Deaths of fish from all causes except fishing (e.g. ageing, predation and disease). It is often expressed as an annual rate that indicates the percentage of fish dying in a year; e.g. a natural mortality rate of 0.2 implies that approximately 20% of the population will die in a year from causes other than fishing. Scientists use the instantaneous natural mortality rate, M, which may also be split into one part due to predation and the other due to all other natural causes (disease, ageing).