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Fish traveling in schools are a wonder of nature


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POSTED: Monday, November 16, 2009

DOG BAY, Isla Tiburon, Sea of Cortez » One of my goals in sailing the Sea of Cortez is to see marine life I've not seen before. Blue and sperm whales, two species that frequent this northern stretch, are high on my wish list, but so far those giant marine mammals have been elsewhere.

That's OK, though. My encounters with fish balls and coyotes are making up for it.

A fish ball here is (to me) a school of hundreds of thousands of 2-inch silvery fish that swim together in the bays. Countless numbers of these enormous schools hang in the water column from 3 to 20 feet deep, each fish swimming an exact distance from its neighbor.

Usually the plankton-eating school moves slowly forward, an amoebalike cloud in these nutrient-rich waters. But when a seabird plunges from above, a predatory fish rises from the bottom or a snorkeler splashes in from the side, the fish explode in a confusion of sparkles and swirls. An instant later the fish form great silver rivers as they work to get back in formation.

When I swam slowly toward a ball, with my arms still and barely kicking, the fish, amazingly, accepted me into their clan and off we all went, each fish maintaining a precise space from my body. Often my swimming companions were so dense I couldn't see the white sand bottom only four feet below. Talk about psychedelic. It felt like I was swimming inside a lava lamp with eyes.

These huge fish schools, however, can be a pain. After I drove my sailboat into this remote bay, I aimed for a sand patch where I planned to drop the anchor. As the boat advanced, though, the sand wasn't where I'd thought it was. I moved farther toward the beach, but the sand eluded me there, too.

Finally, I anchored in something that looked like rocks or seaweed, thinking I'd dive on the anchor to make sure it was well set. By the time I jumped in the water, though, I found my anchor buried in white sand. A moving fish ball had kept me guessing.

Schooling fish can keep such a specific distance from one another with a sense organ called a lateral line, present in most fish and often visible. This line is a narrow strip of pores running the length of the head and body on both sides that detects pressure, vibrations and current.

The lateral line takes advantage of water's ability to carry and sustain pressure waves. The water delivers its messages through the pores into a fluid-filled canal where the vibrations move tiny hairs, stimulating nerves that go to the fish's brain.

With this system, a fish can detect a moving predator and also maintain its distance from school

members.

Sometimes, though, this line of defense fails. Using surprise and speed, diving seabirds and carnivorous fish snatch individuals from these fish balls all day long. And occasionally, in efforts to escape, the school scatters too close to the shore break, and the unlucky fish on the outer edge get grounded.

The coyotes on Tiburon know about such groundings and come to the beach at first light to search for stranded fish. I counted six of those golden brown scavengers one morning poking along the shoreline. When they found a fish pile, the coyotes ate it, cleaning the beach in the process.

Whales? What whales? I'm busy communing with fish and counting coyotes.

Susan Scott can be reached at www.susanscott.net.