Investigating Fish Contamination Leads to Questions About Genetics
The benefits of seafood are well known. Omega 3s from fish are good for metabolism, while fish oil is thought to help with inflammation in the body. But consumption of certain species of fish can pose health risks, particularly for pregnant women and children.
In a marine biology lab at Roger Williams University, Professor David Taylor placed a small, bite-sized chunk of fish inside a counter-top piece of equipment called a DMA-8 mercury analyzer, which will determine how much mercury this piece of scup contains in its flesh.
"So basically I’m going to put this tissue in a quartz weigh-boat," Taylor said. "It thermally decomposes the sample, basically just burns it, turns it into a vapor."
This DMA-8 has an amalgamator, an atomic absorption spectrometer, and oxygen from a nearby tank is used to carry mercury vapor around the instrument. It is essentially a large, 30,000-thousand-dollar Easy Bake Oven for scientists -- one that reaches 600 degrees celsius to turn this piece of scup into ash.
State and local health officials monitor chemical contamination levels through tests like this one, to determine what breed of fish should be eaten, and which ones should be avoided. With the information, government agencies put out advisories. For example, in Massachusetts, consumers and anglers are warned not to eat the bluefish caught off the coast. Don't eat flounder or lobsters from Boston Harbor, or any fish at all caught in New Bedford Harbor.
As for the scup, Taylor finds that the fish is safe.
"The wet weight concentration is roughly .06 parts per million wet weight," Taylor said, which is well below the EPA threshold."
When It Comes to Contamination, Size Matters
Fish contamination is both a local and a global problem. Mercury is a naturally-occurring chemical, but it is also released by coal-fired power plants. It travels in air streams and ocean currents. It gets into the water, the soil and the ocean sediment. In humans, mercury poisoning can cause brain, kidney and lung problems. In the ocean, it becomes concentrated in phytoplankton, which is at the bottom of the food web. Fish eat the phytoplankton, then fish eat each other. And once it is accumulated in the fish, us human cannot cook it out.
"So in fish, they get their mercury through diet," Taylor said. "The reason that it bioaccumulates in fish is that the intake rate usually far exceeds the removal rate, so that’s why you see fish as they get bigger in size, have higher and higher concentrations of mercury, so that’s what we refer to as bioaccumulation."
When it comes to fish contamination, size matters. Generally the bigger the fish the higher the concentrations of mercury. At the top of the food chain, where the tuna and the swordfish dominate, mercury levels are among the highest. In the least-mercury category are species such as pollack, Atlantic haddock and the scup Taylor tested. But unless you caught the fish yourself, it can be difficult to know just what species you're eating, and therefore, just how much contaminate it may contain.
"It is really tricky as a consumer," said Miriam Rockman Ullman, a staff scientist with the Natural Resources Defense Council. Ullman pointed to several recent investigations have found that often the fish people think they're eating are actually mislabeled, putting them at risk for contamination.
"The laws governing the labeling of seafood in the marketplace are unfortunately not strong enough," she said. "And there have been many investigations when people have gone to the supermarket, done genetic testing, and found that it wasn't the species they wanted."
But shouldn't a fish filled with contamination get sick or die? Shouldn't it be sore-covered and tumor-ridden, obviously inedible? Sometimes they are. But sometimes, as in the case of a species of killifish found in New Bedford harbor, fish just shrug off the contaminants.
Thriving in Poisoned Waters
At Scorton Creek in Sandwich researchers Sibel Karchner and Neel Aluruof the Woods Hole Oceanographic Institution, use a minnow trap baited with dry, Gravy Train dog food to catch killifish.
"This is sort of a pristine site," Karchner said. "As opposed to New Bedford, for example. We use this site as a reference, as a clean site we can compare polluted sites to."
This place -- with its river and meandering streams among wetland grasses and cattails -- is nothing like New Bedford harbor. The harbor is a Superfund designated clean-up site, poisoned with man-made chemicals called PCBs -- different than mercury, but also toxic. PCBs were used in New Bedford's electrical industry.
"If you were to expose the fish from here, from Scorton Creek, to PCB levels the New Bedford harbor fish experience, they would develop all kinds of toxicities -- cancer and other effects, toxic effects," she said. "Where as the New Bedford harbor fish are just fine."
That is the mystery researchers are trying to get at. Why, and how, are the killifish in New Bedford Harbor capable of surviving -- even thriving -- in the poisoned harbor waters?
Woods Hole senior scientist Mark Hahn said the purpose of the killifish study is to understand how, on a molecular and genetic level, the New Bedford fish stay healthy.
"We went into this more than 15 years ago," he said, "thinking that we might have some ideas how they are able to resist the PCBs, and we thought it might be relatively simple to figure out. And what we found is that it is much more complicated than we initially thought. In many ways much more interesting, but we still don't know the final answer as to how they're able to do it."
Just because a PCB-contaminated fish doesn't get sick, doesn't mean the animals that eat it won't. In fact, PCB-resistant fish typically retain more contamination than fish that do not have resistance. So the bigger fish who eat the resistant fish can get a larger dosage of toxic contamination, and then go on to pass that contamination right up the food chain to people.
But the killifish mystery could have some positive human implications. Information learned from studying the genetic differences in fish populations could lead to a basic understanding of how genetic differences in people influence how our own bodies respond to chemicals.
VIDEO: Researchers Sibel Karchner and Neel Aluru of the Woods Hole Oceanographic Institution use a minnow trap baited with dry dog food to capture killifish at Scorton Creek in Sandwich. The fish are used as part of a long term science project looking at contamination impacts on fish.
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