National Geographic
Menu

The Mystery of the Migrating Fishes: Swimming the Gauntlet to Green Bay

 

Growth of young-of-the-year (YOY) northern pike over the course of our field season. (Solomon David/Shedd Aquarium)

Growth of young-of-the-year (YOY) northern pike over the course of our field season. (Solomon David/Shedd Aquarium)

Dr. Solomon David, Postdoctoral Research Associate
Daniel P. Haerther Center for Conservation & Research, Shedd Aquarium
Center for Limnology, University of Wisconsin-Madison

The ice and snow of early spring in northern Wisconsin had come and gone. Also departing with the frigid weather were the adult northern pike our team had been tracking as the fish migrated inland from Green Bay to spawn. Now we were looking for evidence of the next generation to find out if they could successfully navigate the many challenges on their migration to the safer waters of Green Bay.

We recently completed the first field season of a 3-year project with Shedd Aquarium, the University of Wisconsin – Green Bay and University of Wisconsin – Madison studying the effects of dam removal and habitat restoration efforts on migratory northern pike in northern Lake Michigan tributaries. My previous post documented the spawning migrations of adult Northern Pike from Green Bay into streams and restoration ditches. Another objective of our study was to investigate the success of young of the year (YOY) northern pike as they hatched, grew and out-migrated to Green Bay.

As noted in our last entry, the natural tributaries and agricultural ditch habitats leading to Green Bay differ in terms of length, temperature and flow regime, as shown in previous work by Dan Oele from the University of Wisconsin – Madison. Previous work by Oele (2011-2012) also indicated that in some years, water levels in agricultural ditches drop so low or completely dry out where YOY pike are unable to reach Green Bay and instead perish. Would YOY pike successfully reach Green Bay this season?  What other challenges might they face along the way?

Trap and Release

Solomon David with Shedd's Conservation Intern Jon Bitner and the YOY northern pike traps. (Solomon David/SheddAquarium)

Solomon David with Shedd’s Conservation Intern Jon Bitner and the YOY northern pike traps. (Solomon David/SheddAquarium)

From May to early July, we were able to estimate the abundance, growth and potential success of the pike as they migrated to Green Bay by setting wood-frame screened box traps at several locations in both stream and ditch habitats. These traps complemented those set by Brown County Conservation (Mushinski et al.) and Oneida Nation (J. Spiegel), which collectively made for a more accurate population estimate. The box traps captured a sample of young pike as they moved downstream toward the bay. Every other day, the traps were checked to count and measure the YOY pike before releasing them to continue their downstream journey.

In early May we began collecting YOY pike in our traps. It was hard to believe that these tiny 1 inch fish could grow into the 44 inch beasts we saw earlier in the season! Equally impressive were the number of pike emerging from the spawning grounds – a notable day in May saw over 10,000 young pike collected from the traps in a particular ditch. Ongoing updates from Brown County, Wisconsin Department of Natural Resources and Oneida teams suggested this would be a good year for pike recruitment, given the more consistent water levels and number of fish they were collecting.

A Treacherous Path

Fluctuating water levels and temperatures both posed a major challenge to the young pike moving down the streams and ditches to Green Bay, but at one particular ditch system we were able to observe an additional hazard to the journey. At a site we called “Brown Road Ditch,” each of the restoration habitats and agricultural ditches in the local area connect and funnel down to one path through a large culvert. On the other side of the culvert is a short channel leading to Green Bay, causing all the young pike in the area to move through this culvert in order to reach the bay – but it wasn’t an easy route for the pike. Just below the culvert was a school of thousands of yellow perch that were likely very hungry! We seined this “perch cloud” several times during the spring and early summer and found that abundance and species diversity increased over time. As the season progressed, the perch were joined by large golden shiners, common shiners and juvenile bullhead catfish. These fishes are not normally considered piscivores (fish-eaters), but to tiny YOY northern pike they are quite threatening! With the added peril of predators on the home stretch to Green Bay, the outlook for migrating pike seemed bleak at best.

Even a quick dip-net sample through the "perch cloud" reveals several yellow perch and other potential pike predators! (Solomon David/Shedd Aquarium)

Even a quick dip-net sample through the “perch cloud” reveals several yellow perch and other potential pike predators. (Solomon David/Shedd Aquarium)

By late June, however, we found several YOY pike (about 3-5 inches long) in our traps at the mouth of Brown Road Ditch and the entrance to Green Bay, indicating that some pike did survive the migration gauntlet and reached the relative safety of the bay. The Green Bay nursery area was ripe with YOY yellow perch, so pike that survived the journey had now become the predators instead of prey, showing how quickly the tables can turn in nature.