mark recapture

Mark Recapture

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

• 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 rookeriesThese finding improved population models and helped focus conservation efforts(Kaplan et al. 2006)

“Sucka”

Central question to this method: What proportion of the population does a

sample represent?

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

Petersen Mark-Recapture Method

Catch several animals

Survey 1:

Mark all (M) animals

Return animals to population

Catch (R) animals

Survey 2:

Count recaptures (m)

Return animals to population

What is the estimated population size (N)?

Survey 1:

M = 12

Survey 2:

R = 15m = 4

N = ?

R = 15m = 4

What is the total population size (N)?

Note that the proportion marked in the populationequals the proportion marked in the 2nd sample

M = 12

N = M R m

M = mN R

N = 12 * 15 4

R = 15m = 4

What is the total population size (N)?

Note that the proportion marked in the populationequals the proportion marked in the 2nd sample

N = M R m

M = 12

N = 45(estimated)

N = 43

(actual)

M = mN R

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

Closed populations

No individuals enter or leave the population between surveys

Survey 1 Survey 2

Open populations

Individuals enter or leave the population between surveys

Survey 1 Survey 2

N = R M m

M = mN R

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

N = R M m

M = mN R

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

Use paper and cups to test out a mark-recapture accuracy.Test variables that increase or decrease accuracyReport results to the rest of class.