Evaluating the Ecological Benefits, Risks, and Feasibility of Reintroduction of Eastern Sand Darter in Ontario

Reintroduction is identified as a potential recovery measure for the Ontario designatable unit of Eastern Sand Darter (Figure 1; Ammocrypta pellucida; listed Threatened under the Species at Risk Act) due to extirpations from three formerly occupied watersheds. There is uncertainty about how reintroduction could change the survival or recovery of the wildlife species (hereafter species), as well as the ecological feasibility and risk of undertaking translocations from wild populations. The implications of several translocation scenarios for the probability of species extirpation were considered. The successful addition of a reintroduced population lowered species extirpation probability. Models were developed to evaluate potential abundances in source populations, the effect of removing individuals from source populations, and the potential for population establishment in recipient locations. Habitat suitability and threats were also described for recipient locations. The Grand and Thames rivers were considered as potential sources, with model predictions suggesting that Eastern Sand Darter abundance is potentially higher in the Grand River than the Thames River. Model outputs were highly dependent on the assumed availability of suitable habitat and density of fish in waters deeper than 1.2 m. Abundance and the probability of catastrophic events were important variables for predicting harm from removals. Source rivers with higher abundance were better able to withstand removals; however, even without removals, source abundances can fluctuate and fall below thresholds for harm. The total number of individuals released and the carrying capacity in recipient rivers were important variables for predicting reintroduction success; success was most likely to occur with a larger number of additions and a higher carrying capacity in recipient rivers. Sand and fine gravel substrate, as well as water clarity, were previously identified as important factors for the occurrence and abundance of Eastern Sand Darter, with predicted occupancy of Eastern Sand Darter in the Ausable River and Big Otter Creek. Substrate sampling at potential release sites in Big Otter Creek suggest relatively high proportional cover of sand and fine gravel in areas where Eastern Sand Darter previously occurred. The effects of agriculture and Round Goby (Neogobius melanostomus) invasion were identified as important threats that may influence the successful reintroduction of Eastern Sand Darter in potential recipient rivers, but these threats also occur where the species is extant and in good condition. Cumulative threats were not assessed. Disease was also identified as a potentially important consideration. Model outputs and the broader evidence base were considered in a structured expert judgement process (modified Delphi) to evaluate factors related to the overall ecological feasibility and risk of unintended ecological consequences of translocation. Structured expert judgements were generally consistent with model outputs, but experts indicated substantial uncertainty about most factors associated with the ecological feasibility of translocation. Generally, there was greater agreement that: 1) the Grand River could likely withstand low levels of removal (250 fish per year for up to 10 years), and 2) life history, genetic factors, competition/predation, and current threats (Round Goby, agriculture) would likely not impede establishment in recipient locations. The availability of habitat and food supply to support larger abundances of fish in recipient rivers had less agreement, though these factors were unlikely to impede smaller numbers of fish from establishing, with slightly greater confidence around the habitat suitability and availability in Big Otter Creek. The risks of unintended ecological consequences of translocation to source and recipient rivers were deemed to be generally low to moderate by experts, but with some uncertainty. Genetic risks (inbreeding depression, outbreeding depression, founder effects) were of relatively low concern given the proposed experimental reintroduction, but were contingent on where and how fish were collected from source rivers. Transformational ecosystem changes were considered unlikely to occur. The potential for disease introduction was deemed to be low, but if introduced, novel pathogens could have significant impacts. Generally, expert judgements were less certain on factors related to ecological feasibility, including source abundance, the effect of source removals, and abiotic and biotic habitat availability, and were more certain on factors related to the risk of unintended ecological outcomes in source and recipient ecosystems. However, the expert perceptions of potential introduction and ecological consequence of disease had higher uncertainty.

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