Fishing is fun, long, and hard. It can also be well worth it for fighting or selling. If you hunger for the more aggressive approach to Syrnia by slashing your way through every opponent, then catching and cooking your own food will greatly decrease the costs you would otherwise have to pay. If on the other hand you wish to live a more peaceful life, selling fish - raw or cooked – can be a good way to fund your relaxation. While fishing you can even find some items including: leather boots, locked toolboxes, small chests, and locked small chests!
All information on what can be caught with which item at each level of fishing can be found in the official Syrnia Manual found here. I have created a table of the fish that can be caught at each level and the average experience you will gain per fish here. If it is possible to catch two fish then you will catch twice as many of the first as of the second. If it is possible to catch three or four fish you will catch an even amount of all.
Fishing time varies by level and by equipment used. A partial list compiled by the community is available here (scroll down to the third post).