10/21/2006

What's killing off the birds?

By
Record-Eagle staff writer

Record-Eagle file photo
A National Park Service biologist checks a dead seagull on the beach of Sleeping Bear Dunes National Lakeshore. Park officials say that 2,600 dead fish-eating birds have been found since August and blame their deaths on Type E botulism.

EMPIRE — Biologists think two invasive marine species may have contributed to the deaths of thousands of birds found in the Sleeping Bear Dunes National Lakeshore.

Tests conducted by the Michigan Department of Natural Resources determined that Type E botulism killed a collection of bird species recovered this month from along the Lake Michigan shoreline. In August, a separate set of tests on dead birds also came back positive for the naturally occurring toxin that's primarily found in decaying fish.

Ken Hyde, a biologist at Sleeping Bear, said the outbreak primarily impacted fish-eating birds, including cormorants, gulls, horned grebes, mergansers and common loons. Park officials estimated the total number of birds lost to Type E botulism in the Lakeshore since August is close to 2,600.

Bird die-offs caused by Type E botulism first were reported on Lake MIchigan in 1963 and have become common in shoreline locations across the Great Lakes, Hyde said. Outbreaks are often related to turnover of lake bottom sediments containing the botulism-causing bacteria, which are ingested by fish that die and poison birds that eat them.

Hyde hypothesized that zebra mussels and round gobies, both invasive species, contributed to this summer's outbreak by concentrating the botulism. Mussels filter the lake sediments and concentrate the botulism. The mussels in turn are eaten by round gobies, which are then eaten by the affected birds.

"The gobies are recent invaders that are steadily increasing in number throughout the Great Lakes and are becoming a significant food source for many fish-eating birds," Hyde said. "These fish can concentrate PCBs and apparently, botulism from the zebra mussels as well."

Hyde said biologists will continue to research a direct link between the invasive species and the botulism outbreak at Sleeping Bear.

Park officials said Type E botulism generally isn't a health risk to humans because people don't eat dead fish carcasses, and thorough cooking destroys the toxin. The risk to dogs that eat the dead birds is unknown.

See related story:

• Biologists try to figure out what's killing lake birds - October 3, 2006