If the concern is that they could escape and reproduce, why can't they just sterilize the stock like triploid trout? Obviously that does nothing for introducing disease and competing for food supplies, but wouldn't that be a step in the right direction?
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In the legend of King Arthur, the Fisher King was a renowned angler whose errant ways caused him to be struck dumb in the presence of the sacred chalice. I am no great fisherman, and a steelhead is not the covenant of Christ, but with each of these fish I am rendered speechless.