Winter Kill!

Winter Kill

By Doug Collicutt

Death can teach you a lot about life, especially in nature.

Most animals spend their lives trying NOT to be seen by predators. Being noticed, getting seen, gets you eaten. So most of the time when we humans go for a walk in nature we end up being oblivious to the many animals all around us; they see us, but remain hidden. Death often strips away that invisibility and can provide surprising insight into the biology of species and the natural history of the world around us.

In mid-March of 2012 I got an email from biologist Laura Reeves, who lives near Gardenton, Manitoba. This was after our strange, early warm spell that set record high temperatures across southern Manitoba and melted all our snow and the ice on most ponds. Spring was here over a month early! Laura had visited one of the local ponds, known to be a breeding site for Eastern Tiger Salamanders, and had disturbing news. The pond was full of dead Leopard Frogs and she even found some dead larval Tiger Salamanders. She sent me pictures of the carnage along the pond shore, hundreds of dead Leopard Frogs and, scattered here and there, dead larval Tiger Salamanders. What had happened?

Well, it wasn't really much of a mystery to either of us, the pond had suffered a winter kill. Over winter the oxygen in the water had been depleted, and all those living things in the pond dependent on oxygen had died. Sadly, winter kill is not that unusual in small bodies of water in Manitoba, or elsewhere for that matter. Anywhere ice covers small water bodies for a substantial length of time, winter kill can occur. Large water bodies, big lakes and rivers are not completely immune to winter kill, but it would take unusual circumstances to produce a winter kill in a large water body.

Winter Kill

So, what happens to cause a winter kill? As a pond cools in autumn, of course the water temperature drops, but wind and wave action and the mixing they cause actually increase the oxygen content of the water. Remember your high school chemistry, more gases can dissolve in cold water than in warm water. By the time ice finally forms over the pond the water is usually quite rich in oxygen, but that is often it's 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. Animals like Leopard Frogs and larval salamanders don't use very much oxygen as they lie dormant in the cold water, but they must have some available oxygen or they suffocate and die.

When Laura had let me know about the winter kill and the presence of dead Tiger Salamander larvae I had immediately thought, "Here's a chance to survey ponds in that locale for more dead salamander larvae!" (I was counting on the fact that when things die in water they usually float to the surface, then accumulate along the shore and are easy to spot. Icky, yes, but an effective survey tool no less.) Eastern Tiger Salamanders have only recently been re-discovered in this part of Manitoba and, so far, only three ponds down around Gardenton have been confirmed as sites where this species breeds. If other ponds had experienced winter kills it presented an opportunity to find other dead larval salamanders and expand our knowledge of the range of this species in Manitoba.

Manitoba hasn't done a great job of keeping tabs on a lot of its wildlife species, outside of the ones people like to shoot or hook. That's why NatureNorth started up the Manitoba Herps Atlas, to try and get regular Manitobans collecting data about our biodiversity. Check out the MHA at this link: Go to the Manitoba Herps Atlas.

So I packed up my gear and headed to Gardenton where Laura and I visited a number of ponds in the area, gathering information provided by the winter kill, trying to make the best of a bad situation. We visited the known Tiger Salamander breeding ponds first. The one where Laura had found all the dead critters looked even worse when I saw it. The water level was continuing to drop, dried out frogs and salamanders ringed the drying mud around the pond. The near-drought of the previous summer and fall was, no doubt, a large factor in the winter kill. The pond had gone into winter with very little water and hence far less available oxygen to begin with. It seems this pond had also gone into winter with a very large populations of frogs.

I was struck by the sheer number of Leopard Frogs that lay dead around the edge of the pond. We didn't do a formal count, but I estimated there were more than 300 adult Leopard Frogs that had sought winter refuge in that pond, a pond which was only about 15 m across and perhaps no more than 1 m deep in the middle. Winter kill had provided us with a quick estimate of how many frogs could live around and depend on such a small body of water. The presence of the dead salamander larvae also confirmed an aspect of that species' biology, perhaps for the first time in Manitoba: that larvae of Eastern Tiger Salamanders can overwinter. I had thought that the young from eggs laid in spring were all transforming to adult form by fall. Any ponds where large larvae had been found in previous years had been empty of larvae by the end of the year. It's pretty well established that their cousins, the Barred Tiger Salamander, found west of the Red River in southern Manitoba, regularly have larvae that overwinter before transforming to adult form and moving to a terrestrial life. However, it's an aspect for which we previously had no evidence for in Eastern Tigers. Now we had a direct observation on record.

Tiger Salamander larvae can remain as larvae for a year or more before transforming to adult form and leaving the pond. Sometimes they even stay in their larval form with external gills and tail fin and attain sexual maturity, a condition known as "neoteny".

Another thing had startled us on our arrival a the pond, the sight of something still alive and swimming about. I guess I shouldn't have stated before that everything in the pond was dead. We found ourselves watching the bobbing head of a Painted Turtle for a few seconds before it ducked below the surface and disappeared. A turtle had survived the anoxia that had killed nearly everything else in the pond. How could that be? Actually, according to fairly well established turtle biology, it wasn't that surprising either. Painted Turtles spend winter under the ice just like the Leopard Frogs, but they have an amazing tolerance to anoxia. Once oxygen is depleted they can use anaerobic metabolism to produce energy in their bodies to stay alive. I had just never encountered such a graphic demonstration of their ability to go without oxygen. Later that day I stumbled across, and almost into, another small pond full of dead frogs only to witness another large Painted Turtle swimming about happy and healthy. The first sighting wasn't a fluke, they really could survive over winter in a tiny pond with no oxygen!

Winter Kill

The other thing that struck me about the turtles, and added to the kit of knowledge I was acquiring that day, was that there were turtles in such small ponds so far from any large permanent water bodies. The Roseau River was more than 5 km away in both cases. While it is known that Painted Turtles, and Snapping Turtles, too, for that matter, do get up and go some distances over land to find new water bodies to live in, I assumed that they would choose larger ponds or small lakes to inhabit. The second pond I found with a resident Painted Turtle was only 3 m across, 15 m long and no more than 1 m deep.

Now, I can't be absolutely certain that both these turtles had spent the winter in those particular ponds. The larger ponds in the region and the Roseau River were well open by this point, so it's not inconceivable that turtles could have trekked out across the land in mid-March and made it to these isolated ponds. (After all they had to have done it at some point in their lives!) I just think that it's more likely that I really did find them where they had spent the winter.

Anyway, back to salamanders. We visited both the other known Tiger Salamander breeding ponds. One was just a dry patch in a gravel pit this year; it was about a 1/2 acre pond last spring. At the third pond we found evidence of winter kill again, dead frogs and fish, but happened upon some live adult salamanders getting ready to breed. It was broad daylight in early afternoon and there were 3 or 4 adult salamanders cavorting about. That's unusual as we had never seen adults in the ponds before when it wasn't night time and completely dark. Salamanders don't like being out in the open and generally shun bright light, except, perhaps, when it's the start of the breeding season? Something else we else we learned that day.

We had also used the day to try out some underwater filming and had some great success. You can see the results on NatureNorth.com's YouTube Channel, check out the Tiger Hunting video. We got video of Eastern Tiger Salamanders in a pond on March 22. Laura had seen a live adult at the other pond on the 19th. These are likely the earliest dates that have been recorded for breeding activity of this species in Manitoba.

After visiting ponds around Gardenton with Laura, I went off by myself further east. Just about every pond I checked out had suffered winter kill. I think I was at about 20 different ponds and I estimated I saw over 1000 dead frogs and at least as many dead fish. At one tiny pond in a gravel pit I counted more than 100 dead Leopard Frogs. The pond, or perhaps I should have called it a deep puddle, measured only 4 m by 2 m and was about 1/2 m deep! I was piling up lots of location records of sites where Leopard Frogs had TRIED to overwinter, but didn't find any more with dead salamander larvae. The day was also turning into an abject lesson on why most of our amphibian species don't overwinter in small ponds; it's a risky proposition!

The Green Frog, Mink Frog, Leopard Frog, Mudpuppy and larval Tiger Salamanders are the only amphibians that overwinter under water. Green and Mink Frogs do so in larger water bodies in eastern Manitoba, Mudpuppies stick to rivers and large lakes; so these three species don't likely have to worry about winter kill. It's mainly Leopard Frogs and salamander larvae that face this problem. The rest of our frogs and toads, along with adult salamanders overwinter on land. The toads and salamanders burrow down below the frost line while the Wood Frog, Treefrogs, Boreal Chorus Frog and Spring Peeper survive winter on the surface of the ground, freezing solid under the snow, then thawing again in the spring. (Some other overwintering articles in NatureNorth: Frog-cicles, Coping below Zero, Frozen Alive!)

Winter Kill

Of course, the day wasn't all just about documenting death. The live turtles were a pleasant surprise; there was lots of birds around: wild turkeys, sandhill cranes and various returning migrants. There was a number of butterflies flitting about, mainly Mourning Cloaks (see Mourning Cloaks), and pussy willows (see Pussy Willows) were popping out, too. Given the warm temperature that day, I had expected to see some Plains Garter Snakes out, but didn't. That seemed a little odd, but I reminded myself that even though the snow was gone it still takes a while for the ground to warm up enough for hibernating snakes to emerge. Later that week, Laura did report that she had seen a road-killed snake, which she figured had died the day we were out or perhaps the following day, but I had expected to see more of them.

Winter Kills Good For Amphibians?

In the long-term, occasional winter kills may be good for amphibian populations in general, even if they are bad for Leopard Frogs. Fish are a definite problem for breeding amphibians. Eggs and larvae (or tadpoles) are gobbled up as nutritious food. Many small fish species have an amazing ability to spread to seemingly isolated bodies of water. As short-term-thinking humans we forget that in times of high water, spring melt or after heavy rains, it's not that unusual for streams and rivers, even ditches, to overflow their banks and spread water across the land. And where the water goes, so do the fish or perhaps even their eggs. It's not unusual to find several varieties of small fishes, such as Mudminnows, Stickleback or Creek Chub, in ponds or dugouts far from any permanent waters. Sometimes people may be stocking these ponds, intentionally or unintentionally, by transporting water or water plants from one site to another. I suppose it might even be possible for birds or other animals to act as vectors for fish migration, transporting eggs on bits of vegetation or perhaps even accidentally dropping a meal of fish in the pond as they fly over? However they got there, a healthy population of small fish in a pond or dugout is bad news for amphibians trying to use that pond for reproduction, so knocking off the fish population in a pond likely benefits local amphibians in the long run. Remember, most amphibians that would use such a pond for breeding don't overwinter there. In the short-term the few species like Leopard Frogs that do overwinter in the pond are going to be wiped out, but new Leopard Frog colonizers will find their way to the pond, hopefully faster than fish populations can re-establish. And in the mean time all other amphibian species will benefit from a fish-free environment.

So, one lovely, unusually warm day in March spent driving and wandering around in southeastern Manitoba ended up providing me with a lot of new knowledge about some of my favourite critters. Laura and I witnessed first hand the effects of winter kill in ponds and how widespread the die-off had been in that region. We documented overwintering use of tiny ponds by Painted Turtles, but didn't manage to extend the known range of Eastern Tiger Salamanders beyond three small ponds, though we did get some cool video. Perhaps the most important thing I learned that day was that, as a biologist, I have to get out of my office and onto the land to truly learn about nature. Despite being triggered by winter kill and death, it turned into a grand day of witnessing life in Manitoba!

Bye for now.

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