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The Bass With The Built-In Motion Detector

If you saw the hit movie Jurassic Park, you will surely remember the scene where Tyrannosaurus rex breaks free of its electric cage and proceeds to go on a rampage. Paleontologist Alan Grant and the park owner's daughter, Lex, get concerned. Death looks imminent. Lex is about to let loose a final, hysterical scream when Grant clasps his hand over her mouth and whispers, Don't move! He can't see us if we don't move! Lucky for the pair, he's right, and the T-rex stares through the two as though they were bumps on a log.

Gaping holes in the movie story line aside, Grant's assertion that things had to move for T-rex to see them is not as outrageous as it may first seem. Motion-based visual systems are fairly common, particularly among predators.

Frogs, for example, have retinal wiring to detect flying insects. A dead fly suspended motionless in front of a frog doesn't register with the frog's visual system. A hungry frog surrounded by motionless flies will starve to death.

Much the same goes for bass, they are virtual motion detectors. Motion signals living and edible. Motionless objects are quickly classified as non-living and ignored.

That does not mean bass are blind to stationary objects. Bass don't constantly ram into rocks or stumps. Bass see and maneuver around these obstacles quite well. Motionless objects simply do not signal food to a bass' brain. To a bass, food moves. A bass' motion detection may be its strongest predatory sense. In some respects, bass are better at seeing motion than humans are.

Research implies that bass are not equally sensitive to all types of motion. Some motions are better seen and preferred than others simply because of the way the eye is built. Trout, for example, are more sensitive to objects moving forward past their nose than to objects moving toward the tail, as long as the object is moving quickly. The reverse is true if the object moves slowly. In contrast, the visual system of perch is biased toward vertical movements, such as objects falling from top to bottom. Know one knows where bass lie in this spectrum. They should show more genetic overlap with perch than trout or carp.

Fish species vary in how fast they prefer objects to move through their visual fields (measured in the number of arc degrees traveled per second).

In this way, fish are said to be velocity-tuned. Bass and perch show a preferred target velocity in the range of 10 to 50 arc degrees per second, with the most effective speeds lying around 30 degrees per second (about one-sixth of the visual field width for either eye). This contrasts with the northern pike which has two preferred velocity ranges: very slow speeds (0.5 to 5 degrees per second) and more moderate speeds (3 to 40 degrees per second).

So how does all this new knowledge help anglers catch more fish? Simply put, baits presented and moved through the water in a natural prey-like manner will be more readily seen, and then devoured, by bass than a bait simply laid to rest on the bottom of the lake.

But perhaps I oversimplify.

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