Concept of Population unit stock Concept of Population

Concept of Population & unit stock

Concept of Population: Population is all the organisms of the same group or species who live in a particular geographical area and are capable of interbreeding. Sometimes, due to unavoidable bias it may be necessary to differentiate between a ‘sample’ population and the real population, where the sample population consists of the objects that have an equal probability of being selected.

Cont. . o Population is the breeding unit of species having common spawning grounds. o A population is a group of individuals of the same species that live together in the same place, and that posses an average set of properties, such as birth rates and death rates.

Cont… The simplest and least restrictive definition of population 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.

Cont… 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). 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.

Cont… Meta-population 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.

Cont… Size of population is determined by growth which is influenced by natality (‘+’ influence) mortality (‘-’ influence) and dispersion (‘+ & -’ influence). There are various ecological density dependent and density independent factors that are the factors that controls 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.

Cont. . 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.

Malthusian theory of Population Malthusianism is the idea that population growth is potentially exponential while the growth of the food supply or other resources is linear.

Concept of a unit stock: A stock is a sub set of a “species”. The basic purpose of fish stock assessment is to provide suggestions on the optimum exploitation of aquatic living resource such as fish and shrimp. A stock is sub set of a “species” which is generally considered as basic taxonomic unit.

Cont. . Stock is a subset of one species having the same growth and mortality parameter, and inhibiting in a particular geographical area. A stock as “an intraspecific group of randomly mating individuals with temporal or spatial integrity”. (Ihssen at al. , 1981)

Shared stock: Several countries may exploit the same stock. This is the case for many migratory stock, e. g. tunas National stock: o Fish stock exploited by a particular country.

Why does the stock unit often fail. . ? 1. The full distribution area of the stock is not covered by the data collected, so that only part of the stock is considered. This is a typical example where several independent fisheries are exploiting the same stock. 2. Several independent stocks are lumped together, for example, because their areas of distribution overlap.

3. Continuous immigration and emigration of the components of one or more stocks from the fishing ground. Taking into account that most of the exploited marine resources undertake migration, an essential element to perform stock assessment is an understanding and knowledge of migration routes.