what does santiago keep reminding himself to do in the morning when it gets light? why must he do this?

From the text:

"I must surely remember to eat the tuna after it gets light."

He needs to eat the tuna for strength:

“I don’t think I can eat an entire one,” he said and drew his knife across one of the strips. He could feel the steady hard pull of the line and his left hand was cramped. It drew up tight on the heavy cord and he looked at it in disgust.

“What kind of a hand is that,” he said. “Cramp then if you want. Make yourself into a claw. It will do you no good.” Come on, he thought and looked down into the dark water at the slant of the line.

Eat it now and it will strengthen the hand. It is not the hand’s fault and you have been many hours with the fish. But you can stay with him forever. Eat the bonito now."