Overwintering site fidelity and communal hibernation predispose northern map turtles to mass mortality events

Mass mortality events (MMEs) are rapid and severe population declines that can lead to population extinctions. MMEs appear to be increasing in intensity and frequency; thus, understanding their causes and consequences is imperatives for wildlife conservation. We report on a MME that affected a population of northern map turtles (Graptemys geographica) in Ontario, Canada that had been monitored since 2003. At a communal overwintering site, 142 carcasses, or approximately 10% of the estimated population, were recovered in April 2022, seemingly depredated upon by river otters (Lontra canadensis). We compared the size and sex of the carcasses to those of turtles that survived the MME. Smaller turtles, and males, were more likely to be depredated upon than larger turtles and females. Moreover, smaller turtles were more likely to have had their shell crushed than larger ones, which tended to have an intact shell, but were more likely to be dismembered. Using captures made at the same overwintering site between 2003 and 2022, we assessed the vulnerability of this population to MME during winter. Nearly 60% of the 1875 turtles marked between 2003 and 2022 used the overwintering site where the MME occurred at least once. On average, individual turtles were detected overwintering at the site 38% of the winters between their first and last capture. The concentration of turtles at a single large overwintering aggregation site combined with high overwintering site fidelity predisposes northern map turtle populations to MME through fidelity- induced ecological traps. Identifying and protecting overwintering sites and mitigating anthropogenic pressures that may mediate predator access will thus be essential for the preservation of certain turtle populations.