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.
Context: Mary's lake was full of fish, the lake's environment was very healthy. Mary stopped fishing on this lake five years ago and maintained it properly. Jane's lake had high fish mortality both from fishing and pollution, and disease from overstocking.
Question: Which lake had more problems with overstocking?