Minecraft wiki entry for Durability, associated text: Breaking a block or hitting a mob counts as 0 uses.
Casting the line and reeling it in empty, counts as 0 uses.
Reeling in an item counts as 1 use.
Catching the bobber/hook on a block, then reeling it in, counts as 2 uses.
Using the line to yank on an item counts as 3 uses.
Using the line to yank on a mob counts as 5 uses.