A fresh look at estimating mortality rates 

Dr. John Hoenig

Professor Emeritus, Virginia Institute of Marine Science

Abstract

Population dynamics are the results of three processes: growth, reproduction and mortality. In a fishing context, mortality is usually decomposed into fishing and “natural” components with natural mortality being all sources of mortality other than fishing. Natural mortality is often described as being difficult to estimate though in contemporary stock assessment work there is increasing emphasis on how natural mortality varies with time, with age, and over space. Little attention has been devoted to developing general principles for estimating natural mortality.  I maintain that in an exploited population, with very few exceptions, one needs contrast in fishing mortality in the data in order to separate fishing mortality from natural mortality. Variability (contrast) in fishing mortality can be over time (inter- or intra-annual, or both), over space, and over ontogeny. Higher contrast in fishing mortality results in enhanced ability to estimate natural mortality than does lower contrast. It follows that study design can be manipulated to achieve greater contrast. Integrated stock assessment models afford the opportunity to enhance the needed contrast provided the contrast is modeled explicitly. 

The division of total mortality into fishing and natural components can be inadequate when there is mortality that is neither “natural” in the sense of “usual” nor due to fishing. An example is when an emergent disease causes stress that a population is evolutionarily ill-equiped to handle. I will discuss quantifying disease impacts and the implications of such mortality.