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.
best to tame a ichthyornis and carry it with you when looking for one of these guys, put the ichthyornis on hunt and retrieve it will catch a fish and bring it back golden and will have a better taming affect to feed to otter, like this up its very important peeps must know :)))
Just push this cute lil creature in front of a giga and it'll have an existential crisis. Realising the path of destruction it left in its wake and deteriorate in front of the otters blue sparkly marble eyes