1. Mark Recapture

2. An application: Estimating Steller sea lion pup mortality
Steller sea lion numbers have declined 70-90% since 1950s; now listed as endangered
Is mortality of pups an important factor in population decline?
Can’t safely tag newborn pups, so used natural markings of moms to “mark” individual pups

4. If she doesn’t suckle her pup - the pup may be dead
After each birth, “mark” mom by photographing, finding natural marks, and giving her a name
Every day, find mom and see if she suckles a pup. If so, her pup is still alive!
If she doesn’t suckle her pup - the pup may be dead
Use a mark-recapture model to estimate the mortality rate
Results: Pup mortality was generally low, but big storms killed large numbers of pups in unprotected rookeries
These finding improved population models and helped focus conservation efforts
(Kaplan et al. 2006)
“Sucka”

5. When do we take samples?
When we can’t observe the entire
population
Mark Recapture method can tell us…
Population size
Population demographics (birth rate, death, movement, survival, age structure)
Nichols,Janna
Central question to this method: What proportion of the population does a sample represent?

6. Petersen Mark-Recapture Method
Survey 1:
Survey 2:
Catch several animals
Catch (R) animals
Count recaptures (m)
Mark all (M) animals
Return animals to population
Return animals to population
What is the estimated population size (N)?

7. N = ?
Survey 2:
R = 15
m = 4
Survey 1:
M = 12

8. What is the total population size (N)?
Note that the proportion marked in the population
equals the proportion marked in the 2nd sample
M = m
N R
N = M R m
M = 12
R = 15
m = 4

9. What is the total population size (N)?
Note that the proportion marked in the population
equals the proportion marked in the 2nd sample
M = m
N R
N = M R m
N = 12 * 15 4
N = 45
(estimated)
N = 43
(actual)
M = 12
R = 15
m = 4

10. When would mark-recapture give you a bad estimate?
Unequal chances of being caught
Marks fall off or go unnoticed
Marked animals don’t disperse evenly back into the general population
Population is open
S. Schultz

11. Closed populations
No individuals enter or leave the population between surveys
Survey 1
Survey 2

12. Individuals enter or leave the population between surveys
Open populations
Individuals enter or leave the population between surveys
Survey 1
Survey 2

13. M = m N = R M N R m N = Estimated population size
M = Number of animals marked
R = Number of animals captured in resampling event
m = Number of “R” that were already marked

14. Use paper and cups to test out a mark-recapture accuracy.
Test variables that increase or decrease accuracy
Report results to the rest of class.
M = m
N R
N = R M m
N = Estimated population size
M = Number of animals marked
R = Number of animals captured in resampling event
m = Number of “R” that were already marked