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Estimating AbundanceReading: Chapter 10

– Survey design– Visual censuses– Acoustic methods– Trawl surveys– Depletion estimates– Mark-recapture estimates– Egg Production Methods– Fishery-dependent CPUE

Estimating AbundanceWhy do we need to estimate abundance?

To estimate:1. Stock size2. Recruitment3. Mortality4. Spatial distribution

Estimating AbundanceSurvey design

– A central problem is obtaining an abundance index that is proportional to stock size

– Well-designed survey should provide estimates of:• average fish abundance or density and • Spatial distribution (survey boundaries?)

– Accuracy vs. Precision

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Accuracy Precision

Estimating AbundanceSurvey design

– A central problem is obtaining an abundance index that is proportional to stock size

– Well-designed survey should provide estimates of:• average fish abundance or density and • Spatial distribution (survey boundaries?)

– Accuracy vs. Precision– Bias vs. Variance– ↑ precision (↓ error) = ↑ $

Sample size (n)5 10 15 20 25 30 35

Sam

ple

Erro

r (%

)

40

50

60

70

80

90

100

110

Sample error vs. sample size

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Estimating AbundanceSurvey design

– Stratification by habitat type or depth– Combine abundance estimates across strata– Increases precision– Systematic vs. Random sampling– Systematic can be more precise and generally

reduces costs

Estimating AbundanceVisual censuses

Require clear, shallow watersBest with non-cryptic fish that don’t avoid diversCan see fish and habitatTransects most commonPoint counts (timed or instantaneous)Behavior

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Estimating AbundanceAcoustics

Use of sound waves to detect fish (swim bladder)Best for pelagic fishesTarget strength is species-specific and must be determined experimentallySimultaneous trawling to ‘ground-truth’ catchProblems with acoustic shadows and avoidanceVery promising for well understood pelagic stocks

Estimating AbundanceDepletion (or Removal) estimates

Relation between abundance and catch rateRequires:

Closed populationShort fishing period (no recruitment)Catchability proportional to abundance

CPUE (C/f) = qNt

Nt = N0 – Kt

CPUE (C/f) = qN0 – qKt

Plot CPUE vs. cumulative catch (K) (known as Leslie method)

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Kt

CPU

E

“Leslie Method”

Estimate of N0Slope = -q

Consecutive sweeps with 100ft. seine (Fall 2003)

1st haul 2nd haul 3rd haul

Num

ber c

aptu

red

0

5

10

15

20

25

30

35

40

45

Pinfish

Cumulative Catch40 50 60 70 80 90 100

CPU

E

0

10

20

30

40

50

Mullet

Cumulative Catch20 25 30 35 40

CPU

E

0

5

10

15

20

25

Spot

Cumulative Catch5 10 15 20 25

CPU

E

0

2

4

6

8

10

12

14Shrimp

Cumulative Catch5 10 15 20 25

CPU

E

0

2

4

6

8

10

Depletion estimates of abundance (Fall 2003)

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60ft seine pulled inside 100ft seine (Fall 2003)

1st haul 2nd haul 3rd haul 4th haul

Num

ber c

aptu

red

0

5

10

15

20

25

MulletSpotPinfishBlue crab

Fort Fisher Field trip 2004Depletion estimation using 100ft. seine

Seine haul1st haul 2nd haul 3rd haul

Num

bers

cap

ture

d

0

20

40

60

80

100

120

140

Pinfish Mojarra Atl silverside Ladyfish Total fish

Pinfish

K70 80 90 100 110 120 130

CPU

E

0

20

40

60

80

100

All species

K75 100 125 150 175 200 225 250 275 300

CPU

E

0

20

40

60

80

100

120

140

160

180

200

Mojarra

K0 50 100 150 200

CPU

E

0

10

20

30

40

Atl. silverside

K0 10 20 30 40 50

CPU

E

0

5

10

15

20

Depletion estimates of abundance (Fall 2004)

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Pinfish

Cumulative catch (K)500 520 540 560 580

CP

UE

(# p

er h

aul)

-100

0

100

200

300

400

500

600

Atl. silverside

Cumulative catch (K)70 80 90 100

CP

UE

(# p

er h

aul)

0

10

20

30

40

50

60

70

80

Atl. croaker

Cumulative catch (K)7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4

CP

UE

(# p

er h

aul)

0

2

4

6

8

10

All species

Cumulative catch (K)850 900 950 1000 1050

CP

UE

(# p

er h

aul)

-200

0

200

400

600

800

1000

Depletion estimates of abundance (Fall 2005)

Total Length (mm)0 20 40 60 80 100 120 140

Rel

ativ

e fre

quen

cy (%

)

0

10

20

30

40

50

60

20 ft seine

Total Length (mm)0 20 40 60 80 100 120 140

Rel

ativ

e fre

quen

cy (%

)

0

2

4

6

8

10

12

14

16

60 ft seine

n = 71

n = 210

Size-selectivity of beach seines (Fall 2005)

Estimating AbundanceDepletion (or Removal) estimates

DeLury Method

CPUE (C/f) = qNtCPUE (C/f) = qN0(Nt/N0)ln CPUE = ln qN0 + ln (Nt/N0)

Substitute Nt/N0 = e-qE

ln CPUE = ln qN0 –qE

Plot ln CPUE vs. cumulative effort (E)

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Cumulative effort (E)

lnC

PUE

DeLury Method

slope = -q

y-int. = ln qN0

Estimating AbundanceTrawl surveys

Very widely used, most commonMesh size regulates fish sizeConstant catchability (q) essential; lack of standardization is major problemConsistent gear design, tow speed, duration help to maintain q

C = qfNCPUE = qDStock biomass = D x area

Estimating AbundanceTrawl surveys

Many factors affect catchability (q)Tow speedDepthTime of dayVessel noise

Mostly, q is unknown, but…..If q is constant, then estimated stock biomass will be proportional to actual stock size

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Estimating AbundanceMark-recapture methods

Successful in terrestrial and freshwater systemsCan also provide growth and movement dataAssume:

Tagged fish mix randomly with untagged fishCatchability equalNo tag loss or mortality due to taggingRelatively closed population

T/N = R/Cso, N = TC/R

Estimating AbundanceEgg production estimates

Provide estimate of size of spawning stockUsed for large pelagic fish stocksAnnual method for determinate spawnersDaily method for indeterminate spawners

Prod = BiomassRatioFecundityso, B = P/RF

Need to account for atresia, mortality, age

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Surveys

Annual method

Daily method

Estimating AbundanceWhat’s wrong with using CPUE from fishery?

It provides catch and effort data from large areas over long time scales, so why not use it?Often times it is used, only data availableLandings data omits discards (bycatch, undersize)Catch/effort data hard to get for every boatCPUE (LPUE) rarely proportional to abundance

No gear standardizationCapture efficiency increases with timeFishers don’t fish randomly

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Fig. 10.15. Spatial distributionof commercial trawling effort (hours per year) in the North Sea

Fig. 4.16. Distributionof Atlantic cod in theGulf of St. Lawrence, showing range expansionand contraction over twenty years

Fig. 4.17. Occurrence oflow, medium, and high catches of Atlantic cod in research vesselsurveys as thefishery collapsed

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Fig. 4.18. How the calculation of meancatch rate can affectthe interpretation offishery trends, examplefrom northern cod

0

1

2

3

4

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6

7

8

9

0 1000 2000 3000 4000

Biomass (Tons)

Cat

chab

ility

Coe

ffici

ent

Abundance

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Hypoaggregation

Hyperaggregation

Abundance

Den

sity

Abundance

Cat

ch p

er E

ffort

(CPU

E)

CPUE remains high due to aggregation of fish