1. Deaths of fish from all causes except fishing (e.g. ageing, predation, cannibalism, disease, and perhaps increasingly pollution). It is often expressed as a rate that indicates the percentage of fish dying in a year; e.g. a natural mortality rate of 0. 2 implies that approximately 20 percent of the population will die in a year from causes other than fishing; 2. The loss in numbers in a year class from one age group to the subsequent one, due to natural death.