The Answer in a Fish’s Ear

Can examining a fish’s ear lead to more effective conservation?

That question is being answered in Green Bay, Wisconsin, where Nature Conservancy scientists and partners are examining the ear stones of northern pike to help figure out where the fish spawn.

I’m here checking northern pike traps with Nicole Van Helden, the director of conservation in the Green Bay watershed, and Dane Oele, a University of Wisconsin graduate student studying pike.

Northern pike are large, toothy fish that typically live in lakes, where they hunt smaller fish in weed beds and shallow bays. In Green Bay, the largest bay in Lake Michigan, pike are the top predator; each spring, many pike migrate from lakes to rivers, streams and even ditches to spawn.

Conservationists in this area focus on restore blocked or degraded streams and ditches for spawning pike. But if a stream is restored, will pike actually return?

At times, restoring wildlife habitat seems like the Field of Dreams: Build it, and the wildlife will come. That may not be the case for pike.

There is some evidence that, at birth, pike may imprint on the chemicals of a specific stream – what biologists call natal homing. They use the scent of these stream chemicals when they return to spawn.

“If pike have natal homing, it totally changes where we work,” says Nicole Van Helden. “Just doing lots of restoration is not going to be enough to restore pike populations. If fish return to the same spot they were born, they may simply ignore restored streams. We need to focus our efforts on where our work will have the most benefit in this watershed.”

Pike don’t talk, but their ears may tell conservationists what they need to hear.

In the ear is a small “stone” composed of calcium carbonate known as the otolith. The otolith, found in all vertebrates, help fish with balance and sound detection.

Otoliths have annual growth rings, like trees, and accumulate trace chemicals from the surrounding water column as they form. Many streams have a specific—and unique—combination of chemicals, and this chemical profile shows up in the otolith when fish move from one chemically distinct water body to another.

As such, researchers can determine where pike spent different years of their lives – and if they return to the streams where they were born, or if they use different streams.

“Conservationists working on stream restoration used to make best guesses on which streams would be best for pike,” says Van Helden. “By examining otoliths, we can gain a much clearer picture of where pike move.”

From Stream to Lab

To examine otoliths, researchers first have to catch pike – an often cold, wet and muddy process. For random sampling, researchers go electrofishing—sending voltage into the water that disorients fish and brings them to the surface—in Green Bay. They also set funnel traps that capture pike in small streams and roadside ditches, and determine if pike are successfully spawning in those small streams by capturing newly hatched pike a few weeks after the adults have returned to the bay.

“I’m running all over the place, checking the lay of the land and figuring out where pike are – and where the could be,” says graduate student Dan Oele. “I’m spending a lot of time in streams. And I’m collecting pike from as many different types of places as possible. Pike have rows and rows of sharp teeth, and even have teeth in their gill rakers. I’ve gotten poked a few times.”

It’s the kind of work many people envision when they picture field biology. But once the otolith is collected, it’s a different picture.

Otoliths are smaller than a penny. To analyze each annual ring, researchers must cut them with a special saw with a one millimeter diamond blade. The micro-thin section is then sanded with ultra-fine sandpaper in a spotlessly clean work area.

The otolith ring is then scanned with a laser sensor that analyzes the chemical composition.

The laser cuts a path from the otolith’s center to the edge, and analyzes the chemicals. Thus, scientists can compare the different life stages of the fish chemically and determine if the fish has migrated to the same place that it was born, or if it utilized different habitats.

Microanalysis like this is helping conservationists determine fish populations and fish migration patterns around the world.

A Future for Pike

In places like outdoors-obsessed Green Bay, pike are a popular gamefish. They’re also a top predator in the Great Lakes, so protecting them protects a whole host of other species in the ecosystem.

The area around Green Bay has almost as many streams and water channels as it does Packers fans. The area are hundreds of road culverts, each potentially blocking pike from spawning. There are seemingly endless streams that could be restored.

Microanalysis will help conservationists determine where conservation can have the most impact—making your investment in organizations like the Conservancy go farther and achieve more lasting results.

Additional research is being conducted to identify the best places to improve fish habitat for a variety of migrating fish species in the Great Lakes.

“In some areas, simply changing one roadside culvert might open up miles and miles of habitat for pike,” says Van Helden. “But we have to know if pike are going to use such habitat if it’s opened up. The research being conducted now is an important component of determining where we work. It means that we will be restoring streams based on evidence, not guesses.”

[Image: University of Wisconsin graduate student Dan Oele with an adult northern pike. Image source: Dan Oele]

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