Are fish merely followers?

By John Kelly
Sunday, July 25, 2010

My husband and I took our daughters to the National Aquarium in Baltimore and did the walk-down-the-ramp thing, where you go from the top of the tank to the bottom, down four or five stories. The fish were all -- I mean ALL -- swimming in a clockwise direction. When we got to the lower depths, where the sharks are, they were all swimming in a counterclockwise direction. I asked my rocket scientist husband why this was and he said, with a straight face, "The Coriolis Effect. If we were in the Southern Hemisphere, they'd all be going the opposite." He had me going for a second before he cracked up. What gives? Are there maybe some water currents in that huge tank that encourage this conformist behavior?

-- Tracy Thompson, Bowie

Wasn't it Shakespeare who wrote, "Oh silv'ry fish: What thoughts within thou brain doth swim?/What inner voice doth compel thee in thy ceaseless wand'rings?"

No, it was Answer Man, just now.

First of all, when visitors to the National Aquarium in Baltimore walk down that dramatic ramp, they are passing two tanks, not one. The upper tank is called "Atlantic Coral Reef." The lower tank is called "Open Oceans." If you stand there long enough, you will see fish swimming in all sorts of directions.

Explained Richard Lerner, curator of fishes, "If it's a fish that schools, they'll follow each other and go in the same direction, simply because there's protection in numbers."

That direction can change from moment to moment as a fish at the front or the back changes its pattern and its schoolmates follow suit.

The tanks -- and there are close to 100 exhibition tanks at the aquarium and twice that many behind the scenes -- do have subtle currents, not to urge the fish in any one direction but just as a result of filtration or to keep the water mixed. In some tanks a solenoid engages periodically to change the flow of the current.

Fish do not like still water, Richard said, and they prefer to swim against the current. That's because they are hydrodynamic: designed to cut through water. Imagine how uncomfortable it would be to have all your fins and scales pushed open from behind like an inside-out umbrella in the wind.

Schooling species at the aquarium include lookdowns, grunts and surgeon fish. "Fish school to get food and for protection against predators," Richard said. "Even though there are no predators for them here in the aquarium, all the years of evolution can't take that out of fish."

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