MARQUETTE - Understanding the Lake Michigan food web could go a long way to understanding a disease that's affecting aquatic birds.
David Essian, a master's candidate in the Biology Department at Northern Michigan University, is part of a study in this great lake about Type E avian botulism, which is causing mortality among common loons and other birds.
NMU biology professor Jill Leonard said Essian's work is important in understanding the basics of science involved with the changes in the Lake Michigan food web. It's also important to discover, she noted, the causes of what is considered a "real conservation issue" for the protected and native species involved with the avian botulism outbreak.
Three dead common loons among 236 that washed ashore along a seven-mile stretch of Lake Michigan beach near Gulliver in fall 2012. Researchers think Type-E botulism is to blame for the die-off. (Damon McCormick photo)
"The Lake Michigan food web has undergone and continues to undergo dramatic changes that have the potential to affect all organisms using the system, including humans and some species that we highly value," Leonard said. "It is important for us to understand this dynamic system in order to anticipate changes and manage resources."
The disease is caused by a toxin and coincides, Essian said, with the introduction of exotic zebra and quagga mussels and round gobies. It also coincides, he said, with warmer lake-surface temperatures in the last 10 years.
However, Essian pointed out, "Most of the birds that are dying are mussel-eating birds or fish-eating birds."
That's where understanding the Lake Michigan food web comes in.
Essian said mussels, by filtering out the water column, allow light to penetrate deeper, which lets a certain type of algae, Cladophora, become more abundant. That algae attaches to hard surfaces at the bottom and grows upward from there. However, the algae at the very bottom can't get enough light and dies off.
It just so happens round gobies live and forage at the bottom.
"That's where this algae is sloughed off," he stressed.
This means gobies are exposed to the toxin found in the algae.
So what happens when a bird consumes a goby?
"I have found round gobies in a very high percentage of a lot of botulism-positive birds," Essian said.
However, Essian pointed out what birds eat in Lake Michigan is not fully understood.
"I'm trying to kind of bring to light of what it is in the diets of these birds," he said. "There's a lot of birds dying, and they don't eat all the same things."
Essian said he needs to find out what round gobies are eating. When he cuts open a bird's stomach, Essian said he doesn't find full-sized gobies but measures bones to calculate fish size.
He's found the diets of gobies that are fewer than 70 millimeters long consists mostly of free-living macroinvertebrates, like fly larvae. The diets of gobies more than 70 millimeters consist mostly of mussels.
Essian said it then can be inferred the toxin is passed from the mussels to the gobies to the birds.
"We can't know for sure," he said, "but it does gives us a clue of how the toxin is in the environment and how it's transferred through the food web."
Essian, who's been involved in the study since May 2012, said he should be finished with his research next May.
He said the U.S. Park Service and the U.S. Geological Survey provided the funding for the project and are involved as well. USGS volunteer groups, he said, locate dead birds, which are sent to the National Wildlife Health Center where they're tested. Those birds then are sent to him, and he compares botulism-positive birds with botulism-negative birds.
Gary Palmer, an NMU master's candidate in biology, helps Essian with data collection. It's his job, he said, to go through the gizzards - the avian versions of stomachs - of the birds he's received, mostly double-crested cormorants shot in Bay de Noc near Escanaba as part of the state's control program.
In a system Essian devised, Palmer uses a series of sieves to separate and rinse the gizzard contents, which helps to get rid of the finely digested organic material of no use to the study but still making up a sizable proportion of the overall contents.
Palmer examines the materials under a dissecting microscope, looking for things like parasitic worms, whole fish, mussel shells and, most importantly, identifiable fish bones. He then collects the items in small jars and preserves them in ethanol.
After the bones have been collected, he measures their lengths to estimate size. It can take anywhere from two to 12 hours to complete an individual bird, depending largely on the species and whether it was a euthanized bird, Palmer said.
"It's been interesting getting to learn more hands-on about the different diets of a few species sharing a habitat," Palmer said, "and I'm excited to see what David and the Leonard lab are able to do with the data I'm helping to collect."
Loons seem to be most affected by Type E avian botulism, according to Essian, who surmised more loons are in the affected areas during migration. However, other birds experiencing mortality include long-tailed ducks, white-winged scoters and red-necked grebes.
Whatever the underlying cause is for the Type E avian botulism, its long-term effects are unknown, Essian said.
"We're just trying to understand what's causing it right now," he said. "We're just trying to find a way to mitigate the effects of it."
Christie Bleck can be reached at 906-228-2500, ext. 250. Her email address is firstname.lastname@example.org.