Task: Question Answering
Input (English): Background Paragraph: Fish mortality is a parameter used in fisheries population dynamics to account for the loss of fish in a fish stock through death. The mortality can be divided into two types:

Natural mortality: the removal of fish from the stock due to causes not associated with fishing. Such causes can include disease, competition, cannibalism, old age, predation, pollution or any other natural factor that causes the death of fish. In fisheries models natural mortality is denoted by (M).[1]
Fishing mortality: the removal of fish from the stock due to fishing activities using any fishing gear.[1] It is denoted by (F) in fisheries models. 
Story: The fisherman had 2 choices: he could go to Silver Lake situated at a higher altitude and fish there; or he could go to Red lake, a few miles further down the road. Both lakes were natural, but Red Lake had a few pollution problems last year, and the oxygen levels in the water decreased, leading to many fish dying. Silver lake was still pristine, untouched by human activities such as pollution. 
Question: Which lake had a lower natural mortality last year?
Output (English): Silver Lake.