It's one of the fish you can catch by hand. I've caught a couple in the hand-fishing spot closer to Konohana village. Over toward the left I think, but not sure if that matters. --- Please insert witty quote here.
I always catch crabs around dusk at the 1st area outside of Konohana. Go to the larger pool of water (most south-west section of the stream that turns into a pool basically) and grab them. If there aren't any, just leave, then come back and it should reset.
*Bonus* If you want some Small Coins, do some hand-fishing in the area closest to Bluebell. Go to the most northeast section right where the stream splits (close to that grassy bridge) and grab the fish. I've gotten around 12-14 Small Coins in that area so far. --- http://www.amazon.com/shops/A258UKMK08DSV0 Playing: DS- HM: Tale of Two Towns, DQ Monsters: Joker 2 / PC- Rome: Total War - Total Realism (Mod for RTW)