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FLUCTUATION IN POPULATION ABUNDANCE (POPULATION DYNAMICS)
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Fish population dynamics helps to understand how the fish population in a particular region grows and shrinks over time, as controlled by birth, death, and emigration or immigration. It is the basis for understanding changing fishery patterns and issues such as habitat destruction, predation and optimal harvesting rates. It helps to determine the sustainable yeilds
The size of the total stock of a race, species vary to a great extent from year to year and from one region of the world to the other. These fluctuations in abundance are caused by natural factors, apart from those caused by man’s activities, i.e., fishing and pollution. This population dynamics in general is considered to reflect the biological characteristic of a given species and its adaptations in the face of changing ecology. Survival reflects the adaptability of the species, mortality during the adversity of the ecology.
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Natality versus mortality
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If natally is equal to mortality at any instant, the stock abundance remains stationary.If it is more, the population increases in size; and if less, the population shows a preponderance of young ones, whereas the regressing one is dominated by aged adults.
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Stock-recruit relationship
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Position of the species in the food chain
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How close a species of fish is to the primary production in the food chain, and what is the variation in the distribution of the primary production at any time or place are important. Supply of nutrients and light are common factors influencing distribution and composition of the primary production of phytoplankton. It has been generally assumed that one trophic level in a food chain has production about 10% of the production of the previous trophic level. The proportion of the food consumed which actually is converted into growth and the efficiency of conversion, at the individual level or at the level of the population are limiting factors. This efficiency (ecological efficiency) of food uitilization is maximum when large year-classes occur, whereas it is less than maximum normally, it has been calculated that fish production comes to about 1% of the primary production. Thus, the population size does not depend on supply of food (primary production) but on the ecological efficiency of the species as well.
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Inter-specific interaction
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Population abundance of any one stock or if any species is also importantly influenced by interaction between this and other species belonging to an ecosystem. The two notable factors controlling population dynamics of one species are its competitors fishery biology, when the competitors and the predators are themselves commercially valuable fish stock. For example, in North sea, herring has mackerel as its competitor for food (zooplankton) and cod as its predator, both of which are objects of commercial fisheries.
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Competition: This form of interaction arises when more than one species compete for same food, i.e., at the same trophic level. This has been found to cause serious fluctuations in important food fishes.
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Predation: This is one of the most powerful forces controlling population dynamics. A majority of marine commercially valuable fishes are predators (such as tuna, salmon, cod, halibut). Their prey includes some of the important commercial fishes. This is one of the most powerful forces controlling population dynamics. Predators may have a selective effect, other may not be selective. Predators range from larger individual of the same species of fish to any other species of fish, amphibian, reptile, birds and mammals, on one hand, and of invertebrates on the other, the extent of damage done to the prey depends upon the species and age-groups of the prey.
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Interspecific interaction (competition) on the spawning grounds: This form of competition may also become so severe as to influence abundance, in the case of Pacific salmon, it so happens often that several species use the same place for spawning and at the same time.
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Interspecific interaction (competition) for space: This form of competition arises from overpopulation, and it is an important limiting factor for abundances. Overpopulation leads to competition for food and for the spawning grounds. In case of overpopulation since the availability of food is limited, less amount of food is converted into growth and more of it is used to maintain basal metabolism.
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Behavioural response to density of stock: Pheromones (fright substances) exist in fish and these are released in the even of crowded conditions by carps and gouramies.
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Physical factors
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