The natural mortality of the plaice

Abstract

During their life histories fish grow up through the trophic levels of the marine ecosystem and it can be argued that their mortality due to predation is a density-dependent function of age. Such a function is derived formally and is applied to the plaice; a curve of mortality was calculated from larval life to the critical age of sixteen years, at which the specific growth rates and mortality rates become equal. For older fish an attempt was made to show that a senescent mortality takes place, i. e. after the critical age. In a study of stock and recruitment, if the trend of natural mortality with age can be established, then cohorts can be considered to have replaced themselves on average by the critical age. Therefore replacement is contained within the system as it must be and there is no need to establish a replacement stock, as for example in the Ricker curve.