Age 10, group or independent
45 minutes, indoors or outdoors
Discover the effect that ocean currents have on seafood and fisher folk.
A Current Event.
For sailing activity:
A Current Event
Fisher folk know that the ocean has many streams of moving water. These streams are called Currents. A current is a flowing body of water or air. Like rivers on land, ocean currents usually, but not always, keep the same course. There are warm-water currents and cold-water currents.
Fisher folk know the directions that the warm and cold water currents flow. A warm water current called the Davidson Current flows northward along the
southern California coastline. A cold-water current called the California
Current flows from north to south along the coast to Point Conception, then offshore into the central Pacific. When the Davidson Current meets the California Current, the Davidson Current turns counterclockwise, forming a gyral current flow that circulates around the Southern California Bight.
Movement of Currents
The earth is like a huge ball. If we divide the ball into two equal parts, we have the Northern and Southern hemispheres. The major currents flow around these hemispheres in circular patterns called gyres (JY-urz).
North of the equator, the movement of the currents is usually in a clockwise direction. (The Southern California Bight is one exception to this rule. )
South of the equator, the movement is generally counter clockwise.
There are two other special water movements that fisher folk track to help them make decisions about where they should look for fish and which species they should target.
The first special water movement is upwelling. In California, upwelling occurs in spring and fall when water on the ocean surface is displaced by strong northwest winds. When the winds push away the surface waters, deep, cold water rises to take its place, carrying with it nutrients from the deep ocean. These nutrients (primarily nitrogen and phosphorus) provide food for microscopic plants and animals (phytoplankton and zooplankton), and the plankton in turn provide food for larger fish and shellfish. Upwelled water is often green or coffee colored due to the abundance of microscopic life it harbors. Bait fish congregate in the upwelled water to feed on the plankton, and predator fish such as salmon and shark follow to feed on the bait fish. The result of upwelling is abundant and healthy sea life. Fisher folk seek out areas of upwelling to help them find fish.
Upwelling also helps to cool the California coast during summer. Upwelling causes summer fog along the coast as warm winds pass over the cold, upwelled water. Without upwelling, San Francisco would be as hot as Sacramento in the summer.
El Niño is another special water movement that fisher folk watch carefully. El Niño is a warm-water flow that originates south of California and moves northward along the California coastline. The warm water of El Niño causes fish populations that prefer cooler water to move north, away from the El Niño influence. This shift also causes fisherfolk to move their fishing grounds or target other species.
Bait fish generally move north to cooler waters in an El Niño cycle, and larger fish follow. Albacore, swordfish, mackerel and halibut are usually abundant in southern California waters, but during El Niños their numbers increase in northern California and even farther north and decline in southern California. Fisher folk must be able to find and follow the fish in order to be successful in their business.
Experiment 1 - Spinning Earth and Spinning Water
The sun warms up different parts of the earth faster or slower than other parts. Hot air rises and colder air moves in to take its place, creating winds. The earth's rotation affects the direction and speed of these winds. The wind creates currents.
To see how this works, place a basin of water on a lazy susan tray or on a rotating piano stool. Spin the basin gently in a counterclockwise direction (right to left). Observe. Let the water rest until it is still. Rotate the basin in the opposite direction. What happens?
Experiment 2 - The Sun and the Ocean's Currents
The equator of our earth receives direct sunlight. This makes water near the equator warmer than water near the North or South poles. Fisher folk call the warm water "tropical waters." These differences in water temperature also create currents. The warm water near the equator expands and moves toward the poles. The colder, heavier water near the poles sinks and moves toward the equator. Currents caused by temperature changes are called convection currents.
To see how convection currents work, fill a baking pan with water and gently heat one end of the pan with a Bunsen burner. Place several drops of food coloring on the cool side of the pan. Which way does the cold water move? When the colored water reaches the warm end of the pan, which way does it move?