I'm not clear about what you mean. Do you mean the last two rays of the dorsal fin? I would think that the most likely explanation would be a mild run-in with a seal, sea lion or some other sharp-toothed critter. I've never heard of dorsal fin clipping as a means of identification. I have caught steelhead with some really savage looking wounds; I've caught a couple with half of the caudal fin bitten away and one with a large chunk bitten out just behind the head that went almost down to the spine. It was fully healed but certainly looked bad enough to have been lethal.