Aquatic environments globally are experiencing increases in intense modification from human activities, which can result in severe consequences for freshwater fish. One key stressor is that of turbidity, which is associated with increased urbanization and run-off, as well as increased anthropogenic nutrient inputs. While elevated turbidity is likely to result in negative physical consequences for fish, it is also likely to result in disruptions to sensory mechanisms of species in these ecosystems. Changes to the visual environment from increased turbidity are expected to result in disrupted visual ecology and are hypothesized to lead to community-level shifts in freshwater ecosystems; however, the proximate mechanisms underlying such shifts and the implications for populations and ecosystems remain to be investigated.
My main objective was to determine the effects of elevated turbidity on the visual ecology of native Lake Erie fishes. Turbidity influences visual abilities differently across different types of turbidity (e.g., algal vs. sedimentary), as well as within and across trophic level (e.g., planktivores vs. piscivores). Due to its relatively shallow bathymetry and high levels of urbanization and anthropogenic inputs, Lake Erie is one system that has been experiencing high levels of both sedimentary and algal turbidity in recent years. I tested if elevated sedimentary and algal turbidity alter the visual ecology of two Lake Erie fishes, a forage fish, emerald shiner (Notropis atherinoides), and a top predator, walleye (Sander vitreus). The specific objectives of this dissertation were (1) to determine the differential effects of elevated turbidity on visual sensitivity through assessment of the optomotor response, (2) to determine the effects of elevated turbidity on visual acuity using reaction distance tests as a behavioral proxy, (3) to determine the effects of elevated turbidity on prey consumption by emerald shiner, (4) to test for long-term (i.e., decadal) changes in visual morphology of emerald shiner associated with prevailing environmental conditions, and finally, (5) to determine how elevated turbidity, particularly algal turbidity, influences lure color successes in the recreational walleye fishery using a citizen science approach.
I used a suite of visual ecology tools to test the species-specific effects of both elevated sedimentary and algal turbidity. First, to test for differences in visual sensitivity between turbidity types and species (Chapter 2), I used the innate optomotor response to determine the turbidity levels at which individual fish could no longer detect a difference between a stimulus and the background (i.e., visual detection threshold). I predicted that visual detection threshold in algal turbidity would be lower than detection threshold in sedimentary turbidity. Detection thresholds were significantly higher in sedimentary compared to algal turbidity for both emerald shiner (meansediment ± se = 79.66 ± 5.51 NTU, meanalgal ± se = 34.41 ± 3.19 NTU) and walleye (meansediment ± se = 99.98 ± 5.31 NTU, meanalgal ± se = 40.35 ± 2.44 NTU). Second, I tested if elevated algal and sedimentary turbidity influenced visual acuity using reaction distance, or the maximum distance at which a fish can see and react to a stimulus, experiments (Chapter 3). I predicted a decrease in reaction distance in both types of turbidity; a more severe decrease was anticipated in algal turbidity compared to sedimentary turbidity. In both emerald shiner (n=40) and walleye (n=27), reaction distance across turbidity type (sedimentary, algal, sedimentary + algal; 20 NTU) was approximately 50% lower relative to the clear treatment. Reaction distance was further reduced in algal compared to sedimentary turbidity for wild-caught walleye. Finally, I tested for a link between prey consumption, turbidity type and level, and visual morphology with the expectation that consumption of prey items would be suppressed in all types of turbidity (Chapter 4), however, I expected that consumption in elevated sedimentary turbidity would be greater than in algal turbidity due to the differing nature of these turbidity types. I found prey consumption was highest in moderate (20 NTU) sedimentary turbidity, while consumption was suppressed at high (40 NTU) algal turbidity.
In Chapter 5, I describe a correlative study that tested if there is a relationship between the relative size of visual morphometry (i.e., eye and optic lobe size) and prevailing water clarity conditions in Lake Erie using historical samples extending back nine decades to 1928. I expected to find a positive relationship between increase size of visual morphological features and turbidity, with increased eye size found in those decades of increased turbidity. I found a strong negative correlation between algal turbidity intensity and eye characters (eye diameter, pupil diameter, and axial length) during 1928-2018. Further, in a fine-resolution examination of yearly emerald shiner during 2010-2018, I found that the relationship between harmful algal bloom severity and visual features was not as strong; however, I did find a relationship between elevated turbidity and decreased size of eye characteristics the subsequent year.
Lastly, to connect my work on the visual ecology of walleye and the Lake Erie recreational fishery, I developed a mobile phone application, Walleye Tracker. This `app’ was used by 19 charter captains in 2017 and 2018 to gather real time photographic data on lure successes under different water clarity conditions, as calculated from submitted photographs combined with a suite of environmental data (Chapter 6). I hypothesized that lure colors that were successful in clear water would differ from lure colors that were successful in turbid water. I found that in moderate and high levels of turbidity, depth at which walleye were caught was significantly shallower (mean ± SE = 5.46 ± 0.323 m) than in clear or low turbidity conditions (mean ± SE = 10.94 ± 0.625 m), regardless of turbidity type (i.e., algal vs. sedimentary). Discriminant function analysis was used to determine that in clear conditions white lures were the most successful, in sedimentary turbidity yellow lures drove catch successes, and in in algal conditions black lures were the most successful.
Overall, my research demonstrates that while both turbidity types studied are likely to have detrimental impacts on fish sensory physiology, algal turbidity disrupt visual ecology more and at lower levels compared to sedimentary turbidity. My work highlights the importance of understanding the species-specific responses of fish to anthropogenically altered turbidity regimes in impacted freshwater systems. Not only is turbidity a stressor that acutely influences visual abilities of fishes (i.e., visual sensitivity, visual acuity, prey consumption), it can also can have chronic lasting consequences (i.e., morphological development) for fish populations. This can lead to distinct population changes but can also influence the recreational fishery that relies on these fish populations.