Biologists fear mercury is silent killer of loons

Deceased loons from throughout New England end up on the examining table, above, of Dr. Mark Pokras, where the birds are dissected to determine what killed them. Staff photo by David A. Rodgers.

NORTH GRAFTON, Mass. - As Mark Pokras pressed the blade of his scalpel into the dead loon's breast, the mystery of its life unfolded.

In the bird's thick layer of down, he saw warmth in the cold Atlantic winters. In its plump red heart, he saw endurance for deep underwater dives. And in the specks of dirt lodged in its respiratory system, he saw a final battle with death at the edge of a lake in central New Hampshire.

''I suspect this bird beached itself when it died, put its head in the water and snorted this stuff down with its last breath,'' he said.

Pokras is a veterinarian, director of the wildlife clinic at Tufts University, and a top expert on loon mortality. Since 1988, he and his colleagues at the university's school of veterinary medicine have dissected nearly 400 loons to learn what caused their deaths.

His studies reveal that most loons die from lead poisoning caused by swallowing fishing sinkers and jigs, from collisions with boats and personal watercraft, or from injuries from gunshots and other loons.

None of the birds examined at the clinic has been killed by mercury poisoning. But Pokras has found high levels of the toxic metal in the blood of loons that died from a fungal respiratory disease known as aspergillosis.

Dr. Mark Pokras examines the wing of a loon as he begins a necropsy on the bird at his Tufts University lab. Staff photo by David A. Rodgers.

His discovery suggests that mercury or some other unidentified toxin in the blood may interfere with the bird's ability to fight off disease.

That's the problem with mercury. While it doesn't appear to kill loons outright, some wildlife biologists believe mercury may be a ''final insult'' that weakens loons, making them vulnerable to disease, severe weather, a lack of food, or other stresses they might otherwise overcome.

''Mercury is one of those hard ones,'' says Drew Major, a biologist at the U.S. Fish and Wildlife Service. ''It doesn't outright kill things, but it may be the straw that breaks the camel's back.''

Dead loons from all over New England are delivered to the wildlife clinic, near Worcester, Mass. Some are found by biologists, game wardens or fishermen. Others are picked up by amateur naturalists or loon enthusiasts from groups like the Maine Audubon Society or the New Hampshire Loon Preservation Committee.

At the clinic, each bird is given an X-ray to detect broken bones and swallowed fishing gear. The loons also get a close external examination to record their weight, dimensions, general health and any injuries.

The external exams provide the kind of basic information that scientists can't get from living loons, which are wary and resist life in captivity.

No one has ever been able to keep a loon alive in a wildlife park or a laboratory because the birds won't eat unless they're in the wild.

Loons also respond poorly to rehabilitation efforts. In 1996, when an oil spill fouled the Rhode Island coast, rescue workers took 80 loons to a wildlife clinic for emergency treatment. None of the fragile birds survived.

''That's why it's important for us to collect as much data as we can,'' says Rose Miconi, a wildlife biologist who works at the clinic.

An X-ray of a dead loon shows the likely culprit in the bird's demise: a lead fishing weight in the stomach. Staff photo by David A. Rodgers.

The internal examination of a loon, called a necropsy, is carried out in a veterinary hospital lab equipped with stainless steel tables, a powerful ventilation system and numerous floor drains.

It's a smelly, unpleasant but fascinating process that yields vital information.

Cutting and prodding methodically, Pokras examines the heart, air sacs, intestines and other organs for signs of disease or injury.

He draws blood with a syringe and slices pieces of liver into small jars of preservative formaldehyde for shipment to testing laboratories. In the labs, the samples will be analyzed for toxic chemicals.

Invariably, mercury turns up in the loons' blood tests - sometimes in concentrations as high as 8 or 9 parts per million. Those levels, no longer uncommon, are a big concern.

Researchers believe that 10 parts per million may be the level at which mercury begins to affect the birds.

''I think mercury is something to be concerned about,'' Pokras says. ''What we have not been able to do is tie the levels we've found to a specific (problem).''

After hundreds of dissections over the last decade, Pokras still retains a capacity for surprise and wonder at the loon.

In late June, while examining a bird found in New Hampshire, he marveled at how loons annually build up a thick layer of fat under their skin.

The natural insulation allows the loon to maintain a body temperature of 103 degrees, even while the water in its winter habitat dips into the 30s.

''You have to admire that,'' he says.

Cutting deeper into the bird, he finds a large, broken fishhook poking through its upper stomach wall. Attached to the hook is a piece of monofilament line and a metal swivel that fishermen use to keep the line from kinking.

It's a gruesome sight, but Pokras says the hook probably wasn't fatal. Fish are the mainstay of a loon's diet, so their digestive systems are probably adapted to dealing with punctures from sharp objects.

The true killer of this loon he finds later, in the lower chamber of the stomach: A lead fishing sinker an inch long.

The highly toxic metal from the sinker enters the bloodstream and damages the loon's nervous system. Fifty-seven percent of the loons brought to the clinic have died from lead poisoning.

Pokras picks up a camera from the examining table and presses it against his bearded face. He snaps a few slides of the sinker, swivel and fishhook, still embedded in the loon's body.

Eventually, the slides will be used by loon advocates in an ongoing public campaign to eliminate lead fishing weights and publicize the dangers of discarded fishing gear.

Sally Stockwell, a staff member at the Maine Audubon Society, says Pokras' work with loons has opened the eyes of many sportsmen.

''Because of the work he's done, we've been able to spread the word to fishing groups,'' she says. ''They're always astounded to see the damage.''

Back in the Tufts lab, Pokras finishes his exam and gathers up his instruments. Miconi, his colleague, cleans up the bird and enters the last bit of information on a data sheet.

At one end of the lab awaits an incinerator, capable of reducing the bird's remains to ash. But loons rarely meet a fiery end.

Museums, tourist attractions and zoology departments are waiting to receive the remains from the clinic. They'll be preserved and mounted for further study or admiration by researchers and the public.

More than most wild creatures, the loon seems to inspire fascination and respect - even in death.