On Course: Tides and Waves
Age 10, group or independent
45 minutes, indoors or outdoors
Discover the effect that weather, moon phases, and time of day and year have on water movement (tides and waves), as well as on the decisions of fisher folk.
On Course Facts & Trivia
What are tides?
Ancient mariners thought tides were caused by the breathing of an earth monster. Later in history when humans began recording the events around them, they found that tides were closely related to the movements of the moon and the sun.
Tides are caused primarily by the gravitational pull of the moon. Although the sun is much larger than the moon, the moon is closer to the earth and exerts more gravitational pull than the sun. The moon's gravitational pull causes a movement of water like a massive "wave." One such movement occurs on the side of the earth facing the moon and other "wave" occurs on the opposite side of the earth.
The earth makes a complete revolution once every 24 hours. This constant motion puts different sections of the earth's oceans under the moon's gravitational influence during the course of a day, resulting in a daily cycle of two high tides and two low tides. However, this tidal cycle occurs (on the average) not every 24 hours, but every 24 hours and 50 minutes. The extra 50 minutes is due to the rotation of the moon around the earth once each month, moving in the same direction as the earth revolves. Therefore, the moon has changed position in relation to a spot on the earth during the 24 hours in which the earth makes a complete revolution. As a result, it takes an average of 50 minutes extra each day for a spot on the earth to "catch up with" (or pass under) the moon.
Spring tides are the highest and the lowest of the tides. Neap tides are the opposite of the spring tides and show the least difference between high and low tide. The change from spring tides to neap tides is gradual, depending on the positions of the moon and the sun in relation to the earth.
Spring tides occur at the time of the new moon and the full moon. Neap tides occur when the moon is in the first quarter and the last quarter. At these times, the sun and the moon form a 90-degree angle with each other, resulting in the least amount of gravitational pull on the water.
What is the Intertidal Zone?
The waves, tides, air and sun create a special environment along the seashore that is known as the intertidal zone. The intertidal zone is the area that is between the high tide line and the low tide line. It is alternately covered with water and exposed to air. The intertidal zone is habitat, or home, for animal and plant life that can live in sunlight, breaking surf and waves and changing tides. Most of the animals hide or hang onto rocks or other things in the sea because of the crashing surf. Most of the animals and plant life adapt well to changing environmental conditions.
Tidepools are formed in the intertidal zone during low tides. Tidepools are very delicate areas, and care should be taken when exploring tidepool areas. In California, tidepools are protected by law. Anyone caught removing or harming tidepool life may be fined as much as $500.
During low tide, intertidal creatures (organisms) are exposed to the air. They must not dry out, and they must withstand air temperatures which vary from hot in summer to bitter cold in the winter. Some organisms have adapted to prevent drying out. How do you think they protect themselves to keep from drying out?
What are waves?
Waves are swells of water that eventually end their journey on some beach where they become breakers, or surf. Waves have a crest and a trough. The crest is the highest part; the trough is the lowest. The vertical distance between crest and trough is the wave height. Waves you commonly see at the seashore are wind waves. They are caused by wind blowing across the water. The size of wind waves depends on three factors: the distance over which the wind blows, the strength of the wind, and the length of time the wind blows. If all three factors are large, the waves are large.
Waves affect the behavior of sea life. Large waves cause sea urchins and abalone to clamp down on rocks. Large waves and crashing surf create turbulence and cause crab and lobster to move to deeper water offshore.
The most spectacular kind of wave is the seismic wave, or tidal wave. Seismic waves are caused by earthquakes, usually underwater but sometimes on land. Seism means earthquake in Greek. The scientific name for a seismic wave is tsunami (soo-nah'-mee), the Japanese word for seismic wave.
People on a ship at sea would hardly notice a tsunami because the crest and the trough are so far apart. But the situation changes in shallower water where the waves may become breakers as high as 100 feet. When a seismograph station detects an earthquake, a warning is sent to all areas that might be hit by a tsunami.
Intertidal plants and animals must survive the action of the waves. Intertidal organisms protect themselves from being smashed against rocks or cast up and stranded on beaches in different ways.
On Course Questions & Answers
1. What can a fisherman or fisherwoman with a permit or license harvest in the low tide zone near rocky areas? ANSWER: Sea urchins, mussels
2. If a fisher folk goes to sea at high tide and leaves at 5 a.m., what time will he/she leave the next day to leave at high tide? ANSWER: 5:50 a.m.
3. If wind from winter storms causes large waves near shore, where would you go to catch spiny lobster? ANSWER: Offshore
4. If a tidal wave occurs when a diver is gathering sea urchins near shore and a fisherman is fishing 20 miles offshore, who would most likely feel the tidal wave, the diver or the fisherman?
ANSWER: The diver, because offshore where the water is deeper the crest and trough of waves are much farther apart. In shallow water they are closer together and the waves could be 100 feet high during a tidal wave.
5. California halibut eat anchovies. If anchovies like cloudy, green water caused by plankton blooms, would you fish for halibut in clear water or in water with plankton? ANSWER: In water with plankton.
6. If tuna like clear, blue water so they can see other fish that they want to eat, would you fish for tuna in deep water or shallow wavy water?
ANSWER: Clear, deep blue water that is sometimes called tuna water.
7. Spiny lobsters do not like rough water in winter when storms come. Lobster move to deep waters in stormy weather. If you were a fisherman or fisherwoman, where would you put your lobster traps, if you know a storm was coming? ANSWER: In deeper offshore water.
8. Sardines migrate north along the coast in spring and summer. The largest fish go as far north as Canada. In fall, schools of sardines return to Southern California waters. If you wanted to catch sardines during winter, where would you fish? ANSWER: In Southern California because that is where sardines concentrate in winter.
9. Some fishes are active at dawn and at dusk. Gillnets catch active fishes that swim into the net. What times may be best for fishing with a gillnet?
ANSWER: Either dawn or dusk.
10. Squid are attracted to light. Fisher folk shine bright lights from their boats at night to attract squid to the surface. When is the best time to catch squid?
ANSWER: At night.
SIX EASY STEPS TO MAKE A WATERSCOPE
What I might see with my waterscope