The impact of roadway mortality on a long-lived freshwater turtle, Chrysemys picta bellii

Description

Road mortality poses a significant threat to freshwater turtles globally, including the western
painted turtle (Chrysemys picta bellii). Even low levels of road mortality can severely impact the
long-term survival of these long-lived reptiles. While mitigation infrastructure is employed to
reduce mortality, its effectiveness may vary depending on the location and species involved.
Population viability analysis can be a valuable alternative for wildlife managers to assess the
viability of the populations they monitor and the potential impact of management decisions, but
are most useful when data from target populations is available. This research project aimed to
evaluate the long-term persistence of a rural western painted turtle population experiencing road
mortality. The study first investigated the effectiveness of a turtle tunnel and drift fencing in
reducing road mortality. Parameters derived from the population were then used to model the
long-term viability of the turtle population in the program Vortex under various scenarios of
ongoing road mortality. This study analyses three years of mark-recapture, ratio telemetry
tracking, and road mortality surveys done by contractors and summer hires of the Ministry of
Water, Land, and Resource Stewardship at Baynes Lake, BC. The turtle population was
estimated to be 1,451 (SD= 75) individuals, with a female-biased sex ratio (1.5 females: 1
male). Only a quarter of sexually mature females were found to be gravid in a given year, with
gravid females laying only one clutch. Subadult and adult survivorship were calculated using the
same dataset employed for the population estimate, resulting in a survivorship rate of 96.4%
(SD = 1.3%) per year. Radio telemetry was used to assess the number of road crossings made
in a given year. Gravid females were found to make a higher average number of road crossings
(1.6 road crossings per season) than males (0.1 road crossings per season) or non-gravid
females (0.7 road crossings per season). In 2021, an under-road turtle tunnel was installed in
Baynes Lake, followed by the installation of drift fencing in the spring of 2023. Over time and
after the implementation of drift fencing, more turtles were observed encountering the installed
turtle tunnel. However, evidence regarding the complete traversal of the turtle tunnel was not
conclusive. Before the installation of the turtle tunnel, casual community-led surveys in 2021
estimated a mortality rate of 1% of the population. Mortality rates, calculated over the duration of
the survey periods, decreased after the installation of the turtle tunnel (0.1 mortalities per day in
2022) and further decreased after drift fencing was installed (0.03 mortalities per day). The
long-term viability of the Baynes Lake population was modeled in Vortex, with additive road
mortality modeled as a percentage loss of the population. The road mortality rate of 1% additive
mortality, seen before road mortality reduction strategies, resulted in a 60% decrease of the initial population after 100 years. This indicates that even low levels of additive adult road
mortality can lead to declines in the Baynes Lake western painted turtle population. Sensitivity
testing of the model indicates that the Baynes Lake population is highly sensitive to changes in
adult/subadult and juvenile mortality. Pre-mitigation mortality levels had the potential to
dramatically impact the population of Baynes Lake. It is crucial to acknowledge and address
what may appear to be relatively low levels of roadway mortality in rural areas, as even these
seemingly modest rates can have a significant impact on western painted turtle populations., Western painted turtle, Chrysemys picta bellii, roadway mortality, population viability modeling, population estimate, scavenge rate