The fish swims upstream with force of purpose toward its goal. Its goal is the reconstruction of life of its species. Instinctively it knows without perseverance and fortitude this will not happen. Therefore, there is no other option.
So it swims, and it swims, and it swims, struggling at times, dodging dangers and sometimes even moving forward peacefully. It becomes tired yet it doesn’t stop. Because it knows to abandon the mission would be to abandon itself to the perils of extinction. And so, to perpetuate life, it moves ever forward.