I think catching them on a dart is legal.
Here's the DMF site:
http://www.state.ma.us/dfwele/dmf/re...m#restrictions
They do not list a size limit, but under prohibitions this is listed:
"Prohibitions
Snagging, Snatching of Anadromous Fish: including shad, smelt, white perch, striped bass, trout, and salmon, but excluding alewives or river herring is prohibited."