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.
If you are starter tame an otter it will give you silica peral by killing fish it will give 1 for 1 fish but good for a starter if you didn’t see a silica on the beaches send him to kill and bring silica and it can tame by raw meat or fish meat. Sorry for my bad English but enjoy the silica and otter
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.
On genesis if you teleport arctic north you can find otters on the frozen lake it's a lot easier since they tend to run around on the lake and not swim in the water, they also usually have blue,brown, or black in their color schemes