Originally published in Lake of the Woods Area News, Volume 54, Number 4, Fall 2024
Why do we use non-invasive fish sampling methods?
Here at IISD Experimental Lakes Area—the world’s freshwater laboratory, just southeast of Kenora—we are monitoring our lakes year-round, trying to figure out the impacts of human activity and pollution on freshwater lakes, from water chemistry to fish.
Traditionally, when evaluating fish health that might be impacted by human activity, scientists collect wild fish and euthanize them. This allows the entire body of the fishes to be analyzed to determine their status and for their tissues to be tested for chemicals, such as mercury, to make sure that they are safe for humans to eat.
Frequent euthanizing of wild fish can, however, reduce population numbers. Scientists want to sample often to keep an eye on the health of populations, but with frequent euthanasia, it can become a problem instead of a solution.
How can we sacrifice as few fish as possible during sampling?
At IISD Experimental Lakes Area, we primarily use non-lethal fish sampling methods, which allow us to get to know the fish as they live their lives in the experimental lakes. We know that they didn’t ask to live in an experimental lake—we understand what a privilege it is to be able to study these fish in their natural habitat and we know that respecting each fish as a unique individual is essential.
In fact, we are developing new, and championing existing, ways to measure fish health without the need for sacrificing.
Once we have collected fish from the lake and brought them to shore in tubs of fresh water, we anesthetize them and then collect a whole host of information on their health and development, including species, weight, length, sex, and maturity status.
After collecting these measurements, we may use one or more non-lethal methods to collect additional sample(s) from the fish before releasing them back into the lake.
Here are just a few ways:
PIT tags
First, we want to identify our large-bodied fish over time.
So much of the monitoring of fish health and populations that we do requires us to track (and use these non-invasive sampling methods on) the same fish over months and years to see how they are getting on and to determine the effects that human activity and pollution may be having on them.
We do this by inserting a tag—just like the ones you use to tag your pet dogs and cats—into the muscle of their back. We can then scan the fish before we start sampling them to determine who is who in our long-term database.
Aging structures
Whenever we insert a PIT tag, we also collect a fin ray to determine its age.
Back in the lab, we count the annual summer (wide growth) and winter (narrow growth) rings to assess the likely age for each fish, just like a tree!
Fish muscle biopsies
The muscle tissue of a fish contains lots of information about its health, including what it is eating, its contaminant load, and how much energy it is using.
The process involves removing a small core of muscle through a simple biopsy procedure, sealing up the wound, which heals easily, and then analyzing that sample. We sometimes resample the same fish after a year or two.
Mucus sampling
You might be surprised to learn that the molecules found in fish mucus can reveal everything from how stressed they are to the contaminants they contain.
This process is simple and very non-invasive. We simply swab the side of a fish to capture some of its mucus, put the tip of the swab in a vial, freeze the sample, and then analyze the mucus. We can even return to the same fish later to compare results.
Gastric lavage
The contents of a fish’s stomach can teach us many things, including a snapshot of what other fish, zooplankton, benthic invertebrates, etc., that fish has been eating.
We do this year-round, which means we can compare seasonal differences in fish diets and note changes that may be a result of the whole-lake manipulation.
Acoustic telemetry
The areas that fish occupy in the lake are just as important as how healthy they are, especially when you consider how climate change is affecting the size and location of fish habitats.
After surgically implanting acoustic telemetry tags (or transmitters), information about fish location and depth gets sent to receivers spread around the lake.
Twice a year, we grab those receivers from the lake, download the data, and place them back so we can keep spying on the fish. This helps to reveal their movements and seasonal usage of freshwater habitat when they are not hanging out in our nets awaiting sampling.
For more information about the world’s freshwater laboratory in our very own backyard, visit iisd.org/ela.
