Char-Sighted: Near-Shore vs. Off-shore Predators: Morphotypical Variation in Vision?

Within Lake Superior there are currently 4 different morphotypes of Lake Trout identified (Figure 1); morphotypes are the same species, but show phenotypic and genotypic variation. We wanted to investigate whether the 2 morphotypes, Siscowet and Lean Lake Trout, had adapted different visual properties owing to their different habitats (Figure 2). The Lean Lake Trout is a littoral or near-shore morphotype that occupies water less than 80 meters deep and historically has less body fat. In contrast, the Siscowet Lake Trout is a pelagic (off-shore) morphotype that occupies waters greater than 80 meters and can have up to 70% body fat. Both are apex predators within their respected zones.

Figure 1. The 4 morphotypes of Lake Trout within Lake Superior

The two morphotypes also display different behavior. Lean Lake Trout perform Diel Bank Migration (DBM) where they move into shallower, near shore waters in the evening, while Siscowet perform Diel Vertical Migration (DVM) characterized by moving higher in the water column at night (Figure 3).

Figure 2. Characteristics of Lean vs. Siscowet Lake Trout

With the help of data published in a previous study entitled Visual Sensitivity of Deepwater Fishes in Lake Superior by a former lab mate, Kelly Harrington (photo), we were able to compare the visual properties of Siscowet Lake Trout to the data we collected on Lean Lake Trout. Additionally, L.J. Rogers collected Lean Lake Trout with the help of the DNR (photo), helped analyze and collect the data and was able to complete a UROP project (Undergraduate Research Opportunities Program).

Figure 3. Diel (Daily) migration patterns for Lean and Siscowet Lake Trout within Lake Superior. A, Day; B, Night.

For this study we wanted to determine which colors of light Lean Lake Trout see and at what intensities. To do this, we conduct electroretinography, or ERG, for short. These ERG's essentially tell us when neurons are firing in response to a color and intensity of light; we can then quantify based on the response. The ERG setup is outlined below (Figure 4 and in short, involves flashing monochromatic (one color) light at different intensities and recording the neural response.

Figure 4: Schematic drawing of electroretinography setup. From left to right: (A) Constant current power supply, (B) electric shutter, (C) quartz-tungsten halogen lamp, (D) neutral density filters, (E) monochromator, (F) Faraday cage, (G) fiber optic light pipe, (H) recording electrodes, (I) signal amplifier (G-I within circular enlargement), (J) PowerLab, (K) personal computer, (L) chilled water lines.

Left: L.J Rogers holds a Lean Lake Trout he collected this past fall with the help of the MN DNR. Right: L.J. Rogers, Kelly Harrington and myself taking a break from conducting ERG trials.

When we analyzed our Lean Lake Trout data and compared to Siscowet data in Harrington et al. (2015), we see that there are some differences in visual sensitivity between the two morphotypes. Lean Lake Trout have peak sensitivity at 550 nm (green) while Siscowet have peak sensitivity at 525 nm (blue-green) (Figure 5). We also see that Leans have green-shifted vision with higher sensitivity to the upper wavelengths. In contrast, Siscowet are blue-shifted and show higher sensitivity to the blue range of wavelengths.

As blue green wavelenths penetrate deepest in freshwater, this supports the sensitivity hypothesis that a fish will be most sensitive to the light available in its environment. The siscowet is most sensitive to 525 nm blue-green light which is the only light available at depth. In shallower waters, the green wavelengths have yet to attenuate and can be sued by a visual predator like the Lean Lake Trout. Green wavelengths also penetrate furthest in turbid waters and may aid Leans in shallower, wave agitated areas.

Figure 5.Visual sensitivity of Lean and Siscowet Lake Trout.

By determining the lowest intensity light each morphotype can detect, we can equate this to what depth a fish might be able to see. We determined that Siscowet and Lean Lake Trout have the same depth profiles for vision and both are able to detect light ~350 m within Lake Superior in the summer. In fall, with an increase in lake turbidity, vision is limited to ~100 m. This data suggests that while Siscowet and Leans have evolved different sensitivities to different wavelengths, they have yet to develop absolute sensitivity differences.