The mustardfish can see bait from a normal distance, longer in the front and shorter behind it. It will chase after small hooked fish, but not at a speed that makes it dangerous to the catch.
The mustard fish can be found near the top of the map, sometimes near bulbfish.
The mustard fish is not normally used for money, even early game, as it is only worth $250 when fully intact. The mustardfish is instead normally used to obtain large bait to catch more valuable fish. Note that the mustardfish can still steal large bait.