for safety as they go. Most ice fishers advise checking that the ice is at least 4 inches thick. Using sonar, the angler can determine the depth of the water, bottom content, weed and structure cover, and even see if there are fish there. Also, by using sonar, they can place the bait according to where they think the fish are. If they are using "tip-ups" they can carpet the area at different depths and with different presentations (the number allowed being determined by local laws) and see what is the most productive. Modern ice anglers can also use modern reels mounted on shorter (18"-36"/45–90 cm long) fishing rods to actively fish by watching, by using their sonar, where their lure is relative to the fish, and jig accordingly to entice a bite. Ice fishing can be done at any time of day, and is typically most active around dusk and dawn. Different fish are active at different times of day, so anglers need to fish for them accordingly. There are fish houses large enough and comfortable enough to spend many days in a row out on the lake, fishing the entire time. One can even fish in one's sleep, by using audible alarms on one's lines to tell when a fish is biting. There are also many lightweight and highly mobile portable shelters that mount on plastic sleds and collapse for transportation. These can vary from small, one-person shelters (commonly called "Fish Traps") to large and complex shelters able to fit up to 20 people at once. Dangers and Safety Precautions Ice Thickness and Type There are many variables which dictate whether or not ice is safe to walk on, however there are some widely accepted parameters. For example, newly frozen ice is always safer than older ice. "Good ice" is ice which has frozen without the interruption of large temperature fluctuations, rain, or snow and is clear and free of large lumps and cracks. Additionally, the safest ice is that which sits upon a lake without moving water. Ice on the surface of rivers can be extremely unpredictable. Many anglers will go out with as little as 2.5 inches (6.4 cm) of "good ice" for walking on. The recommended thickness of ice to support an average person is 4 inches (10 cm), 5-6 inches (13-15 cm) for sleds (snow machines, snowmobiles) and most ATVs, 7-12 inches (18–30 cm)