Mass Mortality in a Community of Headstarted (Emydoidea blandingii) and Naturally Occurring (Chrysemys picta marginata) Freshwater Turtles in Protected Urban Wetlands

Turtles experience numerous threats and high mortality in urban areas. Mass-mortality events (MMEs) are localized, sudden events resulting in a catastrophic increase in mortality rate. Turtles are susceptible to MMEs because their long generation times do not permit density-dependent compensation. We retrospectively investigated an MME in 2020 that affected two species, headstarted Blanding’s Turtles ( Emydoidea blandingii ) and naturally occurring Midland Painted Turtles ( Chrysemys picta marginata ) in Rouge National Urban Park (RNUP) in Ontario, Canada, where we monitored the freshwater turtle community from 2014 to 2022. In 2020, 48 juvenile headstarted Blanding’s Turtle and 57 Midland Painted Turtle carcasses were found, most (75%) of which were females. The turtles were likely depredated by Raccoons ( Procyon lotor ) or American Minks ( Neogale vison ). Documented mortalities were highest in release sites for headstarted turtles, possibly due to increased monitoring efforts at those sites. Conservation initiatives such as headstarting are useful to recover turtle populations, but MMEs may disrupt and delay population stabilization, especially in areas where other threats (e.g., subsidized predators, road mortality) are prevalent. We emphasize the importance of long-term monitoring of turtle populations, especially after conservation interventions, to detect challenges that affect their persistence.