Swimming Against the Current: How migratory fish keep streams healthy. During the summer of 2003,  Peter McIntyre  took a week off from his graduate work to join his grandfather on a trip to the Upper Peninsula. His grandfather took him to a stream to catch suckers—freshwater fish common in the Great Lakes region—to use for his bait business. What McIntyre saw there, he says, was “staggering.”

McIntyre was working on his doctorate at the time, studying the productivity of lakes and rivers in the tropics. What he saw in Michigan was a massive fish migration that easily surpassed the migratory spectacle he had witnessed in South America.

Anecdotal evidence suggests that sucker breeding migrations, known as runs, may not be as enormous as they once were. Yet McIntyre estimates that there are still tens of millions of suckers in the Great Lakes basin that make the annual upstream pilgrimage. Unlike other migratory fish, suckers swim up streams both large and small. McIntyre has measured tributaries only 20 feet wide that swell with 3,000 to 8,000 suckers during migration. According to local lore, migrating suckers become “so thick you can walk across the stream on their backs.”  

Despite their humble name, suckers are anything but ordinary. They are the third-most diverse family of freshwater fish in North America, growing anywhere from 14 inches to two feet long and living as long as 25 to 30 years. Carnivores—not mudsuckers, as their name might suggest—suckers are bottom-dwelling fish whose diet ranges from shellfish to snails.

As they pour out of the Great Lakes into streams and small tributaries to breed by the million, McIntyre says the massive migration is a phenomenon rivaled only by more famous migratory wonders like the wildebeest of the Serengeti or the salmon of the Northwest Coast. And unlike the declining species whose scarcity inspires protection, the suckers’ conservation value is born of their sheer volume.

Today, McIntyre studies these enormous migrations in a more official capacity than when he first saw them with his grandfather. As a Smith Fellow of the Society for Conservation Biology, McIntyre has teamed up with Dr. Patrick Doran, Michigan’s director of science for the Conservancy, Dr. David Allan from the University of Michigan’s School of Natural Resources and Environment and Robin Abell at the World Wildlife Fund. Their work on suckers takes place on the Stonington Peninsula in the northwest corner of Lake Michigan. Local residents help the researchers establish study sites and monitor the sucker runs.

One of McIntyre’s overarching goals is to measure the degree to which sucker runs fertilize Great Lakes tributaries. This data is especially important when considering the manmade barriers that can block sucker runs, such as dams, road culverts and fishing nets—barriers which may have a far-reaching impact on overall stream health, not just the immediate surroundings.

McIntyre’s early data shows a clear increase in stream nutrients during the migration. That’s no surprise to anybody who has witnessed the enormity of this event and has seen firsthand the tangible natural link between small streams and the Great Lakes. In a watershed, where everything drains from upstream to downstream, the millions of fish that swim against the current are an important exception to the downward flow of nutrients.

The life cycle of suckers supports the streams they migrate through. Adult fish are an important food source for both birds and mammals, and emerging evidence shows that sucker eggs are a critical seasonal resource for salmonids and other game fish. Walleye and northern pike, which breed in the same rivers as suckers, may benefit because predators are less likely to consume their larvae than those of the more-plentiful suckers.

The waste products and carcasses generated during sucker migration inject new nutrients into the streams. That enhances algae growth, which provides food for insects, which then become food for birds and game fish. Suckers typically survive the breeding season and return year after year to spawn in streams, and although they usually out-populate other fish species, their non-game status has discouraged research and management efforts. But as Conservancy scientist Patrick Doran explains, “what’s good for suckers is almost always good for the entire fish community, including game species.”
McIntyre’s study isn’t just about suckers. His results will provide critical input for the Conservancy’s efforts to protect the health of Michigan’s streams—understanding how suckers impact stream health can help inform basin-wide conservation decisions.

McIntyre and Doran both stress that this study can yield a win-win scenario. Massive sucker runs enhance the health of both stream ecosystems and game fish. The more we understand about suckers, the better The Nature Conservancy can protect and preserve the integrity of the mighty Great Lakes and their tributaries.

To assess the potential importance of sucker runs for the productivity of tea-stained streams in the UP, McIntyre teamed up with David Allan (University of Michigan), Patrick Doran (Director of Science for The Nature Conservancy in Michigan ), and Robin Abell (World Wildlife Fund). The Smith Fellows program, which provides post-doctoral funding for young environmental scientists to partner with academic and NGO mentors to address pressing conservation issues, offered an ideal mechanism for initiating the project.

Nature picture credits (top to bottom, left to right): Photo © Peter McIntyre (Wilsey Bay Creek); Photo © Peter McIntyre (crew); Photo © Peter McIntyre (net in river); Photo © Peter McIntyre (suckers in net); Photo © Jack Biek (sucker census); Photo © Peter McIntyre (dam).