yes u should be leveling off them both but fishing skill ups are very slow not like weapons or magic

try talking to the fishing guild master and join the guild but u shouldn't have to
fish with job over lvl 20 fish ur limit for a couple days if u dont skill up at all after joining guild and doing this i would report bug to SE even at that level you should be getting 1-2 levels if u catch your max limit of fish per JP day