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Public Radio's Environmental News Magazine (follow us on Google News)

Mutant Fish

Air Date: Week of

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A harbor off of Cape Cod, Massachusetts is so polluted with PCBs, it’s been declared a Superfund site. That’s not unusual in this day and age. But what is extraordinary is the fact that one type of fish in the harbor has mutated in response to the pollution.


CURWOOD: Beneath the waters of New Bedford Harbor in Massachusetts lies sediment highly contaminated with PCBs. PCBs, or polychlorinated biphenyls, were used in factories that once operated near the harbor. From the 1940s to the 1970s these chemicals were dumped into the water. The harbor is now a Superfund site with PCB levels more than four times greater than what is considered safe. One species of fish is thriving there despite long-term exposure to these toxic chemicals, but, as Tracy Hampton of member station WCAI reports, that's not necessarily good news.

HAMPTON: Mark Hahn of the Woods Hole Oceanographic Institution enjoys showing off his collection of killifish. His lab is filled with large fish tanks. In each one there's a different group of fish the biologists are studying.


HAHN: This is Fundulus heteroclitis, which is the Atlantic killifish, sometimes called the mummichog. And when the males are mature they have this beautiful yellow color that you might see on some of these guys, whereas the females are more of a dull, but still a pretty, more of a dull white.

HAMPTON: Hahn gets his killifish from nearby estuaries and marshes and he brings them back to his lab where he studies their genetic make-up. He's interested in discovering how these four to five inch fish, as well as other vertebrates, are affected by pollution.

HAHN: It's a very interesting ecological question: What happens when animals are exposed for many generations to very high levels of contaminants? Obviously, one thing that could happen is the population could be eliminated. But another thing that might happen is that the population is able to adapt in some way, through genetic changes, adapt to the high levels of pollution there.

HAMPTON: And that's just what Hahn has found at the New Bedford Harbor. There are plenty of killifish living there, and they seem to be eating and reproducing without any trouble. This shouldn't be happening, though. Other fish died once the waters in the harbor became so polluted. Hahn wondered why the killifish could survive, so, after he took a closer look, he found something unusual. Before we hear about that though, let's step back for a moment and look at what happens when a normal fish is exposed to harmful chemicals. In that case, the body steps up production of certain enzymes, in order to break down pollutants.

HAHN: Now, when the PCBs are broken down, some of the breakdown products are less toxic, so that's beneficial for the fish. But other breakdown products are more toxic. So in a normal fish we have this balance between production of less toxic metabolites and more toxic metabolites.

HAMPTON: The killifish in the New Bedford Harbor have somehow managed to turn off this breakdown response entirely.

HAHN: They have shut down this pathway that's involved in all of these events and, for reasons that aren't yet clear, on balance, that leads to fish that are more resistant to the toxicity.

HAMPTON: Hahn theorizes that, since these killifish don't respond to PCB exposure in the normal way, meaning they never step up the enzyme production, they make fewer of the PCB breakdown products. Remember, some of those breakdown products are even more toxic than the PCBs themselves. And this may be the reason they can survive in this poisoned environment. But, just because the killifish have been able to adapt, that doesn't mean they're perfectly healthy. Sarah Cohen is a geneticist at Harvard University who works with Hahn. She says these fish may be even more susceptible to the toxic effects of other chemicals.

COHEN: There's a cost to that kind of alteration, and it can be quite a severe cost. For example, we see that, if there's other kinds of dangerous compounds in the environment, if the organism has shut down that pathway in response to heavy PCB loads, then it can't metabolize other harmful compounds that are present in the environment. So there's really a tradeoff to adapting to these very extreme situations. You can't have it all.

HAMPTON: Another cost is the effect these fish are having on other animals after they are consumed. Now, while normal fish do break down PCBs, some of the PCBs in their body remain intact, so some toxins accumulate in their tissues as well. Of course, when these fish are eaten, these low levels of PCBs are passed onto other animals. But the normal fish can only tolerate so many PCBs before they would be killed by them. So there's an upper limit to the amount of PCBs they can build up and pass on. But, with the New Bedford fish, this isn't the case. Again, Mark Hahn.

HAHN: With these resistant fish in New Bedford, they're able to build up higher concentrations of PCBs than a normal fish would be able to accumulate. Therefore, when these resistant fish are eaten by something else, by a bird or by a larger fish, those higher concentrates of PCBs are then available to the predator. The concentration of PCBs in the killifish in New Bedford Harbor that we have measured and that other people have measured are somewhere around 10,000 times higher than what we see in clean sites nearby.

HAMPTON: So far, Hahn says, reproductive problems in elevated death rates have been found in some birds, such as terns, that feed on killifish. Other fish, as well as birds at other PCB Superfund sites, have developed similar adaptations. Cohen and Hahn are also interested in seeing how their fish studies can apply to other areas of research, including how humans vary in their sensitivity to chemicals.

HAHN: For example, by studying the sensitive and resistant populations of fish, we can perhaps learn something about how the individual humans might differ in their sensitivity. That is, we're using the fish as models of the two ends of the spectrum of sensitivity.

HAMPTON: These researchers are also curious about what will happen to these fish when the New Bedford Harbor is finally cleaned up, a process the Environmental Protection Agency says will take years. The question is, are they so well adapted to their polluted environment, they'll no longer be able to live in a healthy one. For Living on Earth, I'm Tracy Hampton, on Cape Cod, Massachusetts.

[Quabais Reed Ghazala, "Silence the Tongues of Prophecy," GRAVIKORDS, WHIRLIES & PYROPHONES (Ellipsis- 1996)]



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