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I think they experimented to see how many make it up the river. Actually, bears and other animals rely on scavenging the dead fish, and they fertilize the riverbanks and nearby plants, so if all of them make it it would ruin the ecology. Also, the population is carefully managed. The authorities will count the fish and release more from a hatchery if there are too few or further restrict fishing and do the opposite if there are too many.

Howdy today sir abitcoinskeptic! OH that makes so much sense, thanks for expounding on that for me!