Natural mortality and body size in fish populations

Fisheries stock assessments increasingly account for size-dependence in natural mortality rates, usually by modeling mortality as a power function of body length. Various empirical studies have indicated a scaling of mortality with length in the range of - 0.84 to - 1.11, but substantially different scaling exponents ranging from - 0.75 to - 1.5 have been proposed on theoretical grounds or derived from some empirical models. To resolve these controversies and provide a well-supported default estimate of scaling for stock assessments, we reanalyzed two major data sets used in previous studies that supported different scaling exponents, and a combined data set. Both original data sets and the combined data yielded within-population exponents close to - 1 when analyzed using joint-slope mixed-effects models with population as a random effect. When population effects were disregarded, regression models yielded exponents that did not correctly reflect within-population scaling. The greatest deviations from the correct within-population scaling of approximately - 1 occurred in multiple regression models of mortality, size, and growth parameters. We conclude that within- and amongpopulation scaling of natural mortality should be clearly distinguished, and that within-population scaling of natural mortality with length in fish populations is highly consistent at approximately - 1. We also explored empirical models for predicting the intercept of the mortality-length relationship for a given population from growth parameters.

Automated summary

Fisheries stock assessments increasingly consider the relationship between natural mortality rates and body length. This study reanalyzed previous data sets and found a close to -1 scaling relationship between natural mortality and length within populations. The study emphasizes the need to distinguish between within-population and among-population scaling relationships.