Évaluation du potentiel de rétablissment du ptychobranche réniforme (Ptychobranchus Fasciolaris)

Abstract

In Canada, the current distribution of Kidneyshell is limited to the Ausable River (Lake Huron drainage) and the Sydenham River (Lake St. Clair drainage). Current evidence indicates that the species is extirpated from the Detroit, Grand, Niagara, St. Clair, and Thames rivers, Lake St. Clair, and Lake Erie. Kidneyshell is typically found in moderate to swift-flowing small- to medium-sized rivers or shallow lake areas in sand and gravel substrates. In Canada, Kidneyshell occupies medium rivers in relatively shallow depths (<1 m), moderate to swift currents, and a high proportion of gravel, sand, and cobble substrates. Kidneyshell larvae must encyst on the gills of a suitable host fish to survive and metamorphose into juveniles. The functional host fishes for Kidneyshell in Canada include Blackside Darter (Percina maculata), Johnny Darter (Etheostoma nigrum), and Logperch (P. caprodes), but may include other Percina spp. or Etheostoma spp. and Brook Stickleback (Culaea inconstans). Modeling projections indicate that achieving a 99% probability of persistence over 100 years requires ~5,250 (CI: 2,250–9,750) adult Kidneyshell. At current densities, this would require ~260 ha and ~50 ha of suitable habitat in the Ausable and Sydenham rivers, respectively. Habitat area is likely sufficient in both rivers, but with less certainty in the Ausable River. Positive population growth rates were observed in most sites in both rivers. On average, monitored sites in the Ausable River exhibited 7% growth per year (CI: 2–13%) based on five sample areas in each of three time periods between 2006 and 2019, while sites in the Sydenham River exhibited 13% growth per year (CI: 11–15%) based on 10 sampled areas in each of two or three time periods between 1999 and 2022. Overall, the abundances are interpreted to be stable in the Ausable River, and increasing in the Sydenham River. Kidneyshell populations are generally most sensitive to perturbations in adult survival. They may become more sensitive to perturbations to juvenile survival under higher population growth rate and later maturity. The Ausable River population is predicted to be more sensitive to perturbations in adult survival because of its current lower population growth rate. Dreissenid mussels are the likely cause of extirpation of Kidneyshell in the Great Lakes and connecting channels, including the St. Clair River, Lake St. Clair, Detroit River, and Lake Erie. Agricultural and urban land use, dams, and other historical habitat modifications likely contributed to extirpations in the Thames and Grand rivers. Presently, a number of threats, including agricultural and urban sources of pollution, aquatic invasive species, and climate change are contributing to habitat degradation. Current threats appear not to be causing population-level declines in extant localities, but localized impacts below the population level may be occurring. The cumulative effect of threats and their interactions are poorly understood. Recent research suggests greater impacts from certain combinations of threats than individual threats on their own. Key knowledge gaps remain regarding life history parameters (e.g., age at maturity, juvenile survival); larvae-host relationships; habitat preferences by life stage; population structure, abundance, and dynamics; and the magnitude, spatial extent, and impact of threats (individually and in combination).