Some Methods for Estimating Exploited Populations

Abstract

Three methods of estimating population and one of estimating recruitment are described which are suitable for use where data are available on annual age composition, the numbers caught, and the effort expended in taking a known part of the catch.The recruitment rate is estimated, as the proportion of new recruits in the recruited population, from age composition data year by year without regard to the sizes of the parent populations, and from the approximate fishing mortality rate.The "q" population method estimates q, the coefficient of catchability, by comparing the rate of disappearance of a fixed group of year-classes with the rate of accumulation of fishing effort, using the rate of disappearance with age in a similar, unexploited population to eliminate the effects of natural mortality. The annual populations are then estimated from q and the catch per unit effort.The natural mortality and reproduction method is basically a modification of the DeLury method with an allowance for natural reproduction and recruitment. It obtains a series of estimates of the base year population by considering each of the periods beginning in this year and ending in each successive subsequent year, the final estimate being the mean of this series.The expected catch method takes as unknowns the initial population and the value of q, estimates the expected catches in each year, and compares them with the actual catches. It determines the values of the initial population and q which minimize the sum of the squares of the differences.Examples are worked out in full for all methods.