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Fish Population Assessment How many fish do we have?

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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)

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

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Plot Method - Estimated Variance Number of fish counted in i th plot Number of plots used

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

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Plot Method - Example Pond area = 100 m 2 Size of plot = 1 m 2 Average number of fish per plot = 1.5 Pond area = 100 m 2 Size of plot = 1 m 2 Average number of fish per plot = 1.5

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Mark and Recapture - Peterson Method (single) Number of fish initially marked & released Number of fish collected/examined in 2nd period Number of recaptures found in C Bailey modification

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Mark and Recapture - Variance

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Mark and Recapture - 95% confidence interval

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

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

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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

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

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Schnabel Method - example p. 137 (2nd ed.) PeriodRUnmarkedTotal C MCM 10150 00 22220322515033,750 3268611235339,536 …. 439…. Total254457,208

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

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

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Change in Ratio or Dichotomy 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) 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)

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Change in Ratio or Dichotomy Method Two classes, X & Y X/Y 0 Total harvest Zero X harvested per Y Conducted by sport or commercial fisheries or artificial manipulation (selective removal)

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Change in Ratio or Dichotomy Method 1. Total harvest C (C X, C Y ) 2. Sample size before harvest n 1 (X 1,Y 1 ) 3. Sample size after harvest n 2 (X 2,Y 2 ) 1. Total harvest C (C X, C Y ) 2. Sample size before harvest n 1 (X 1,Y 1 ) 3. Sample size after harvest n 2 (X 2,Y 2 )

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Change in Ratio or Dichotomy Method Proportion of X in first sample Proportion of X in second sample Population estimate for X

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Change in Ratio or Dichotomy Method Population estimate for X + Y Population estimate for Y

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Change in Ratio or Dichotomy Method - Example Trout (T) and suckers (S) Sample before harvest: n 1 =90, T 1 =30, S 1 =60 Sample after harvest: n 2 =58, T 2 =14, S 2 =44 Harvest between samples: 160 trout, 160 suckers Trout (T) and suckers (S) Sample before harvest: n 1 =90, T 1 =30, S 1 =60 Sample after harvest: n 2 =58, T 2 =14, S 2 =44 Harvest between samples: 160 trout, 160 suckers

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Change in Ratio or Dichotomy Method - Example Proportions of trout in two samples

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Change in Ratio or Dichotomy Method - Example Sucker estimate Trout estimate Trout and suckers combined

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

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Removal Sampling - Zippin Method Capture probability

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Removal Sampling - Zippin Method Population estimate

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Removal Sampling - Zippin Method - example Slimy sculpin in Garvin Brook t = 3 U 1 = 250 U 2 = 125 U 3 = 65 M = 440 C = (1) 250 + (2) 125 + (3) 65 = 695

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

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Removal Sampling - Zippin Method - example

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