1 Estimating Abundance Reading: Chapter 10 – Survey design – Visual censuses – Acoustic methods – Trawl surveys – Depletion estimates – Mark-recapture estimates – Egg Production Methods – Fishery-dependent CPUE Estimating Abundance Why do we need to estimate abundance? To estimate: 1. Stock size 2. Recruitment 3. Mortality 4. Spatial distribution Estimating Abundance Survey 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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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
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