University of Wisconsin researchers check northern pike traps in a ditch near Green Bay. Matt Miller/TNC
by Matt Miller, senior science writer
It’s well known that some migratory fish species, like salmon, are able to trace their way back to the stream where they were born. However, conservationists have no idea if this is the case for hundreds of other fish species.
Do pike return to spawn in the streams where they were born, a la salmon?
Not necessarily, at least in the Green Bay watershed. If there’s suitable habitat, pike will find it and spawn. That’s the central finding of research conducted by the University of Wisconsin’s Pete McIntyre and Dan Oele.
This result may sound like a let-down, but in reality it’s a relief for conservationists in the Green Bay area. The Nature Conservancy and other conservation groups are focused on restoring streams for pike and other fish, but the best methods to accomplish that goal remain uncertain. If pike returned to their natal streams to spawn, they may never find a restored stream even if it contained suitable habitat.
That’s not the case, which is good news for conservation efforts.
The Answer in the Ear Stone
Northern pike aren’t picky about where they spawn–which presents both challenges and opportunities for conservationists. Julie Hawkins-Tyrizer/ Northeast Wisconsin Land Trust
As reported previously on Cool Green Science, Dan Oele spent the last two springs collecting northern pike around Green Bay to determine if their birth streams were the same as their spawning streams.
The researchers collected the pike’s otoliths, also known as ear stones. Otoliths have annual growth rings, like trees, and accumulate trace chemicals from the surrounding water column as they form. The chemical snapshot is retained through the fish’s lifetime.
Many streams have a specific—and unique—combination of chemicals, and this chemical profile shows up in the otolith when fish move from one water body to another. Scientists in the lab analyze the fine rings of the otolith to determine where pike spent different years of their lives.
Pike were sampled from four natural rivers, including an urban stream called Duck Creek and two roadside ditches (which, unremarkable as they may look, are heavily used by fish to spawn).
“We hoped that the chemical profile of the different rivers would allow us to draw conclusions on a very fine scale,” says Oele. “That wasn’t the case. We couldn’t distinguish between the four natural rivers. But on a coarse scale we could tell the difference between the rivers, Duck Creek and the ditches.”
Even at that coarse scale, researchers could determine important information on where pike spawn.
“Some of them do seem to return to their natal habitat type, but there’s also evidence that pike frequently spawn in another habitat entirely,” says Oele. “We conclude that pike do not return to spawn in the same waters as they were born. It may sound like a cliché, but that means that if we restore access to spawning grounds, the pike will come.”
At the same time, researchers also captured (in fish traps) young pike in the waters they surveyed, to determine how successful pike were in breeding.
“The cool thing is how well our field research complemented the otolith chemical analysis,” says Oele. “We found out important information about the waters pike were using and their breeding success.”
Answers Lead to More Questions
Floodplains offer ideal spawning habitat for pike. Matt Miller/TNC
So yes: If you build it, pike will come. But if they come, will they spawn successfully? The answer to that question is more complicated.
“Research often opens more questions than it answers,” says Nicole Van Helden, The Nature Conservancy’s Green Bay conservation director. “There are so many things we don’t know about the wildlife we study.”
Pike need specific conditions to spawn the next generation successfully. When the adult pike lay eggs, usually in late March or early April, they need a stream substrate where eggs will stick—typically an area with some vegetation. A flooded portion of a river (or the floodplain), with ample plants underwater, is ideal.
That area then must stay wet until the eggs hatch and the young pike mature and can leave that area to return downstream to Green Bay, usually in late May.
Adult pike will often lay eggs in habitat that is flooded at the time, but sometimes that habitat isn’t a good choice because it dries up too soon. And if they don’t find good habitat, they’ll lay eggs in poor quality habitat, or they may not spawn at all.
“Pike, it turns out, are not picky about where they deposit eggs,” says Oele.
If a river, stream or even a drainage ditch is available and connected to Green Bay, the adult pike will come. Yet when it comes to producing new generations of pike, not all waters are created equal.
Ditches attract pike and see heavy adult breeding, but not all ditches have ideal conditions. “Lots of adults are spawning in ditches but in our two years of research we found they produce relatively few young,” says Oele. “The conditions may be fine in the spring, but then water levels in the ditches drop too low, and the young can’t get back to the bay.”
Still, ditches do produce some pike and have been a component of the pike habitat restoration efforts in this area. McIntyre and Oele’s research found that according to the earstone analysis about one-third of the adult pike they collected were born in ditches. The next step is to understand the conditions that are necessary to effectively produce the largest number of young pike in the Green Bay system.
Fishy Challenges and Opportunities
University of Wisconsin graduate student Dan Oele holds a hefty northern pike.
Duck Creek is connected to Green Bay, but due to two small dams, adults were not able to reach suitable spawning habitat. “There are lots of adult pike there, but they produced almost no young that survived,” says Oele. “When pike don’t have the proper substrate for their eggs, they’ll either spawn in poor conditions or return to the bay without spawning.”
These two dams were removed this past fall and new research beginning this spring will look at how pike and other fish respond to restored access to upstream spawning habitat.
The research by McIntyre and Oele provides important information about stream restoration efforts. Connecting streams to Green Bay is vital, but so is ensuring that habitat is good throughout the spawning season.
“When planning restoration, conservationists have to account for the pike’s entire life cycle,” says Oele. “If a restored stream has sufficient water throughout the season, and has suitable vegetation, the pike will find it and produce young.”
The streams and ditches around Green Bay form a mind-bogglingly complex network of waters. Conservationists can’t connect and restore them all. The research project will help to direct efforts toward sites that will be the most beneficial for pike, making conservation funding go farther.
“This research enables us to prioritize where we work,” says Van Helden. “The research provides us an ever-growing picture of pike and their needs, so that we can restore the most habitat for the least cost.”
Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.