Methods for Estimating Natural Mortality in Forage Fish Population Modeling: An Analyst's Perspective

Forage fishes can present unique problems for fisheries managers that are different from other fish being managed.  Estimation of natural mortality can be an important step in estimating stock status, trends, and reference points.  For analysts the problem can be a difficult one to deal with as traditional approaches often are unsatisfactory for forage species.  Managers are often interested not only in status, but availability of forage fish for commercially and recreationally important predators as well as relative removals of predators compared with the directed fishery. Here we will examine some current methods in use to estimate natural morality; from relatively simplistic life history approaches, to direct measures, and model estimations. Understanding the data requirements, pitfalls, and value of various methods can be helpful when deciding on which of the various methods are most appropriate to a given stock.