I was trying to use the execute command but I was never able to find something where I could trigger it at the use of the fishing rod. I'm trying to make my own weapon using commands and I want to make a line of particles that do damage and deal knockback I thought a bow or fishing rod was a good place to start but I can't find any command that works.
You can execute a command whenever a
fishing_bobber (end of fishing line, only spawns when you use your fishing rod) is spawned. An example:
/execute at @e[type=minecraft:fishing_bobber] run particle minecraft:happy_villager
If you put that command in a repeating command block it will show the happy villager particle along the path the fishing bobber takes.