Fishin Blues is indeed happy, unless you consider the lot of one who has nothing better to do than fish only to be outdone by his baby brother, as blues worthy.   It may be a bit more raggish than bluesy, but it is a great song.   Taj's version is very similar to Henry Thomas' original other than the latter's use (at least in the recording I heard) of a fast high chord accompaniment with a break on the quills, a sound reproduced by Canned Heat in Going Up the Country (which appears to have been a reworking of another Thomas tune)