Fish Population Assessment

1
Fish Population Assessment

• How many fish do we have?

2
(No Transcript)
3
Fish Population Assessment

• Estimating population size

• 1) Plot method
• 2) Mark and recapture (Peterson method)
• 3) Mark and recapture (Schnabel method)
• 4) Change in ratio or dichotomy method
• 5) Removal sampling (Zippin method)

4
Plot Method
Total population area
Size of the plot
Average number of fish per plot
Population estimate
5
Plot Method - Estimated Variance
Number of fish counted in ith plot
Number of plots used
6
Plot Method - 95 confidence interval
for s-1 df, p0.05
7
Plot Method -Example

• Pond area 100 m2
• Size of plot 1 m2
• Average number of fish per plot 1.5

8
Mark and Recapture - Peterson Method (single)
Bailey modification
Number of fish initially marked released
Number of fish collected/examined in 2nd period
Number of recaptures found in C
9
Mark and Recapture - Variance
10
Mark and Recapture - 95 confidence interval
11
Mark and Recapture - Example
M 550 C 500 R 157
12
Mark and Recapture - Example
M 550 C 500 R 157
13
Mark and Recapture - Schnabel Method
Multiple episodes of mark and recapture
CM total captures X marked fish available for
recapture R recaptures of marked fish
14
Schnabel Method - Variance 95 C.I.
Then invert for 95 C.I. for N
15
Schnabel Method - example p. 137 (2nd ed.)
Period R Unmarked Total C M CM
1 0 150 150 0 0
2 22 203 225 150 33,750
3 26 86 112 353 39,536
. . . . 439 .
Total 254 457,208
16
Schnabel Method - example p. 137 (2nd ed.)
95 C.I. 1,602 - 2,049
17
Change in Ratio orDichotomy Method

• Requirements
• 1) two recognizable classes
• Species
• Sexes
• Adults vs. juveniles
• Age classes
• 2) different rates of exploitation

18
Change in Ratio orDichotomy Method

• Two assumptions must be met
• 1) All population change is due to harvest
• No mortality, recruitment, migration
• 2) Figures for harvest must be reliable (need for
  GOOD data)

19
Change in Ratio orDichotomy Method

• Two classes, X Y

Conducted by sport or commercial fisheries
or artificial manipulation (selective removal)
X/Y
0
Total harvest
Zero X harvested per Y
20
Change in Ratio orDichotomy Method

• 1. Total harvest C (CX , CY)
• 2. Sample size before harvest n1 (X1 ,Y1)
• 3. Sample size after harvest n2 (X2 ,Y2)

21
Change in Ratio orDichotomy Method
Proportion of X in first sample
Proportion of X in second sample
Population estimate for X
22
Change in Ratio orDichotomy Method
Population estimate for X Y
Population estimate for Y
23
Change in Ratio orDichotomy Method - Example

• Trout (T) and suckers (S)
• Sample before harvest
• n190, T130, S160
• Sample after harvest
• n258, T214, S244
• Harvest between samples
• 160 trout, 160 suckers

24
Change in Ratio orDichotomy Method - Example
Proportions of trout in two samples
25
Change in Ratio orDichotomy Method - Example
Trout estimate
Trout and suckers combined
Sucker estimate
26
Removal Sampling -Zippin Method

• 3-pass removal
• U1number of fish removed on 1st pass
• U2number of fish removed on 2nd pass
• U3number of fish removed on 3rd pass
• Msum of all removals (U1U2U3)
• tnumber of removal passes (3)
• Cweighted sum (1 X U1)(2 X U2)(3 X U3)

27
Removal Sampling -Zippin Method
Capture probability
28
Removal Sampling -Zippin Method
Population estimate
29
Removal Sampling -Zippin Method - example
Slimy sculpin in Garvin Brook
t 3 U1 250 U2 125 U3 65 M 440
C (1) 250 (2) 125 (3) 65 695
30
Removal Sampling -Zippin Method - example
Slimy sculpin in Garvin Brook
31
Removal Sampling -Zippin Method - example