In Ernest Hemingway's The Old Man and the Sea, the Old Man has hooked a huge marlin, which is pulling his boat out to sea. Meanwhile, using another line, he has caught a "dolphin" (meaning a dolphin-fish or dorado):
The old man unhooked the fish, rebaited the line with another sardine and tossed it over. Then he worked his way slowly back to the bow. He washed his left hand and wiped it on his trousers. Then he shifted the heavy line from his right hand to his left and washed his right hand in the sea while he watched the sun go into the ocean and the slant of the big cord.
"He hasn't changed at all," he said. But watching the movement of the water against his hand he noted that it was perceptibly slower.
"I'll lash the two oars together across the stern and that will slow him in the night," he said. "He's good for the night and so am I."
It would be better to gut the dolphin a little later to save the blood in the meat, he thought. I can do that a little later and lash the oars to make a drag at the same time. I had better keep the fish quiet now and not disturb him too much at sunset. The setting of the sun is a difficult time for all fish.
He let his hand dry in the air then grasped the line with it and eased himself as much as he could and allowed himself to be pulled forward against the wood so that the boat took the strain as much, or more, than he did.
What does the Old Man mean by the sentence in bold? What exactly is the Old Man worried over here? Why would fishes be "worried" at sunset?