Dr. Solomon David, Postdoctoral Research Associate
Daniel P. Haerther Center for Conservation & Research, Shedd Aquarium
Center for Limnology, University of Wisconsin-Madison
I had received word from colleagues that northern pike (Esox lucius), an apex predator in freshwaters throughout the northern hemisphere, had begun migrating inland from Lake Michigan, and I had come to track them. Upon my arrival, snow fell steadily and heavily with ice coating most of the trees around the water. This wasn’t a rain forest jungle or a tropical coast; this was northern Wisconsin in the Great Lakes. Parker, my field tech, and I put on our heavy neoprene waders and climbed into the frigid water.
I’m Dr. Solomon David, postdoctoral research associate at Shedd Aquarium studying the ecology of migratory fishes in the Great Lakes, and how these species may reveal patterns reflecting the health of this important ecosystem. Recently, our team completed the first field season of a 3-year project studying the effects of dam removal and habitat restoration efforts on migratory northern pike in northern Lake Michigan tributaries. One of the primary questions we are investigating is how does pike habitat use differ between artificial restoration sites and natural streams? The answer could help shape future habitat restoration strategies for other systems in the Great Lakes region and beyond.
Our study takes place in northern Wisconsin where several tributaries and agricultural ditches flow into Green Bay, a sub-basin of Lake Michigan. In this region, adult pike travel from Green Bay upstream into connecting waterways in the spring to spawn. These waterways may be rivers and streams or agricultural ditches. Some of these ditches are connected to newly-constructed restoration wetlands; part of a collaborative effort by state, county, tribal, and non-profit partners to improve pike spawning habitat. Previous research on these systems (D. Oele, University of Wisconsin-Madison) suggests that pike tend to move into the agricultural ditches earlier than natural streams since the ditches warm up faster in the spring, but water levels and temperatures are comparatively much “flashier” (higher fluctuation) in the ditches. After spawning, the adults migrate back to Green Bay. Depending on temperature, the young pike hatch after 10-14 days and grow rapidly before out-migrating to Green Bay.
The field team, consisting of myself and University of Wisconsin-Green Bay (UWGB) undergraduate and graduate students, set out in early April to survey adult pike as they moved inland to spawn. These surveys would allow us to estimate the relative size, number, and spatial distribution of adult spawners in ditches and streams. Amidst cold temperatures, frequent rains, and sporadic heavy snowfall, our team managed to survey spawning pike at several sites. We had giant nets typically used for landing lake sturgeon (Acipenser fulvescens) to trap the surprisingly wary and elusive pike. Once caught, we collected our data and returned them to the water. It was amazing to see numerous large pike moving through such narrow waterways – some fish were longer than the width of the ditches!
Perils, Persistence, and More Questions
We found that pike faced many challenges along their migratory journey. In some cases, pike had to navigate culverts and riprap which injured several fish. Some ditches lacked adequate cover, leaving pike exposed to predators such as bald eagles and red-tailed hawks, further evidenced by talon marks on some fish. As if jagged connective structures and large birds of prey were not perilous enough, water levels in the ditches, already highly prone to fluctuation, dropped so low at one point that pike were unable to migrate from spawning grounds back to Green Bay. This prompted an emergency “pike rescue” by Wisconsin DNR and Brown County Conservation officials in which they rounded up the stranded pike, loaded them into containers on a truck and drove them back to Green Bay.
Although the migration of northern pike from Green Bay to spawning grounds and back was a perilous journey, we would soon find evidence that the persistence of these fish had paid off. The first phase of our pike research provided us with some exciting days in the field, and raised even more questions. We observed that the ditch-restoration networks were attracting other migratory fishes. Were these artificial sites actually drawing fishes away from more stable natural streams and typical migration routes? We’ve observed the challenges facing adult pike, but how do the newly hatched young pike fare between the streams and ditches? What other species are yet to be uncovered in these complex connective waterways? Stay tuned for more from our first field season “down by the bay!”