Why did Santiago cut the line that a second fish had been hooked on?

Santigo is fixed on the Marlin. He is not concerned with other fish. In the night, Santiago catches another fish on one of his other lines but cuts it loose before he even knows what it is. He also cuts away the other leader line that is still in the water, so he can use all the reserve coils of line to bring in the marlin that he is joined in battle with.