speed
videos, he measured how long it took the minnows to curve their bodies into a C
shape أ¢â‚¬â€ an escape behavior known as a C-start. "They are preparing to dart
away," he explained. He found that, even at environmental levels of estradiol
(20 or 100 nanograms per liter), the minnows' reaction time was significantly
slowed compared to control larvae that had not been exposed to estradiol.
In a second experiment, he raised hundreds of estradiol-exposed and control
larvae, and repeatedly put ten larvae from both groups together in a tank with a
predator, a bluegill sunfish (Lepomis macrochirus). When half of the larvae in
each experiment had been eaten, Rearick looked at which types of larvae were
left.
The result agreed with the C-start experiments: of the surviving fish, only
about 45% were from the estradiol-exposed group, with the majority of survivors
coming from the control group (55%).
Population crash
That difference might not sound like much, but using a multi-generation
population biology model, Rearick found that it would be enough to produce a
rapid population crash in the estradiol-exposed fish. Even if the fish weren't
as badly affected, there would still be a slow, steady decline, he found. "There
is probably a need for concern," he said.
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