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Hatchery-Wild Salmon Hybrid Survival in Doubt by Associated Press Seattle Post-Intelligencer - March 14, 2003

GRANTS PASS, Ore. -- Scientists have found that salmon raised at a Canadian fish farm rapidly evolved smaller and more numerous eggs, adding to concerns that using hatchery fish to rebuild endangered populations may produce offspring less able to survive in the wild.

The big eggs characteristic of fish spawning in the wild become smaller when wild fish interbreed with hatchery fish carrying the genetic trait for smaller eggs, researchers found. Smaller eggs generally produce smaller fry, or juvenile salmon, which are considered less able to compete for food.

"It's sort of a cautionary tale for salmon enhancement efforts," said Daniel Heath, holder of the Canada research chair in conservation genetics at the University of Windsor in Ontario and lead author of the study published in this week's issue of the journal Science.

"On the bigger question of captive breeding in general, it could potentially drive evolution because it's in a benign environment," he said.

Researchers examined the size of eggs produced by chinook salmon over the past 12 years -- four generations -- at Yellow Island Aquaculture Ltd. in British Columbia. The size of eggs declined by 25 percent in that time. The smaller size of eggs meant salmon could produce more eggs.

Rapid Evolution of Egg Size in Captive Salmon Daniel D. Heath, John W. Heath, Colleen A. Bryden, Rachel M. Johnson, and Charles W. Fox Science 2003 March 14; 299: 1738-1740.

Summary:
"Captive rearing and release programs designed to supplement populations of endangered or commercially exploited species can actually reduce population fitness because the animals are reared under low-mortality conditions that can favor maladaptive traits. Heath et al. (p. 1738) show that rearing chinook salmon under aquaculture conditions, in which juvenile mortality is intentionally minimized, reduces selection that naturally favors large eggs. The result is that hatchery supplementation of wild populations has substantial negative impacts on chinook salmon by driving nonadaptive traits (small egg size) into the fish populations. The evolutionary change to the maladaptive state occurred in under 20 years. "