Chapter 2: Fish Population Dynamics: Mortality, Growth, and Recruitment
Micheal S. Allen and Joseph E. Hightower
Fisheries management is a rewarding career because it is challenging and fun and, most importantly, has a real impact on the quality of people’s lives. Decisions made by fisheries managers about a commercial fishery directly affect the income of fishers. Decisions about a recreational fishery can influence angler satisfaction and the level of participation, which has direct economic effects on tackle shops, motel and restaurant owners, and fishing guides. Because these decisions can have an impact on a community or region, it is critical to have the best available information about fisheries resources, including habitat quality and species interactions, as well as the needs of human users of a resource. Methods to evaluate many of these factors are described in other chapters of this text.
The focus of this chapter is the use of quantitative methods to evaluate how management actions regarding harvest may influence fish abundance, the size of fish in a population, angler catch, and total yield (i.e., biomass of fish removed from a population). Assessment of these basic population characteristics enables a fisheries manager to detect changes occurring in a population in response to fishing. Diagnosing the condition of overfishing is an important step in fisheries management, and identifying management actions that can improve fish abundance and angler catches is obviously required for sustaining and improving fisheries. Thus, fish population dynamics and assessment are literally where “the rubber meets the road” in fisheries management.
Assessment of fish populations usually contains much uncertainty. John Shepherd’s adage that “fish are like trees, except they are invisible and they move,” provides a first look at the difficulties in evaluating fish populations. Fish are not typically visible, and thus our “view” of a fish population usually comes from a variety of sources, including anglers, commercial fisheries, and different sampling gears. All sampling gears have inherent sampling biases, and fisheries managers almost always work with incomplete information about fish stocks.