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Fish mortality
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).〔Sparre, P.; Ursin E.and Venema S. C. (1989). Introduction to tropical fish stock assessment. Part 1- Manual. FAO Fisheries Technical Paper No. 3061. Rome, FAO. p. 337.〕
* Fishing mortality: the removal of fish from the stock due to fishing activities using any fishing gear.〔 It is denoted by (F) in fisheries models.
(M) and (F) are additive instantaneous rates that sum up to (Z), the instantaneous total mortality coefficient; that is, Z=M+F.〔Gulland, J.A. (1969). Manual of Methods for fish stock assessment. Part 1. Fish population analysis. FAO Man.Fish. Sci., (4):154.〕 These rates are usually calculated on an annual basis. Estimates of fish mortality rates are often included in mathematical yield models to predict yield levels obtained under various exploitation scenarios. These are used as resource management indices or in bioeconomic studies of fisheries.
== Estimating mortality ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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