According to Wikipedia, the sea trout is the anadromous form of the brown trout. What's anadromous, I hear you ask. Well: an anadromous fish is one that migrates from the sea into fresh water to spawn – as distinct from a catadromous one, which migrates from fresh water into the sea to spawn, or a potamodromous one, which migrates from lakes into rivers and streams to spawn.
Most brown trout are potamodromous, apparently; and it's only the anadromous type that's known as the sea trout, sewin or peal.
Given all this, I would suggest that "brown trout", or even just "trout", should have been allowed as a correct answer.
Sewin is the Welsh word for the sea trout, and peal is the name by which it's known in the West Country of England. It's also known as the finnock in Scotland, the mort in North–West England, and the white trout in Ireland.
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