Population models are an essential part of the stock assessment framework because they provide three essential elements: (1) a theoretical framework for understanding the dynamics of a natural population, (2) a quantitative framework to make inferences about the historical and current status of a population, and (3) a framework to make quantitative predictions about the effects of future harvest on the status of the population. There are literally hundreds of different types of population models that have been developed to describe the dynamics of a single population, and even dozens ofmodels that have been developed to describe the dynamics of whole communities. Here, we restrict the descriptions to three general types ofpopulation models: (1) production models, (2) age- or stage-based models, and (3) multispecies or multistock models. A common thread among all of the population models and the ecological basis of sustainable harvesting is the concept of density dependence.
The dynamics of any natural population is determined by four vital rates: mortality, births (or recruitment), immigration, and emigration. In fisheries stock assessment, it is common to assume a unit stock, or a closed population, and simply ignore the effects of immigration and emigration; therefore, there are only two important vital rates of concern (mortality and recruitment). The ecological basis for sustainable harvesting requires that either recruitment rate increases or natural mortality rate must decrease, otherwise the population will head toward extinction once harvesting has initiated. In other words, the recruitment rate must increase or the mortality rate must decrease as the density of fish decreases, or these rates are simple density dependent. There is very little empirical evidence in fish of density-dependent mortality rates; however, there is a tremendous amount of evidence for density-dependent recruitment rates.
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