I believe that there are two ways to look at it...

I think that a jack, biologically speaking, is a fish (mainly salmon) that return earlier than is normal. For a king, that would be after a year or two instead of three to five years. For a coho, that would be one salt fish rather than a two to four salt. I'm also pretty sure that they are sexually mature, just young.

The other, of course, is the legal definition, which is based on size, i.e., coho under 20 inches, etc.

I don't think that there are any that return early, don't spawn, and return to the salt.

Fish on...

Todd.