Seasonal and environmental effects on physiological tolerances and gill adaptations of imperilled Eastern Sand Darter (Ammocrypta pellucida)

Abstract

In the absence of accurate biological information, conservation and recovery strategies carry the risk of failing to meet their objectives. Understanding species-specific physiological tolerances is important for several aspects of conservation, such as better understanding habitat requirements, determining the significance of future environmental change, and evaluating the suitability of recovery activities such as reintroduction. Currently, there is relatively little scientific information on the physiological tolerances of Eastern Sand Darter (Ammocrypta pellucida), a small benthic fish listed as threatened under the Species at Risk Act in Canada. The goal of this thesis was to define temperature and oxygen tolerances of Eastern Sand Darter and determine the role of temperature and turbidity, two common environmental stressors, on these tolerances. I also aimed to determine the response of three co-occurring darter species to the same environmental stressors. All objectives were conducted to evaluate physiological thresholds, thereby improving our understanding of species habitat, which allows us to identify restoration priorities and evaluate the suitability of candidate sites for reintroduction. To achieve this goal, field trials were conducted between June and November 2019 in the Grand River, Ontario, to encompass a range of ambient water temperatures (7-25℃) and August 2020 in the comparatively more turbid Thames River, Ontario. Critical thermal maximum (CTmax), agitation temperature (Tag), hypoxia tolerance (loss of equilibrium, LOE; and critical oxygen tension), and metabolic rate were measured to test the effect of seasonal temperature change and turbidity. Gills were collected from incidental mortalities of Eastern Sand Darter, and from three co-occurring darter species (Blackside Darter, Percina maculata; Greenside Darter, Etheostoma blennioides; and Johnny Darter, Etheostoma nigrum) to assess gill morphometrics. Chapter 2 demonstrated that Tag and CTmax significantly increased linearly with water temperature but there was no significant influence of turbidity. However, there was a significant interaction between turbidity, mass, and CTmax, indicating that turbidity has a hypothesized, indirect effect on CTmax by reducing growth. Overall, study results better define the sensitivity of Eastern Sand Darter to temperature and turbidity fluctuations and provide a better understanding of suitable thermal habitat. In Chapter 3, I demonstrated that temperature alone had no impact on metabolism but significantly and positively impacted hypoxia tolerance metrics (Loss of equilibrium and critical oxygen tensions). However, temperature explained very little of the variability seen in the response of metabolism and both hypoxia tolerance metrics across seasons, while environmental and fish-specific factors (reproduction and condition) explained more of the variation. Overall, temperature may be a weak predictor for Eastern Sand Darter metabolism and hypoxia tolerance, but the identification of a relatively high hypoxia tolerance helps to explain why the species persists in sand substrates that may have low availability of oxygen. In Chapter 4, I determined that turbidity significantly increased gill interlamellar space, filament width, and loss of equilibrium (i.e. decreased hypoxia tolerance) but had no significant influence on any other gill morphometric measurements, or on species metabolism. The findings suggest that turbidity has limited impact on Eastern Sand Darter gill physiology and that current turbidity levels do not surpass most physiological thresholds of Eastern Sand Darter. Chapter 5 demonstrated that temperature and turbidity have limited impact on the gill morphology of three common darter species, only impacting ionocyte number, lamellae width, and hematocrit. Findings suggest that the three darter species have limited plasticity in gill morphology under the range of temperature and turbidity tested. Having data on other darter species lets one put Eastern Sand Darter in context of other members of the ecosystem that are phylogenetically related. Overall, my thesis provides information on the physiological tolerances of Eastern Sand Darter that can be used to inform species recovery under Canada’s Species at Risk Act, such as refining the description of critical habitat, identifying habitat restoration objectives, and evaluating the suitability of candidate sites for reintroduction. Additionally, the discovery that current turbidity levels have limited impact on Eastern Sand Darter physiology allows for a refined understanding of how siltation impacts the species survival.