I'll be calling someone tomorrow who might be able to clarify, but at least at the moment, it looks like aquarium trade fish as defined by: "Aquarium Trade Fish means those freshwater fish which cannot survive year-round in a wild environment above 30º north latitude (approximately from Jacksonville, FL west to Ensenada, CA) or below 30º south latitude (approximately from Puerto-Alegre, Brazil west to La Serena, Chile). Notwithstanding the foregoing, aquarium trade fish shall also include the goldfish (Carassius auratus), koi or Japanese carp (Cyprinus carpio), and guppies (Poecilia spp.), but shall not include tilapia (Tilapia spp.) which require a permit." and fathead minnows are it for lawful pet fish. They specifically list 'native and established fish in Massachusetts' as unlawful. (http://www.mass.gov/...s.html#domestic - for the info about unlawful natives and lawful fatheads)
I'm unable to tell how difficult something I found about a propagater's license would be to obtain, as I think it might be the way to go for those of us wishing to keep native fish. It certainly seems very complicated, though, and unnecessarily so for people just wanting pets... (http://www.mass.gov/...00-fishing.html)
*sad* Somehow it didn't occur to me that it would be illegal to keep fish that are common enough that they can be eaten or used as bait. I guess I'm only going to be catching, identifying, and releasing, not catching and keeping.

I kind of want to know what is involved in attempting to get laws like these changed, but a) I'm in grad school, and b) I feel like it's a _huge_ uphill battle.