Having a mount that can attack in all directions can save you from a piranha swarm! Highly recommend the Baryonyx for it’s spinning attack. Won’t stun them like larger creatures but if melee is high enough it can 1 shot and harvest them all in one swoop. If that’s hard to find, a Moschops can kill piranhas swimming behind it but it won’t harvest fish for some reason. Useful in caves!
These things will spawn in your water pen and kill your tames if they're on passive. Make sure you have Dino's to kill them or foundations all around your pen, they've killed so many of my Icthyosaurus' it's not even funny.
I'm around lvl 30 and ever since I started the game I can honestly say "I hate these creatures!" I've died by them more then anything else lost two parasaurs, a raptor, and who knows what else. Advise keep glancing under water to see if any are around, avoide rivers crossing on foot. I like rock hopping and dino swimming they can take more damage