If you are playing on mobile you don't bring it a dead fish instead you have to feed it raw fish meat. Similar to what is done with the diplo or other dinos of that type you just wait for their hungry to reset as indicated above their head and then refeed them.
These little guys are so much easier to tame in mobile, but harder to find. You still need to kill a megipirahna, but you don’t need to grab its corpse to feed the otter. After harvesting its meat you can easily walk up to it and feed it until it’s tamed. It doesn’t run away in mobile, and I’m sure you can use any fish meat.