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Of the two fundamental characteristics of a population, one, the genetic composition, is discussed in Chapter 10. The other is numerical density. The abundance of a population of sticklebacks will depend on the rate at which new animals are added to the population by births or immigration and the rate at which fish are lost from the population by deaths and emigration. Sticklebacks are seasonal breeders, so the increase in density because of births in the population takes place only during a limited period of the year, whereas deaths take place throughout the year. In principle, immigration and emigration can also occur throughout the year, but the extent of such movements between populations is not known. Some studies on resident freshwater populations have suggested that movement in such populations can be very restricted (Hagen, 1967), but the migratory behaviour of anadromous populations makes possible some movement between them, particularly during the overwintering period in the coastal waters. Analysis of the genetic composition of anadromous populations from along the same coast may indicate the degree of mixing between the populations. Both birth and death rates and immigration and emigration rates also depend on the age structure of the population and its genetic composition. Thus, although the question, ‘What determines the abundance of a population of sticklebacks?’, looks deceptively simple, the answer involves many aspects of the biology of the stickleback.
KeywordsBreeding Season Sexual Maturity Genetic Composition Sockeye Salmon Maximum Lifespan
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