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Estimates of population size play a vital role in many fisheries management decisions The numbers of fish in a stock are used to identify influences of environmental factors human exploitation and ultimately to identify the effectiveness of management strategies Van Den Avyle Hayward 1999 The three most common methods of population estimation among biologist are sample plots mark and recapture and removal Counts on sample plots is based on the principle that an estimate of population size can be obtained by determining the average density of animals per unit area and multiplying its value by the total area covered by the population This procedure is done by setting up a pre-determined number of circular square or rectangular plots These plots should be randomly placed and should not over lap This method is used when all members of the target population can be counted with reasonable certaintyVan Der Avyle Hayward 1999 The formula used with this method is Where A is the size of the study area a is the size of the plot and n is the average number of animals counted per sample plot Van Der Avyle Hayward 1999The mark and recapture method is simply preformed by collecting fish marking them releasing them and at a later time collecting fish from the same area and examining them for marks This is based on the principle that the number of marked fish in the second sample is proportional to the total number of fish in the population This is called the Peterson method and the equation is as followsWhere M is the number of fish initially marked and released C is the number of fish collected and R is the number of recaptures Van Der Avyle Hayward 1999 The Peterson index can give biased estimates when the numbers of fish sampled are low so several modifications have been made to correct this One being Baileys modification which is used when sampling
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