If you have an AFK farm (example: for gold, mobs, etc), then it would be even more efficient to also have the player automaticaly fish while waiting for the drops of the farm.

In my mob farm, I built an automatic fishing machine.

The benefits are:

• I get loot from the farm and loot from fishing (like name tags and enchanted books) at the same time
• I also get exp
• My mending tools get automaticaly repaired

Bonus Tips:

• Use a fishing rod that has Mending, Luck of the Sea III and Lure III

• If possilbe, try to have the water explosed directly to the sky, so that it's more efficient (also if it happens to rain, it would be even more efficient)

Recomended design: Panda's Simple Automatic Fishfarm (for 1.9)