What I do to kill these things is just get it to the shoreline, then it can't hurt you while you stand in front of it and beat it to death with a bat. Literally the second species I killed after some dodos.
Went on a killing spree with my spino to pass time to breed aga. Decided to go kill piranhas and my spino legit got stunlocked permanently until I finally managed to kill all of them with my spino hanging on by a thread because of these godforsaken things. Hunt these things into extinction or they'll murder everything you love and the neighbors.
These things spawn on The Island at the bottom right of the map near the Lava Cave. These cheeky little buggers are more than happy to ruin your day by swarming and stunlocking you and your tames nonstop unless you are on a Basilosaurus, which is the only creature that is immune to their attacks. If you don't have a Basilosaurus, lure them close to shore and cheese it with a spear or pike, better yet pray that they beach themselves and die.