Principles for estimating natural mortality rate

In contemporary stock assessments there is increasing emphasis on how natural mortality varies with time, age, and space. But, little guidance is available for estimating this parameter. We maintain that in an exploited population, with few exceptions, contrast in fishing mortality in the data is needed to separate fishing mortality from natural mortality. Variability (contrast) in fishing mortality can be over time (inter- or intra-annual), space, and ontogeny. (The exceptions where contrast in fishing mortality is not needed are cases where information is available on population size and total catch.) High contrast in fishing mortality results in enhanced ability to estimate natural mortality. It follows that study design can be manipulated to achieve greater contrast and make more effective use of existing contrast. Integrated stock assessments afford the opportunity to enhance the needed contrast, provided the contrast is modeled explicitly.