Fish Population Dynamic and Stock Assessment

1.2.1. Concept of population

Population is the breeding unit of species having common spawning grounds. A population is a group of individuals of the same species that live together in the same place, and that posess an average set of properties, such as birth rates and death rates. This definition recognizes that populations are made up of individual organisms but does not require that we know which individuals give birth or die, or where they are located in space. Instead the population is characterized by average birth and death rates, and variability in these averages is treated as a statistical property of the population. Most definitions of population have some kind of spatial reference. The simplest and least restrictive of these is that a population is a group of individuals of the same species that live together in a particular area (e.g., Roughgarden, 1989). However, even though this definition is widely used by ecologists, it gives rise to serious difficulties and misinterpretations. A more rigorous definition should define the spatial dimension more precisely; for example, a group of individuals of the same species that live together in an area of sufficient size that all the requirements for reproduction, survival and migration can be met (e.g., Huffaker et al., 1984). It may be helpful to conceive an idea of sufficient size such that the rates of emigration out of the area and immigration into the area are roughly balanced. Whatever method is used to define the appropriate size of an area within which the population of a particular organism exists, it is important that most of the change in population size or density is due to births and deaths rather than immigration and emigration because the theory of population dynamics is based on this assumption.

Local population is a group of organisms of the same species that live together in an area where there is a high probability of interbreeding. Also called a subpopulation or, in systematics, a deme. Emigration and immigration from local populations need not necessarily be balanced and there may be fairly high probability of local extinction.

Metapopulation is a group of populations that share occasional migrants.

Absolute population is an estimate of the total number of organisms in an area.

Population density is an estimate of the number of organisms per unit area (e.g., hectare) or unit of habitat (e.g., kilogram of soil).

Relative population is an estimate of the number of organisms caught in nets or traps but which cannot be related to area in any way.

Size of population is determined by growth which is influenced by natality (‘+’ influence), mortality (‘-’ influence) and dispersion (‘+ & -’ influence). Various ecological density dependent and density independent factors are the factors that control the growth.

For the purpose of fitting yield models, males and females of the populations are treated as separate units, if differences exist in all morphological and biological characters.

For modelling purposes, population of different species coexisting in a particular area having similar characters can sometimes be treated as a single stock.

Assessment could be made on stock of a species, only when biology of species is clearly understood which includes its feeding, growth, spawning and migration habits.