Turtles killed by Winter Kill in Toronto
On a spring visit to Milliken Park in Toronto, we were shocked to come upon a number of dead turtles along the shoreline of the pond that had been killed by what is called Winter Kill.
The pond at Milliken Park is fairly shallow according to members of a park’s work crew that we spoke to on our last visit there. They said that, in all likelihood, due to the prolonged sub-zero temperatures this past winter, the water would have frozen solid almost to the bottom of the pond reducing the amount of oxygen required by the turtles to survive.
Last summer, Milliken Park was home to a large number of turtles including native Painted Turtles and non-native Red-Eared Slider Turtles.
It was a shock this spring to find close to a dozen dead Red-eared Sliders lying in the water or along the edge of the shore. Our first question, of course, was what might have killed these reptiles.
To get a more conclusive answer, we contacted Adopt-A-Pond Wetland Conservation at the Toronto Zoo. This organization is working to collect, record and store location and species information on turtles in Ontario including species at risk.
Within a day, we heard back from Julia, the Program Coordinator at Adopt-A-Pond. She explained that, due to the extremely harsh winter across Ontario, a large number of turtles had been killed by what biologists call ” Winter Kill “. She said that, “Many other people are reporting similar circumstances as yourself. As the snow melts and the water levels return to normal, these dead turtles are exposed. This will similarly affect other species of animals such as fish. On the bright side, this leaves less competition for the animals that did survive, and their ‘hardy’ genes will be passed on to future generations. I will make a note of this in our program Ontario Turtle Tally, which monitors turtle populations and is used for conservation research. Thank you for your sighting and for your concern.”
Julia explained that many ponds across Ontario such as this one at Milliken Park in Toronto normally do not freeze to the bottom in winter. She said “normally there exists a large area of water beneath the ice which is oxygen-rich. As the winter gets colder, this area shrinks as does the oxygen”. Julia went on to say, “the record-breaking temperatures of this past winter resulted in some small wetlands such as this pond completely freezing, and killing everything in the area.”
Bob and I were witness to the fact that a good number of the Koi fish in Milliken Pond were also dually affected by the lack of oxygen caused by the thick ice. This one lay at the shoreline, but others could be seen floating near the bottom of the pond.
On the internet at Nature North, we learned that, in Manitoba, large numbers of frogs were killed by Winter Kill in 2012. That spring, biologists out there came upon large numbers of dead Salamanders and close to 300 dead adult Leopard Frogs that had sought winter refuge in a pond. They were all killed by the freezing over of the pond.
As Nature North explains, “By the time ice finally forms over the pond, the water is usually quite rich in oxygen, but that is often its oxygen complement for the winter. Unless there are plants that receive enough light under the ice to produce more oxygen, the dissolved supply of this vital gas is fixed and will only decline from the point in time when the ice sealed off the water from the air. Gas exchange cannot occur through ice. Most aquatic plants die-back for winter anyway and can actually add to the winter kill problem as their decomposing parts are consumed by animals and bacteria, using up oxygen in the process.”
So, as disturbing as it was for Bob and me to find close to a dozen dead turtles at Milliken Pond this spring, it is not so unusual as to warrant concern.
Julia reminds us that, even with all the loss, there is still a bright side to the story. The unfavorable competition for nesting locations and food caused by the proliferation of non-native species such as Red-eared Sliders has now been put in check and will perhaps lead to a healthier population of native species that were being threatened.
Bob and I hope that none of the Painted Turtles are found belly side up. The chances are pretty good because this native species is specially adapted to survive without oxygen for extended periods of time. They can alter their metabolism and can remain submerged for up to 3 months with zero blood oxygen. This is why it is not wise to release pet turtles into the wild. They are ill-equipped to deal with Canadian winters. To report a sighting of either living or dead turtles across Ontario, contact Adopt-A-Pond Wetland Conservation
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