Here at IISD Experimental Lakes Area, we are monitoring our lakes year-round, trying to figure out the impacts of human activity and pollution on a wide range of aspects that affect 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 condition and for their tissues to be tested for chemicals like mercury to make sure that they are safe for humans to eat.
Frequent euthanizing of wild fish can, however, reduce population numbers, as well as prove to be an inhumane way to treat animals. Multiple small- and large-bodied fishes, both male and female, need to be collected each time to ensure that a representative sample is taken. Scientists want to sample often to keep an eye on the health of populations, but if monitoring requires frequent euthanizing, it can actually become a problem instead of a solution.
At IISD Experimental Lakes Area, we reduce and refine the sampling methods that require euthanizing fish.
In fact, we are developing new, and championing existing, ways to measure fish health without the needing to euthanize them (And yes, we acknowledge the parallels with alien abduction).
Once we have collected fish from the lake and brought them to shore in tubs of fresh water, we 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. The fish are under mild anesthesia for the whole process, ensuring the health and safety of fish and handlers.
Here are just a few ways we sample fish non-lethally:
Firstly, 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 9 mm passive integrated transponder (PIT) tag—just like the ones you use to tag your pet dog or cat—into the muscle of their back where it is least likely to cause harm. The needle insertion is a simple process, sterilized with iodine, heals quickly, and allows us to simply scan the fish before we start sampling them to determine who is who in our long-term database.
Whenever we insert a PIT tag, we also collect a fin ray to determine its age.
We clip off the leading ray on the pectoral fin of most large-bodied fish species. 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 you would do with a tree!), which can then be increased in our database each time we recapture that same fish.
A bonus of this collection is the bottom end of the fin can be used for further stable isotope analysis to see where in the lake (nearshore, offshore) and the food chain (higher, lower) the fish has been feeding!
The muscle tissue of a fish contains lots of information about its health. We can see a range of contaminants that the fish has ingested (such as mercury), model its metabolic rates, determine its place in the food web through the use of stable isotopes and fatty acids, and examine the tissue energy density.
The process involves removing a small core of muscle (4 mm diameter) 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!
You might be surprised to learn that the molecules found in the mucus swabbed from a fish can reveal a whole host of secrets about their health, from how stressed they are to the contaminants they contain.
This process is pretty 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 come back to the same fish later to compare results.
We can also take blood samples from the fish to determine how healthy and stressed they are and feces samples to learn about their gut microbiome.
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, collecting up to 10 lavage samples from lake trout in a selection of our active experiment and reference lakes each spring, summer, fall (and now winter)! This means we can compare seasonal differences in fish diets and note changes that may be a result of the whole-lake manipulation.
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.
We use acoustic telemetry to keep tabs on where the fish are moving and hanging out, 24/7, to build a picture not just over days, but over months, seasons, and years.
After surgically implanting acoustic telemetry tags (or transmitters) into the larger fish that we want to track in some of our lakes each spring (~10 active tags per lake), information about fish location and depth gets sent to receivers that are spread around the lake.
Twice a year, we grab those receivers from the lake, download the data, and place them back around the lake 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.