Find out who are the people selling most fish on your realm. You can see it on the AH or the trade channel. People with the "Salty"-title are also usually botters or total nolifers. Ask bank alts who is their main.
Make a lvl 1 alt and add these people to your friends list. Spam them with tells when they are online (probably botting) in the quiet hours of the night. Their bot may automatically disconnect them when they have received too many tells. They'll also become nervous and bot less. Unless responses really convince you that he is not botting, report the guy to the GMs. Botters may get a ban.
Less fishermen -> Less supply -> More profit for each fish you sell.