In lieu of an abstract, here is a brief excerpt of the content:

chapter 6 Population Dynamics and Recruitment population dynamics Population Characteristics Fecundity Longevity Individual Size Growth Rate Population Size Specificity of Habitat Requirements Spawning Frequency Parental Investment in Individual Offspring Early Life-History Traits population fluctuations Scales of Population Fluctuations Ranges of Population Fluctuations Concordant and Disconcordant Fluctuations Causes of Population Fluctuations spawner-recruit relationship General Theory Density Dependence Environmental Influences Density-Independent Effects Empirical Relationships Recruitment Theories Critical Period Theory Survival Curves for Natural Populations of Fishes Evidence for Starvation of Larvae at Sea Evidence for the Sensitivity of Larval Fish to Lack of Food Offshore Transport-Retention Theory Growth-Mortality Theory factors affecting survival of eggs and larvae Physical Factors Temperature Salinity Storminess and Turbulence Currents and Flow Features Biological Factors Feeding Predators importance of juvenile stage When Is Recruitment Set? Factors Affecting Survival of Juveniles Food Predators 181 POPULATION DYNAMICS In order to manage harvested fish populations, the sources and magnitude of changes in population size must be known. How much of the variation in population size is due to natural causes and how much is due to fishing? How much of the population can be harvested without affecting its ability to reproduce itself? When considering the impact of pollution or other anthropogenic habitat changes, it is essential to know how fish populations are affected; are changes due to these factors, or can such changes be expected without these environmental changes? Population dynamics is the study of fluctuations in abundance of populations. It considers how much populations vary, and over what timescales, and seeks to find the causes of these variations. These are complex issues with most fish populations having high fecundities, complex life cycles, and multiple year-classes in the mature population (Fogarty et al. 1991). Much of the study of population dynamics in fishes has focused on recruitment, since variations in annual reproductive output of populations seem to drive population abundance in most fishes. However, the term recruitment has several meanings in fishery literature. It can mean recruitment to the adult population, to the fished population, to the spawning stock, or to the juvenile stage. In this discussion the latter meaning is used; that is, recruitment is the number of juveniles in a population that annually survives the egg and larval stages. Populations of most nontropical fishes are composed of several distinct yearclasses (cohorts) (Figure 6.1). The variable contribution of annual recruitment to the population produces year-classes of varying abundance (Figure 6.2). Once a year-class has recruited to the population (i.e., reached the juvenile stage), there is limited interannual variation in its natural mortality rate, 182 Population Dynamics and Recruitment Currents Juvenile Habitat Requirements (Nursery Grounds) recruitment studies History of Recruitment Studies in the United States (From Kendall and Duker 1998) Recent Recruitment Studies Fisheries-Oceanography Coordinated Investigations (FOCI) South Atlantic Bight Recruitment Experiment (SABRE) Global Ocean Ecosystem Dynamics (GLOBEC) ICES: Cod and Climate Change U.S. GLOBEC: Georges Bank Future Directions of Recruitment Studies Timescales of Variability and Biological Responses A New Paradigm for Recruitment Research Timescales Multiple Trophic Levels Multispecies Focus however it may also be subjected to fishing mortality. In each successive year, the abundance of a year-class is diminished, however the relative abundance of each year-class maintains itself from year to year in the population. Strong and weak year-classes can be seen as peaks and valleys in age frequency diagrams of the population. Year-classes can be seen moving through the population in successive annual age frequency diagrams of the population. A common feature of population dynamics of fishes is that the number of progeny recruiting to the population is often not proportional to the number of adults (there is little if any relationship between stock and recruitment [Koslow 1992]). However, the range of variations in population fecundity is related to the range of variations in recruitment in some fishes (Serebryakov 1990). In the long term each adult must reproduce itself for population stability. However, with fishes that produce on the order of 105 to 106 eggs per year, most must die or the population will explode. Very small changes in the mortality of early stages will have dramatic impacts on recruitment to the adult population. Given this basic life-history pattern, Population Dynamics and Recruitment 183 Figure 6.1. Example of year-class phenomenon in fishes (from Hjort 1914). Composition in point of age of spring Atlantic herring (Clupea harengus), 1908–1913. 4 5 4 years old (scales showing 4 winter...

Share