How To Estimate Absolute Abundance

1
How To Estimate Absolute Abundance
Extrapolating local samples Depletion
Estimates Mark Recapture methods Stock -
Assessments
2
Depletion Estimates Repeated sampling and
removal.
Block Nets
Block Nets
Stream
3
(No Transcript)
4
Suppose there were 200 trout in the stream
area AND, that during each pass, you catch 30
of the trout present
100
60
42
29
30
21
Catch
14
10
3
1
1
2
3
4
5
Pass Number
5
Some math

• The population size before each pass (Np) is
  equal to the initial population size (N) minus
  the cumulative catch that has already occurred
  (Kp-1)
• NpN-Kp-1
• The number of fish captured on each pass (Yp) is
  a constant fraction, q, of fish present
• Ypq Np
• q (N Kp-1)
• q N q Kp-1
• This is a line that relates catch per pass to
  cumulative catch

6
In Graphical Form
Essentially, youre estimating the expected
maximum cumulative catch (the x- intercept)
7
How would the following effect your estimation ?
Immigration or Emigration Differences in
individual vulnerabilities
8
How To Estimate Absolute Abundance
Extrapolating local samples Depletion
Estimates Mark Recapture methods Stock -
Assessments
9
Mark Recapture Methods

• Helps tell us the catchability (the proportion of
  the population captured in a sampling event)
• Mark Recapture can work well for this because
  the ratio of marked to unmarked individuals tells
  us about the catchability
• Anything that might alter this ratio can impose
  bias

10
How to Choose Depletion vs. Mark Recapture?

• When would depletion estimates be more reliable
  and cost- effective
• When would mark recapture be more reliable and
  cost- effective?

11
How To Estimate Absolute Abundance
Extrapolating local samples Depletion
Estimates Mark Recapture methods Stock -
Assessments
12
Stock Assessments

• Essentially a kind of depletion estimate
• Operates over longer time periods than depletion
  estimates (usually annual)
• So, you must account for reproduction and
  mortality
• The data are catches and catch rates
• More sophisticated methods use many types of data
• Survey catch rates
• Survey or fishery catch-at-age (or size)
• Recruitment Indices
• There are many more parameters to estimate!