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Who's eating whom in Lake Michigan?
It's not an easy question, especially since a few invasive species have
dramatically altered the food web.
Zebra mussels, quagga mussels,
round gobies, none of them were here twenty years ago.
And they've changed everything, that includes the food available to our
favorite sport fish
like steelhead and coho salmon and rainbow trout.
The Sea Grant programs around the lake are trying to sort this out.
Funding from Sea Grant and other agencies is supporting scientists like
Doctor Harvey Bootsma
of the School for Freshwater Sciences at the University of Wisconsin-Milwaukee.
BOOTSMA: And so we're trying to figure out how the near shore
food structures and the food web structure of the whole lake has changed
as a result of these invasive species that have come into the near shore.
NARR: In particular, they're assessing whether round gobies and quagga mussels
might be dead ends in the food chain or whether the changed conditions nearshore
might deplete the food available for offshore species
such as trout and salmon. Bootsma and his crew will sample for invertebrates
on the bottom, like mussels, crayfish and coronimens. They'll measure dissolved oxygen,
pH and other properties of the water. They'll also take water samples back to the
lab. They use nets to gather zooplankton,
the tiny animals living in the water. In the lab, they'll find out what species there
are, how many there are, and what they eat.
Today they're sampling a rocky site near Milwaukee. On other days they'll sample a sandy site.
The bottom type has a big influence on the kinds of organisms that live there.
Dr. John Janssen, also of UW Milwaukee School of Freshwater Sciences, is the other Wisconsin
researcher on the project.
He and his crew are gathering fish samples. They will take various measurements from them,
such as lipids, fatty acids and stable isotopes of carbon and nitrogen.
These, along with analyzing the stomach contents,
tell Bootsma and Janssen what the fish have been eating in the past days and weeks.
The work is fairly tedious,
and a bit messy.
All the fish will be taken back to the lab for analysis.
They'll check nets at four sites today.
They get mostly perch, alewives,
and a few round gobies.
JANSSEN: What we'd hope for is to get a sucker or two,
which is something we don't know much about.
NARR: Back in the lab the sorting,
counting and measuring starts. One job is to separate the muscles in the cladophora algae.
Erin Wilcox is a research specialist in Bootsma's lab.
WILCOX: This filter will be used for chlorophyll analysis,
so giving us a general abundance of algae in the water sample.
These samples are for
carbon and nitrogen isotope analysis, and
each individual organism kind of has their own signature based
on either what they eat or the nutrients that they acquire
and assimilate into their tissue. And so we can actually kind of trace
up the food chain then based on their isotopes,
who's eating who in the food web
and where the nutrients are kind of going, what nutrients they're
retaining as well in their body tissue.
NARR: It's known that mussels feed primarily on plankton,
but Janssen and Bootsma want to determine whether some of that energy
passes through the mussels and gets to other organisms.
Mike Vansistine is an undergraduate working in Bootsma's lab.
Among his tasks, measuring the soluble reactive phosphorus in the water.
Undergraduate Lisa DeGuire
analyzes the gut contents of the fish they gathered.
DEGUIRE: Good content analysis is a pretty traditional technique for looking at the diet of an animal.
But it's really kinda hard to do because
the stomach is pretty fragile and you don't wanna rip it.
And yep, I was right.
This is a quagga mussel.
But it's a very old technique, I mean they've
used it for years because I mean it's a pretty basic concept just
what does something eat, let's open up and take a look inside.
NARR: Janssen and Bootsma will combine their findings with other Sea Grant funded
researchers in Illinois, Indiana and Michigan. They'll also analyze samples taken by other agencies around the lake.
In the end, the project team hopes the widespread sampling and measurements
will help them piece together a reasonable picture of what's happening
nearshore these days. How many of each species is present,
how the available energy cycles through the system, and who's eating whom.
Ultimately, understanding the nearshore ecosystems in Lake Michigan
will help us better manage the lake's top predators,
the salmon and trout, that support the lake's multi-billion-dollar sport fishing industry.