Illustrations by Bill Reynolds ▼ oung Y Fish in the naturalists Zone During the year, fish travel between zones of plant and animal life. Fish all have their favorite spots in a lake. If you fish, you’ll learn where certain fish are most likely to be found. Why does a fish choose to live in one part of the lake rather than another? Where fish hang out depends on the time of year. As weather changes from season to sea- son, conditions in a lake change. For example, as weather gets warmer, some parts of the lake warm up. Then fish might move to a different place, or zone, to find food and shelter. Fish also change zones as they grow. Like babies, larval yellow perch—average newly hatched fish need different kinds of food length 6 millimeters (less than one-quarter inch) and shelter than grown-up fish do. This story follows young yellow perch from spring through winter. As the seasons change and the perch grow, they travel to new lake zones. By Roland Sigurdson May–June 2012 47 grow there and her eggs need plants. After about 15 days, larvae hatch Her eggs come out as long, jellylike rib- from the eggs. Less than a quarter- First Home bons, some more than 6 feet long. The inch long and just beginning to grow In April the sun’s rays warm the lake ice, chunks. The wind blows the ice chunks ribbons get tangled around the under- fins, perch larvae don’t look much like causing millions of tiny cracks. Rocks to the north shore, where they pile up and water plants, so they don’t get washed up fish. They look more like transparent along shore soak up the sun’s heat dur- melt. By early May, the lake is free of ice, on shore by wind and wave action. mini-tadpoles. ing the day and give off heat at night, and new life begins to churn underwater. Aquatic plants can grow in this zone The littoral zone teems with small melting ice several feet out from shore. To lay eggs, yellow perch adults move because sunlight reaches the nutrient- fish, insects, and other creatures that eat A strong, warm wind from the south into the warm, shallow waters of the rich soil at the bottom. In Minnesota yellow perch larvae. The newly hatched pushes down on the cracked ice sheet. littoral (shore) zone. The female perch lakes with clear water, the littoral zone can perch would be safer in another part of Snap! Pop! Boom! The ice splits into comes to the littoral zone because plants stretch from shore to water 15 feet deep. the lake. wild rice yellow lotus white water lily Submergent wild celery plants bulrush Floating-leaf coontail plants Emergent plants Note to Teachers: To learn more about fish biology and lake ecology, see the MinnAqua online guide Fishing: Get in the Habitat! Check out the Fishing in the Neighborhood program and state parks “I Can Fish!” clinics at www.mndnr.gov/fishing. 49 Safer in the Middle When water temperatures rise to grate back to the bottom, so fish about 59 degrees, the tiny larval that could see them in the light perch move to the limnetic (open won’t eat them. Larval perch eat littoral water) zone. In open water away midge larvae in late evening or zone from shore and above deep water, early morning when the larvae are fewer fish are hunting larval perch. moving between the bottom and Daphnia—average Yellow perch larvae find plenty the surface. length 2 millimeters to eat in the limnetic zone. Plants and animals called plankton Home for Summer abound here in the middle of the By mid-June, about 40 days after lake. These tiny organisms are hatching, a perch is about 1 inch limnetic zone the perfect bite size for hungry long. It now looks like a mini-adult. perch larvae. Phytoplankton, also It has fully formed fins, a big mouth, called algae, are plants. Perch prefer and a big appetite. littoral zooplankton, which are nutritious Juvenile perch are still growing littoral zone zone little animals such as rotifers, water and need food with more calories. fleas, and copepods. When it grows To find more food, they migrate bigger, a larval perch will hunt for from the profundal zone back to bigger prey. the littoral zone. The littoral zone was a danger- Dining in Deep Water ous place for perch when they limnetic zone Larval perch find their biggest prey were tiny, but now they are big- in the profundal (deep) zone. These ger and have fins. They can swim waters start below the limnetic zone fast and escape predators. Their and reach all the way to the bottom swimming speed helps them of the lake. Because sunlight can’t catch prey such as mayflies, midg- profundal zone reach this deep, no green plants es, and scuds. Here they also can grow in the profundal zone. Large hide among aquatic plants, rocks, numbers of bacteria and fungi live and fallen trees. in the bottom muck. So do inver- Juvenile perch live in the litto- tebrates such as Chaoborus, also ral zone all summer and into late known as phantom midges. fall. In the late fall, when the lake Each evening phantom midges temperature and oxygen changes, Chaoborus—average migrate from the profundal zone called turnover, the perch will length 10 millimeters up to the surface to feed on plank- travel back to the profundal zone ton. As morning arrives, they mi- for winter. 51 Summer Tucked in for Winter Like all living things, fish need oxygen. mayflies, scuds, and other inverte- epilimnion During summer, the water in the profun- brates—providing young perch with 59 - 68F thermocline dal zone has very little oxygen. In winter plenty to eat until spring. hypolimnion the profundal zone has the same amount 39 - 50F of oxygen as the rest of the lake. Also, the Another Year Spring Fall water near the bottom is about 4 degrees Spring begins another year for the warmer than the water just under the ice. young perch. Once again, the ice melts, With plenty of oxygen and slightly the littoral zone comes alive, and new entire lake entire lake warmer water, the profundal zone in food becomes abundant. With luck, the 37 - 50F 37 - 50F winter has large numbers of midges, perch might live nine more springs. nV Winter ice 32 F Want to Be a Limnologist? 39F Lakes are full of mysteries to ning off a road and into a lake discover and solve. A limnolo- change its chemistry? If some- gist is a scientist who studies one adds minnows or other Lake Turnover In many lakes during summer, water warmed by the sun floats atop a layer of cold water. Water is dens- lakes, ponds, rivers, springs, fish to the lake, what differ- est (heaviest) at 39 F. In fall, the surface water cools to 39 degrees and sinks. The wind helps mix the streams, and wetlands. Lim- ence might they make? lake’s water layers—called fall turnover. When surface water cools to 32 degrees, it freezes. Spring turnover nology is the study of all inland As a limnologist, you would occurs as cold water at the surface warms to 39 degrees and sinks. Once again, wind helps mix the layers. waters—bodies of water not look for patterns—how things considered oceans or seas. repeat—from season to sea- A limnologist asks questions son, year to year, and century Layers, Turnover, and Floating Ice and looks for answers about ev- to century. You’d keep records UMD research assistant Jiying Li Which creatures live in a zone the hypolimnion, is cold and dark to bottom. The water also has a ery animal and plant that lives of your observations of a lake. (littoral, limnetic, or profundal) with only a little oxygen. good supply of oxygen from top in a water body—the biologi- You’d compare them with other anytime. When you go fishing, depends on water temperature, In the fall, as air tempera- to bottom. This mixing of layers cal life. A limnologist also ex- scientists’ observations. Then stand on shore, or swim in a amount of sunlight, and amount tures begin to drop, the water is called turnover. amines other things in the wa- you’d begin to understand what lake, you can think about life of oxygen. A lake has three lay- in the epilimnion gets colder. As the lake water turns from ter and how they mix or relate might happen to the lake in the around and under water. What ers, each with different levels of You will notice this change if liquid to solid ice, the water to each other—the chemistry future. Your study could help questions do you have? Where oxygen, light, and temperature. you see steam rising from a lake molecules line up in a lacy pat- of the water. others see how to take care of can you find answers? In an unfrozen lake, the on a cold morning. tern with many empty spaces. If you were a limnologist the lake so the water is clean epilimnion layer at the top has As the surface water cools, The spaces make the ice less studying a lake, you would look and fish and plants are healthy. plenty of oxygen and sunlight it gets denser. Eventually, the dense than cold water, so it around to find out what might To become a limnologist, and warmer water than the epilimnion becomes as dense as floats instead of sinking to the change the water.