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Natural Curiosities

burping lake . . . dead snails . . . lady's-slippers . . . potter wasps . . . fluttering swallowtails . . . circle hooks

Last summer I was boating on a lake by shore. Through the water I saw the ground being lifted off. Then there were big, black holes where it came off. What is this?

Julia Brogle
Pine River

Dave Wright of DNR Ecological Resourcessays a layer of organic material (once living things) can build up beneath plants and algae at the bottom of lakes. As the material rots, it produces carbon dioxide, methane, and other gases that might build up enough to lift the layer above it, producing an effect like the one you observed. Another possibility is that plants in the scum layer had enough gas inside their cells to cause the pieces to float. You could say that you saw the lake burp!

We live on Big Fish Lake in Stearns County. One evening I walked down to the lake and was surprised to see hundreds of dark snails floating all over the lake. What was going on?

Deb Killmer
Cold Spring

Over the past decade or so, we have been seeing more and more lakes with annual snail die-offs, says DNR aquatic invertebrate biologist Gary Montz. It's not clear why. One possibility is that many lakes have more nutrients that stimulate plant growth, thus providing more food to support higher snail populations. Recent mild winters might also allow more snails to survive. Eventually the snails start to die from some reason, such as old age, lack of food, overcrowding, or low oxygen at the lake bottom due to warm conditions. After the snails die, decomposition causes gas to build up in the shells, and they float to the surface.

I live across the lake from the Lake Robina Wildlife Management Area. Every year I watch for the yellow lady's-slipper and Orchis plants to bloom. How rare are they, and why don't they bloom some years?

Penny Bailey
Maple Plain

Yellow lady's-slippers can be found throughout the forested regions of Minnesota, although they are not common, says DNR botanist Welby Smith. They are becoming even harder to find as roads and housing developments take over the landscape. A lady's-slipper plant takes an average of 12 years to produce its first flowers. It will ordinarily produce flowers every year after that. Deer often eat the flowers soon after they appear, making it seem as though the plants are not blooming every year.

A few years ago I noticed some clay balls about the size of marbles stuck to the bottom of grape leaves. I picked them off and broke them open. Inside were three or four inchworms, maggots, and small brown worms, a different mix in each ball. Some were still alive. What were these?

Dan Sheild

Sounds like the work of a potter wasp, says DNR insect expert Robert Dana. These wasps lay their eggs individually in pot-shaped nests they make from saliva and clay. The contents you describe are critters the adult wasp stung and then put into the nest as food for its larva -- which is the "still alive" one that looks like a small (well-fed) maggot.

At a public landing on a northern Minnesota lake last summer, I saw several yellow swallowtails lying dead on the sand. Many other swallowtails were attracted to the site, fluttering a foot or two off the ground and then landing on the carcasses. Could some sort of chemical have spilled on the sand that was fatally attracting the butterflies?

S. Anderson

Swallowtails are among several kinds of butterflies that engage in a behavior called "puddling." The males converge on a damp surface that contains salt or other substances they need in their diet. The fact that some of the swallowtails you observed were dead suggests the substance they chose in this instance might have been toxic to them. Whatever it was, it certainly is a good reminder of the importance of disposing of waste liquids properly.

When releasing a fish that has swallowed the hook, is it better to cut the line, leaving the hook in, or to try to remove the hook?

Brian Anderson

Attempting to remove a deeply swallowed hook from a fish can do additional harm by piercing internal organs and destroying tissue, says DNR aquatic education specialist Roland Sigurdson. If the hook is in the stomach or gills, simply cut the line, leaving about 2 inches hanging from the fish's mouth. Most hooks will rust out or pass through the gut after a short time. If you're practicing catch-and-release fishing, Sigurdson suggests you invest in circle hooks. These hooks are designed to catch the fish in the lip area, rather than deeper in the mouth or throat. They may also help young anglers catch fish because you don't need to set the hook when using them.

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