For many species of fish, spring spawning migrations are a crucial part of their life cycle. They swim upstream to habitat both more suitable for them to deposit eggs and where young fish that hatch from those eggs can avoid predators and grow. That’s why state agencies, environmental non-profits and anglers groups spend millions each year to restore spawning habitat and remove barriers to these migrations. But one lingering question often remains – if you build it, will they come?
In the case of northern pike, an important top predator and popular sport fish across the northern hemisphere, the answer appears to be “Yes.”
To reach that conclusion, researchers at the University of Wisconsin Madison’s Center for Limnology had to first answer a very basic question about pike behavior. While salmon are famous for making long trips back to their birthplace to spawn, there are hundreds of other migratory species that scientists know far less about. Northern pike were one of those species – did they also return to their home, or natal, streams to spawn or would any suitable wetland do?
“Understanding whether or not adult pike return to the same areas in which they were born is crucial to maximizing restoration efforts,” says Dan Oele, lead author of a report published online by the Journal of Great Lakes Research. If pike showed strong natal homing, then wetland restoration projects, newly constructed spawning areas and fish passage projects like dam removals would all need to take this into account – either focusing on improving existing spawning runs or ‘seeding’ new areas with young pike to imprint on. But, if the fish weren’t homing in on home, then just getting them access to these areas would be enough.
Oele, who is now a research scientist at the Wisconsin Department of Natural Resources, began working on the study during his time as a graduate student at the Center for Limnology. And, unlike the very straightforward research question, the science required to answer it was quite complex.
To determine whether northern pike show homing behavior or not, University of Wisconsin-Madison researchers analyzed “otoliths,” or the ear bones inside the head of fishes. Otoliths grow similarly to a tree – adding a ring for each season of growth. As these bony disks accumulate rings, they are also incorporating chemicals and elements from the surrounding water. By analyzing the chemistry of a body of water and then deciphering an otolith’s “chemical fingerprint,” scientists can map where and when individual fish resided or moved though time.
In this study, says Oele, they used the “chemical fingerprint” of otoliths from young pike that had recently hatched in a habitat to make a map of different pike breeding grounds. For example, young fish born in an urban river showed elevated levels of magnesium concentrations in their otoliths while the element, barium, helped decipher between fish born in agricultural rivers (lower barium) and restored ditch systems (higher barium). If the otoliths of the young fish had similar “chemical fingerprints” to the adults, then it would indicate they had returned to where they were born to start their own new generation of pike.
But, says Oele, what they found was adult pike from all over. Some fish had come from urban waterways, others from streams in more agricultural landscapes and still others from coastal wetlands.
According to additional analysis of their data, Ole says, “young fish likely disperse widely and, as adults, remain near a river mouth and wait for water temperatures to warm in the spring. Then the cycle repeats and adults start spawning in suitable wetlands as they migrate from Lake Michigan into inland river and stream networks.”
“Our data indicate that adult pike will colonize newly created or restored areas,” Oele says, which suggests that restoration and management plans should ensure that rivers and streams allow free movement of the fish – both for heading upstream to spawn and for getting back downstream in search of suitable habitats in which they can mature and grow. According to Oele, the team’s field data indicated that urban and ditch systems produced fewer pike than more natural systems, indicating that restoration efforts on such areas may lead to bigger conservation gains.