Special thanks to Picho, info taken from Nodiatis forum.

Fishing can only be done in water areas and is one of the first trade skills you can do. To fish, you need to only kill a monster in a watery area, then use the pole by clicking its icon on the bottom. You can find areas where it is possible to fish here.
This will start the fishing mini game. While fishing you have to wait till the pole buckles completely over then quickly push the space bar.

This will net you a fish or something worthless. Fishes can be used to make oil, or scales.

Fishing Level Required to Process Scaling
Flounder 0 Flounder Scales
Giant Guppie 3 Giant Guppie Scales
Longhat Trigger 6 Longhat Trigger Scales
Cod 10 Cod Scales
Blue Salmon 13 Blue Salmon Scales
Mudbeak 17 Mudbeak Scales
Skullfish 20 Skullfish Scales
Flying Eel 24 Flying Eel Scales
Crowned Seasnake 27 Crowned Seasnake Scales
Angler Fish 30 Angler Fish Scales
Sockeye Sockeye Scales
Mudflapper Mudflapper Scales
Poison Scale Fish Poison Scale Fish Scales
Froghead Fish Froghead Fish Scales
Tigertail Tigertail Scales
Rock Bass Rock Bass Scales
Bluegill Bluegill Scales
Crappie Crappie Scales
Bluefin Shark Bluefin Shark Scales
Gnomefish Gnomefish Scales
Split Tail Skate Split Tail Skate Scales
Hammerhead Hammerhead Scales
Monkey Tail Monkey Tail Scales
Twitter Fish Twitter Fish Scales
Chub Shark Chub Shark Scales
Angel Fish Angel Fish Scales
Sunfish Sunfish Scales
Pumpkinjack Puffer Pumpkinjack Puffer Scales
Blueghost Fish Blueghost Fish Scales
Arrowhead Fish Arrowhead Fish Scales
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