Chapter 11: Methods for Assessing Fish Populations
Kevin L. Pope, Steve E. Lochmann, and Michael K. Young
Fisheries managers are likely to assess fish populations at some point during the fisheries management process. Managers that follow the fisheries management process (see Chapter 5) might find their knowledge base insufficient during the steps of problem identification or management action and must assess a population before appropriate actions can be taken. Managers will implement some type of assessment during the evaluation step as a means of measuring progress relative to objectives. Choosing how to assess a population is an important decision because managers strive to maximize their knowledge of a population while minimizing the time and money expended to gain that knowledge.
A fish population is defined as a group of individuals of the same species or subspecies that are spatially, genetically, or demographically separated from other groups (Wells and Richmond 1995). A population will have a unique set of dynamics (e.g., recruitment, growth, and mortality) that influence its current and future status. The terms population assessment and stock assessment are used interchangeably by some fisheries managers. In general terms, a fish stock is a portion of a population, or a subpopulation. Stock assessment often refers to that portion of the fish population that is exploitable by a fishery, but we use the more inclusive population assessment throughout this chapter. Distinction is also made between a fish population and a sample of that population. Biologists almost never examine all the fish in a population, but rather base inferences on a sample of individuals from a population. How, where, and when those samples are drawn has a tremendous influence on the quality of data and validity of inferences.
In this chapter, methods for assessing inland fish populations to support management decisions are presented. It is important to consider bias (the unequal probability of sampling members of a population), precision (the degree of reproducibility of results), and the benefits of standardized sampling methods. A variety of population parameters and indices currently used to evaluate fish populations are reviewed, as are their respective strengths and limitations. This chapter will help students understand that proper design, analysis, and interpretation of assessment data are the foundation for appropriate management decisions.