Grab the otter with your argentarvis but not with the claws, instead do it with the beak. That way you can carry him from river to river searching for big fish while he is sitting with your argi. Remember to change your argi to passiv
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.