After reading about eye lens stable isotope analysis work being done by other great fisheries biologists (special shout-out to the fascinating work done by Dr. Julie Vecchio and Miranda Bell-Tilcock!), my mind starting whirring with all the possibilities this technique holds. I teamed up with a fantastic group of researchers to explore the utility of eye lens stable isotopes to describe life history characteristics in char (Lake Trout and Arctic Char). Together, our team secured funding for this work through the Great Lakes Fishery Commission. Our team includes Andrew Muir (Great Lakes Fishery Commission), Heidi Swanson (University of Waterloo), Ben Turschak (Michigan DNR), Mark Vinson (USGS), and Willie Fetzer (University of Wyoming).
Eye lens stable isotope analyses are emerging as a tool to answer questions about lifetime resource use of individual fish. Fish eye lenses develop by forming layers like an onion, with layers added as the fish grows. When a new layer forms, it integrates chemical signatures of the food the fish has been eating. We can use stable isotope analysis, a chemical detection technique, to determine what those food sources were. By peeling layers of the eye lens apart and running stable isotope analyses on each layer, we can recreate the lifetime resource use of a fish. This is exciting because it allows to ask questions like: what does it take to grow a trophy-size fish? how much are fish moving within a lake? and, how are environmental changes like the introduction of non-native species impacting fish throughout their lifetimes?
Why study Lake Trout and Arctic Char?
Lake Trout and Arctic Char are related species that have some interesting characteristics that make them a great species to study to understand applications of eye lens stable isotope analysis.
For one, these species can be quite long-lived. This means that they often survive through periods of ecosystem change. For example, in Lake Michigan, one of our study systems, there are individual Lake Trout that are old enough to have survived through multiple boom-bust population cycles of Alewife, as well as the introduction of Round Goby. Alewife and Round Goby are both non-native species to Lake Michigan, and their introductions have caused changes to the preyfish community and, as a result, the diets of Lake Trout. Because Lake Trout are so long lived, we wanted to look at the eye lenses of Lake Trout in Lake Michigan to understand how much Lake Trout diets are changing to track these prey items. We know that average diets of Lake Trout are changing, but the eye lens technique allows us to ask if individual fish are making changes within their lifetime to key in on new prey resources.
Another reason these species are well-suited to explore the eye lens technique is that they display a range of “life history characteristics.” Life history characteristics are something like lifestyle choices. For example, the Arctic Char we are studying in Nunavut, Canada, migrate between freshwater and saltwater. They are generally anadromous, meaning they spawn in freshwater, then migrate out to the ocean, grow to be adults, and then return to freshwater to spawn. However, the frequency and timing with which they move between freshwater and saltwater varies between individuals. Since food sources are different in freshwater and saltwater environments, we can use eye lens stable isotope analysis to see when fish migrate between the two, look at the ways individuals differ, and what affect that might have on their success (i.e. growth).
Finally, in another version of life history characteristics, Lake Trout can also have different “morphotypes.” Morphotypes are essentially different forms of the species “Lake Trout.” While they’re all the same species, each morphotype looks different, occupies different habitat, and feeds on different food sources. In Lake Superior, there are four recognized morphotypes of Lake Trout: “leans” are a nearshore morph and are the fish generally targeted by anglers, “siscowets” or “fats” are fatter and generally occupy deep habitats, and “humper” and “redfin” Lake Trout are morphotypes that are restricted to habitats on offshore shoals/islands. We are looking at the eye lenses of the two most common morphotypes, leans and siscowets, to see how individuals of those morphotypes diverge from each other over their lifetime. These two morphotypes often occupy similar habitats when they are young, but siscowets then move to deeper waters while leans stay shallow. By looking at the eye lenses, we can investigate the timing of when the two morphotypes separate, and see if it’s a switch in prey that is driving the separation.
Finally, and importantly, we are using Lake Trout and Arctic Char because they are readily available through existing survey efforts. In Lake Superior, we are using Lake Trout that were collected and unable to be released during standard surveys conducted by the Wisconsin DNR and USGS. In Lake Michigan, we are using Lake Trout collected and unable to be released during surveys by the Michigan DNR. In Nunavut, we are using Arctic Char harvested by fishermen and donated for a previous study to the University of Waterloo. Using fish already collected allows us to 1) prevent excess mortality and 2) capitalize on fish that are part of existing, large datasets, allowing us to know more about those fish and the populations they came from.
In summary…
Eye lens stable isotope analysis is emerging as a scientific method that holds tremendous promise for expanding our knowledge of fisheries. With this technique, we are able to ask questions about how individual fish are responding to their environments, rather than relying on averages. This is incredibly exciting! Not only can we generate new knowledge about how fish live, but this knowledge can give us better information about how to manage fisheries to meet objectives.
With funding from the Great Lakes Fishery Commission, we are testing applications of this technique on Lake Trout and Arctic Char. Our specific goals are:
- Quantify the timing and frequency of Arctic Char migrations between freshwater and saltwater habitats in Nunavut, Canada.
- Determine the timing of feeding divergence between lean and siscowet Lake Trout in Lake Superior.
- Evaluate the ability of individual Lake Trout to respond to changing prey populations in Lake Michigan.
Stay tuned! This project is in-progress, and we are excited about the data coming in so far. I presented some preliminary data in 2022 at the Joint Aquatic Science Meeting in Grand Rapids, MI, and we hope to present this work again at future conferences. Please reach out with questions about the project: crosins1@uwyo.edu
Caroline L. Rosinski 