Use a rex and a crossbow. Find a LOW level (20 or less) bird swimming on a river. Use the rex to clear both shores of all nearby carnivores, then have the rex stand in the water and kill any piranha that show up. With your rex on PASSIVE follow (otherwise it will eat your fish), swim in the river and shoot a fish with your crossbow (sabertooth salmon are best, then piranha), then DRAG (do not harvest the meat) the fish to the bird, which is passively tamed. Rinse and repeat. (I find if the bird is swimming, it's easiest to folllow it and try to feed it the fish while you are floating under the bird.)