Great Lakes

Small Tributaries, Big Diversity

The small tributaries of Green Bay pack a quite a biodiverse punch.

A total of 52 species, representing 18 different family classifications, were observed across the 19 study sites.

Lake Michigan’s Green Bay is one of the largest freshwater estuaries in the world and is the most productive part of the lake. A new study of its smallest tributaries has unearthed surprising discoveries that could impact The Nature Conservancy’s efforts to protect fish populations in the Green Bay watershed.

Studying the Small Things

Dr. Patrick Forsythe, a University of Wisconsin-Green Bay professor, was inspired to conduct a comprehensive study of Green Bay’s smallest tributaries after learning of another study that recorded 35 different fish species in Wequiock Creek – a small, unassuming tributary of the Bay.

“I wondered how many more seemingly insignificant tributaries were harboring extraordinary levels of biodiversity. I had a mission to take the first crack at estimating the biodiversity of these tributaries,” said Patrick.

With the help of graduate student Angelena Koosmann, Patrick collected samples from 19 tributaries over a two-year period. Each tributary was “small enough for a person to hop across,” Patrick said.

Don’t Judge a Tributary by Its Size

A total of 52 species, representing 18 different family classifications, were observed across the 19 study sites. Of those 52 species, nearly 42 percent are considered rare and roughly 40 percent were observed using the tributaries for spawning and as nursery grounds.

“It was really impressive to find so many-- and so many rare-- species using such small tributaries. Even large muskie were found! We netted a 50-inch muskie in Wequiock Creek,” said Patrick.

But perhaps the most interesting and impactful finding of all is Patrick and Angelena’s revolutionary discovery about species distribution.

“Each tributary has its own unique ‘biological footprint,’” explained Patrick. “Given a list of species, Angelena could likely tell you which tributary the fish belong to based solely on the unique assemblage. What’s even more interesting is that some species were found only in certain tributaries.”

Understanding which species are using which tributaries can have significant, positive impacts for species-specific conservation efforts, such as the Conservancy’s efforts to protect and restore northern pike migration routes and spawning grounds in the Green Bay watershed.

Protecting Pike and Others

For many years, the Conservancy has been working to remove culverts and bridges, which create barriers for migrating northern pike and other fish species.

“Northern pike are a major focus of The Nature Conservancy’s conservation work in the Green Bay watershed,” said Nicole Van Helden, who directs the Conservancy’s work in the watershed, “because if we can protect and improve pike habitat, many other species of fish, crayfish and mussels will benefit.”

The species distribution data from Patrick and Angelena’s study will tell fisheries managers which tributaries are most used by pike, thus helping the Conservancy better prioritize its barrier removal efforts.

“This research enables us to prioritize our work and make smart, cost-effective decisions about where to invest in fish barrier removal so that we can restore the most habitat for the least cost,” explained Nicole.

Overall, Patrick and Angelena’s study has shown that these modest tributaries are crucially important for Lake Michigan’s fish populations. They’re providing habitat for rare species, important hunting grounds for predatory fish, along with spawning and nursery habitat for many other species.

“When these findings are combined, you’re looking at a bunch of really small systems that are acting like a much larger system,” explained Patrick.

“This study underscores the importance of never under-estimating small tributaries. Even though they appear small and insignificant, this study shows they are some of the most diverse places in Green Bay,” echoed Nicole.



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