WLF 448: Fish & Wildlife Population Ecology 2010

REMOVAL METHODS 

A.  Change-in-Ratio

• Can be inexpensive (i.e., use hunters or fishers to "collect" all the data)

• Animals can be categorized into 2 groups (e.g., males and females, adults and juveniles, or 2 species) and removals are selective (i.e., more of one group is removed from the population than the other group)

• Need an estimate of the proportion in each group before and after removal

• Population must be closed during the removal season

    Example: 2 groups males (m) and females (f)

        P_m1 = proportion of males in population before removal (time = 1)

        P_m2 = proportion of males in population after removal (time = 2)

        m₁ = number of males in population before removal

        R_m = number of males removed

        R = total number of individuals removed (R_m + R_f)

        N₁ = total number of individuals before removal

B.  Catch-Effort Methods

• As more individuals are caught (and removed) from a population, there are fewer available to be 'caught' and catch-per-effort should decline

• If all could be removed, then the expected catch-per-effort would be 0 and the total number caught would equal the initial population size.

• The population is closed

• Probability of each individual being caught is constant throughout the experiment

• All individuals have the same probability of being caught in sample i.

We can estimate N₁ by regressing y_i and x_i

Example:  Blue Crabs (From Fischler 1965)