February 2, 2013

The View from Under the Ice

By Margaret Gillespie

            There was once a class at the Science Center focused on exploring life under the ice. Looking behind the scenes, preparations for this class involved staff going out the day before, lugging an auger to drill a hole through about two feet of ice on Little Squam Lake, preferably over a deep spot. Yes, you guessed it – deep spots are often far from shore and the snow is always deep! A cardboard box secured over the hole kept the opening from freezing overnight. The day of the class might dawn bright and sunny but bring with it a sweeping northwest wind kicking up snow and guaranteed to penetrate with ease anything a middle school student might choose to wear. Most of the scientific equipment involved the instructor, at the very least, to be gloveless. One positive aspect of the class shone through – the driving enthusiasm of the students to get their assignments finished! Another aspect actually stopped them in their tracks. Pulling the plankton net up through the ice, we discovered tiny animals – crustaceans and rotifers – interrupted from their busy lives to be scrutinized later, inside under a microscope. Even in the cold of winter, life continues under the ice!
            The view from beneath the ice certainly shows a different world with its own challenges. Finding microscopic animals or zooplankton tells us that one of the basic links in the food chain is in place. Many zooplankton serve as food for aquatic insects which in turn are eaten by small fish which feed some of the bigger fish. Gatherings of “bobhouses” tell us that at least some people are successfully catching fish.
            So what is a fish to do in winter? Fish have to deal with changes in temperature, light and oxygen levels, all the while foraging for food. Being “cold-blooded” or ectothermic, fish become the temperature of the surrounding lake waters. In comparison to the windswept lake surface, this can be at least temperate, if not exactly toasty . . . for a fish. Where ice is forming at the surface, water is zero degrees Celsius or 32 degrees Fahrenheit. In deep places, water reaches its greatest density as temperatures hover around four degrees Celsius or 39 degrees Fahrenheit. Compared with summer, these lower temperatures result in a decrease in metabolism for fish, causing them to digest their food more slowly and thus to eat less frequently. The combination of shortening day length and snow covering the ice can decrease visibility markedly but fish do not have to see their prey perfectly.  In winter, predatory fish rely primarily on their sense of smell and hearing as well as that amazing lateral line which detects vibrations and provides information about nearby organisms. Another challenge arises as waters cool, resulting in rooted plants dying, depleting oxygen levels as bacteria break down this decaying plant material. Fish can die from what is called “winter kill” if dissolved oxygen levels decline in a shallow lake, a rare occurrence in deep lakes with good water flow. If lakes have deeper sections, fish may move to these areas of higher oxygen.  Fortunately cold water in general holds more oxygen than the warm waters of summer. Fish also travel as they forage. When shallow vegetated areas are swept clean of invertebrates in early winter, fish will search deeper areas and rocky ledges as the winter progresses.
            Yellow perch is a popular catch for those interested in ice fishing. Traveling in schools, these fish eat insects, crustaceans and small fish.  n winter they are able to extract insect larvae burrowed in the mud. While foraging, perch keep an eye out for their own predators – larger fish like small-mouth bass, lake trout and chain pickerel. Plankton populations in winter are lower than in spring and fall but are still fueling the lake’s web of life. As winter winds down, preparations for spawning become the priority with more concentrated feeding. Finally the inevitable happens – ice-out arrives and waters start to warm again. Yellow perch find their way to shallow, vegetated coves and, under the shelter of darkness, lay their eggs by the thousands in strands over aquatic plants and fallen tree branches. They have survived and a new season begins.       

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