Easy Life Makes for Dull Fish

Brain drain. Trout living in captivity have smaller brains.

SNOWBIRD, UTAH--Anglers have long noted that compared with their wild kin, hatchery-raised trout and salmon do not seem to be very bright. According to research presented here last week at the annual meeting of the Ecological Society of America, they also have smaller brains.

Hatchery trout are known to grow faster and larger than wild fish, but to be less wary of predators, say aquatic ecologist Michael Marchetti and neurobiologist Gabrielle Nevitt of the University of California, Davis. To see if these differences were reflected in the fishes' brains, the scientists assembled two strains of hatchery-raised rainbow trout and two of wild fish. The team then took eight different measurements of the brains of the 48 wild and 51 hatchery fish.

For seven of the eight measures, wild trout had bigger brains. The telencephalon--the fishy equivalent of the cortex--was larger, as was the olfactory bulb. Marchetti suspects that as with mice--whose brains develop fewer connections when raised in austere environments--so with trout. Hatchery fish live in a bland and relatively stress-free environment, he said, whereas wild fish must contend with predators, changes in water velocity and temperature, and unpredictable edibles.

John Musick, a marine biologist at the Virginia Institute of Marine Science in Gloucester Point, Virginia, says that although hatchery-raised fish are known to have lower survivorship in the wild, no one had linked it to actual brain differences. "To see such gross differences--that's very surprising."

To see whether they are mostly the result of the environment interacting with the developing brain--as opposed to gradual genetic change in captivity--Marchetti and Nevitt now plan to compare members of a single strain of Coho salmon raised in either captive or wild environments. "We think there's a lot of implications of this work for captive rearing," says Marchetti, because restocking depleted streams with unfit captive-bred fish may be bad for wild populations, for instance. And captive trout fry, he suggests, may need more challenging environments.

Related sites
Northwest Fisheries Science Center
Trout Unlimited, a conservation organization

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