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Fish Population Assessment

Fish Population Assessment. How many fish do we have?. 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). Fish Population Assessment.

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Fish Population Assessment

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  1. Fish Population Assessment How many fish do we have?

  2. 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) Fish Population Assessment

  3. Plot Method Total population area Size of the plot Average number of fish per plot Population estimate

  4. Plot Method - Estimated Variance Number of fish counted in ith plot Number of plots used

  5. Plot Method - 95% confidence interval for s-1 df, p=0.05

  6. Plot Method -Example • Pond area = 100 m2 • Size of plot = 1 m2 • Average number of fish per plot = 1.5

  7. 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

  8. Mark and Recapture - Variance

  9. Mark and Recapture - 95% confidence interval

  10. Mark and Recapture - Example M = 550 C = 500 R = 157

  11. Mark and Recapture - Example M = 550 C = 500 R = 157

  12. 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

  13. Schnabel Method - Variance & 95% C.I. Then invert for 95% C.I. for N

  14. Schnabel Method - example p. 137 (2nd ed.)

  15. Schnabel Method - example p. 137 (2nd ed.) 95% C.I. = 1,602 - 2,049

  16. Change in Ratio orDichotomy Method Requirements: • 1) two recognizable classes • Species • Sexes • Adults vs. juveniles • Age classes • 2) different rates of exploitation

  17. 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)

  18. 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

  19. 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)

  20. Change in Ratio orDichotomy Method Proportion of X in first sample Proportion of X in second sample Population estimate for X

  21. Change in Ratio orDichotomy Method Population estimate for X + Y Population estimate for Y

  22. Change in Ratio orDichotomy Method - Example • Trout (T) and suckers (S) • Sample before harvest: • n1=90, T1=30, S1=60 • Sample after harvest: • n2=58, T2=14, S2=44 • Harvest between samples: • 160 trout, 160 suckers

  23. Change in Ratio orDichotomy Method - Example Proportions of trout in two samples

  24. Change in Ratio orDichotomy Method - Example Trout estimate Trout and suckers combined Sucker estimate

  25. Removal Sampling -Zippin Method • 3-pass removal • U1=number of fish removed on 1st pass • U2=number of fish removed on 2nd pass • U3=number of fish removed on 3rd pass • M=sum of all removals (U1+U2+U3) • t=number of removal passes (3) • C=weighted sum = (1 X U1)+(2 X U2)+(3 X U3)

  26. Removal Sampling -Zippin Method Capture probability

  27. Removal Sampling -Zippin Method Population estimate

  28. 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

  29. Removal Sampling -Zippin Method - example Slimy sculpin in Garvin Brook

  30. Removal Sampling -Zippin Method - example

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