7 minute read
Fish Family Tree
Using parentage to understand fish stocking success
Story by Garrett Dietz; Photos by David Nevala
Garrett Dietz is a public information officer in the DNR’s Office of Communications. David Nevala is a freelance photographer in Madison.
It’s fun to think about the traits we’ve received from our parents and how we might look similar to our siblings or relatives. Unfortunately, with fish, we can’t simply see matching scale patterns or cute little dimples that might signal blood relationships.
When we want to know if two fish are related, we need to rely on genetics.
You might wonder why we care about fish relationships. The answer is simple: Understanding the patterns hidden in fish DNA can allow us to better manage popular species like walleye and northern pike.
One way fish managers use genetics is to track stocked fish. With DNA analysis, the success of fish stocking can be validated using a less invasive method than physical tags.
This cost-effective practice is called parentage-based tagging. By comparing DNA samples from wild-caught fish to known samples, biologists can identify the parents of a particular individual. This tells them whether a fish was stocked or came from the wild.
TESTING FOR FISH DNA
Parentage-based tagging is based on the genetic similarities between parents and their offspring. Knowing the parents' genetic makeup creates a “genetic fingerprint” to identify their children.
Think of it like an at-home DNA testing kit that allows you to learn about your ancestry. To use it, you collect a saliva sample and send it to a lab for analysis. The lab then compares your DNA to data from people around the globe to determine your ancestry.
Similarly, when DNR biologists work in the field, they collect fin samples from fish and send them to a lab that compares the DNA to a database of information on stocked fish. The data comes directly from Wisconsin hatcheries, which partner with Jared Homola, Ph.D., assistant unit leader of the Wisconsin Cooperative Fishery Research Unit in the College of Natural Resources at UW-Stevens Point, to analyze the DNA.
“It has become a routine,” Homola said. “The folks from the DNR hatcheries send us all the fin clips from their walleye broodstock.”
Broodstock are mature fish collected specifically for breeding purposes. By taking fin samples from these fish, managers provide the necessary information to build a database that incorporates all the parents of Wisconsin’s stocked fish. As stocking continues, this database grows.
It's like building a family tree for fish.
“The database has tons of data,” said David Rowe, DNR fisheries team supervisor. “It can be used to know exactly when a fish was spawned, who the parents were and their physical characteristics, like weight, length or age.”
This powerful tool helps track stocked fish and analyze trends that rely on physical characteristics to contribute to their success. Researchers also use it to understand how differences in water conditions impact stocked fish success by examining populations across water bodies.
AN EYE ON EFFECTIVENESS
The DNR mainly uses parentage-based tagging to understand the success of stocked fish, but it can be expanded to understand native populations as well. The goal is to determine how and where stocking is effective to support state fisheries.
There are only so many tools for controlling fish populations. The most common are fishing regulations, habitat improvement, fishery access and stocking. Because stocking takes significant time and resources, the DNR wants to ensure it is making a difference.
Often, the DNR uses stocking to supplement natural reproduction or create fishing opportunities where reproduction is absent. Knowing how stocked fish survive provides insight into which stocking methods and locations are most effective.
For example, an analysis of walleye in Lake Mendota showed that more than 87% of the 5-year-old fish sampled in 2019 were raised in a hatchery, proving the efficacy of stocking to create a robust fishery. However, another study of northern pike showed stocked fish made up anywhere from 0% to 41% of the population in stocked lakes across the southern region of the state.
This level of feedback allows managers to make informed decisions about where to stock fish. It includes considering factors like water clarity, lake habitats, the spawning success of wild fish and the presence of competing species. There are many variables, but getting a detailed look at the makeup of fish populations helps the DNR get it right.
DNA ADVANTAGES
Tracking stocked fish has been happening for decades, but using parentage-based tagging has benefits over other survey methods.
In the 1980s, hatcheries put dye into fish food to color the bones of fish that consumed it so they could be identified if caught. That practice has long since stopped in Wisconsin, but other common survey methods still in use include clipping fins and tagging fish with physical markers, like numbered plastic tags and wire tags.
The downfall of these methods is how time-consuming and invasive they are. Physically handling thousands or tens of thousands of fish to clip a fin or insert a tag takes significant resources and labor. Advances in stocking technology have made this more manageable, but physical tags and fish stocking trucks all come with a cost.
That’s where parentage-based tagging has advantages.
“Instead of physically marking every fish, hatcheries can take a fin sample from 100 broodstock and effectively tag thousands of their offspring because we know their parents’ specific DNA sequence,” Homola said. “You also don’t have to worry about a physical tag falling out or a fin clip growing back.”
Another benefit is that fish aren’t sacrificed to collect samples as they are with dye injections or food colorings. Parentage-based tagging is a minimally invasive technique — biologists cut a small tissue or fin sample and send it to the lab for analysis.
Results not only tell if the fish was from a specific year or batch, as a tag would, but also which hatchery the fish came from and who its parents were.
Parentage-based tagging has yet to replace the need for other methods, and researchers still conduct studies using physical tags across the state. Those efforts are focused on understanding growth, survival and movement — data that require identifying fish years later.
REAL GAME CHANGER
The state is slowly scaling up DNA tagging across different fish species and locations and refining its processes. Parentage-based tagging data enables DNR biologists to monitor fish populations with more detail than ever.
This helps the DNR understand how stocking impacts a fishery, like where fish should be stocked to best contribute to angler success and overall populations. It also provides valuable information on fish movement within different systems.
“If we sample a fish from the Bay of Green Bay, we can generally tell if it was born in the northern or southern portion of the bay,” Homola said. “Most species spawn reliably enough in their home streams that the populations have genetically diverged to a certain detectable extent.”
DNA analysis also can help determine the characteristics of fish that successfully spawn naturally, which can help inform regulations and bag limits. The breadth of data generated by researching fish genetics allows the DNR and other fish managers to track fish in ways like never before.
It’s changing the game for tracking the lineage of stocked fish, supporting healthy populations and ensuring opportunities for Wisconsin fish fries well into the future.
