Hatchery steelhead will be missing their adipose fin, which is the small fleshy one on the back between the dorsal fin and tail.
You’ll sometimes encounter a fish that’s a “tweener” — one that has a partial adipose fin. While this can occur in the wild (rare), it is more often the byproduct of a “miss-clip” by the person doing the fin removal.
Where done by hand, you can imagine that there are going to be some imperfect cuts when people are trying to get through tens of thousands of baby Steelhead.
One other clue to look for, however, is sign of an eroded dorsal fin. When jammed together in fish hatcheries, baby steelies often rub against each other and the concrete walls, resulting in worn down fins.
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