Question

To estimate the population size of crabs in a certain lagoon, traps were laid at random 400 crabs were caught, marked and released back into the lagoon. Four days later traps were laid again and 374 crabs were caught. Out of the 374 crabs, 80 were found to have been marked.

1. Calculate the population size of crabs in the lagoon. 
2. State three assumptions that were made during the investigation. 
3. What is the name given to this method of estimating the population size? 
4. Other than the method named above, state any other two methods that are used to estimate population size. 

Answer 1

1. Population = First Number marked × Second capture
                                Marked recarptured
                  = 400×374 = 1870
                           80
2.  
   1. There was even distribution of crabs.
   2. No movement in and out of lagoon/no migration
   3. Released crabs mixed freely with remaining population.
   4. Marked crabs had enough time to mix with the rest.
3. Capture recapture method
4.  
   1. Body counts
   2. Quadrat
   3. Transect (belt and line)

Answer 2

1870