The fish marks the steer for slaughter, for which the steer will haul the fish into the killing air.
It's this kind of partnership that puts the lie to animals' lack of intelligence and sophistication. Together, in pure and loving symbiosis, each of them—the fish, the steer—ensures the death of the other.
Their relationship serves as a sign that the "food" animals are ready. Ready to form their own society with its own unfathomable morality. Ready to present to the world their warped reflection of humanity.
(Thanks to Dr. Papa Squirrel for the referral, from long, long ago.)