trawl – total effort & catch estimation joël prado - fao/fiit

Trawl – Total Effort & Catch Estimation

http://www.safmc.net

Joël Prado - FAO/FIIThttp://www.whboat.com

Sampling Priorities1. Collect information on fishing

effort 2. Randomly sample for catch

composition 3. Record gear characteristics 4. Collect length-frequency data on

target and non-target catch

ObjectivesDescribe two methods to estimate

total catchDetermine when each method

should be utilizedDemonstrate ability to complete

the Trawl Effort / Catch form

Total Catch Estimation Make an estimate for every haul 5 methods – not all appropriate on

all vessels Choice of method depends on

haul size observer presence when haul

comes aboard Unsorted or sorted first

Total Catch Estimation - Methods1. Weigh entire catch (small, ~400 kg,)

before or after sorting;2. Weigh subsample, tally total baskets and

extrapolate to total catch using average basket weight (moderate, ~400-750kg);

3. Volumetric estimate: Bin or codend (large);

4. Catch/effort ratio – using “similar” hauls5. Captain/vessel estimate – least desirable10.Unable to estimate

Section 6.6.1 of Observer Manual

Tow comes on board

Determine approximate

catch size

<16 baskets

2A – Method #1i. Shovel all catch to basketsii. Weigh each basketiii. Sum weights & record total weight

>16 & < 30 baskets

2B – Method #2

i. Shovel all catch to baskets ii. Weigh ~10 baskets and count the restiii. Multiply total number of baskets by average weight of 10 basketsGo to #3

3B Go to Catch Comp #4

> 30 baskets

Yes

3A Done

Total Catch EstimateFigure 6-13 in manual

2

3Sampling for

catch composition?

No

Observer On deck

Observer Not on deck

1A– Method #4Catch/effort ratio 1

Similar tows?

1B – Method #5Captain’s estimate

Yes

No

2C – Method #3

i. Measure dimensions of bin, deck or codend

ii. Calculate catch volumeiii. Collect 4-6 baskets of unsorted

catch for density sampleiv. Weigh & record basket weightsv. Calculate total catch weight by

multiplying volume by densityGo to #3

Tow comes on board


catch size

<16 baskets


>16 & < 30 baskets

2B – Method #2



> 30 baskets

Yes

3A Done


2

3Sampling for

catch composition?

No

Observer On deck


1A– Method #4Catch/ effort ratio

1Similar tows?


Yes

No

2C – Method #3





Catch/effort ratio

X = (2.53 mt / 800 minutes) * 238 minutes = 0.752675 mt or 0.75mt

Haul Total Catch (mt)

StartTime

EndTime

Duration (min)

17 0.87 22:47 02:20 27318 1.11 02:59 06:38 21919 0.55 07:10 12:18 30820 X 12:51 16:49 238Estimated weight of

unknown haul (X) = (Σ Total weight of similar hauls / Σ Haul duration of similar hauls) * Haul duration of unknown haul

Tow comes on board


catch size

<16 baskets

2A – Method #1 – Weigh all catchi. Shovel all catch to basketsii. Weigh each basketiii. Sum weights & record total weight

>16 & < 30 baskets

2B – Method #2



> 30 baskets

Yes

3A Done


2

3Sampling for

catch composition?

No

Observer On deck



Similar tows?


Yes

No

2C – Method #3





Weigh all catch – before sorting

Σ basket weights = = 258.9 kg

24.3 kg

23.3 kg

27.7 kg

22.8 kg

22.9kg

23.2 kg

24.7 kg

25.6kg

24.6 kg

24.4 kg

15.4 kg

24.3 + 23.3 + 27.7 + 22.8 + 22.9 + 23.2 + 24.7 + 25.6 + 24.6 + 24.4 + 15.4

Weigh all catch – after sorting

Σ retained + Σ discard = +

= 170.5 kg + 88.4 kg = 258.9 kg

Spp A59.3 kg

18.4 kg

20.1kg

22.6 kg

27.3kg

DiscardRetained

Spp B45.5 kgSpp C

25.2 kg

Spp D20.3kg

Spp E7.6 kgSpp F6.8 kgSpp G4.3 kgSpp H1.5 kg

(59.3 + 45.5 + 25.2 + 20.3 + 7.6 + 6.8 +

4.3 + 1.5 kg)

(18.4 + 20.1 +22.6 + 27.3 kg)

Tow comes on board


catch size

<16 baskets


>16 & < 30 baskets

2B – Method #2 – Tally & Extrapolatei. Shovel all catch to baskets (fill evenly)ii. Weigh ~10 baskets and count the restiii. Multiply total number of baskets by average weight of 10 basketsGo to #3


> 30 baskets

Yes

3A Done

2

3Sampling for

catch composition?

No

Observer On deck



Similar tows?


Yes

No

2C – Method #3






Weigh subset of catch – before sorting

Fill evenly – randomly select 10Σ basket weights (orange) / # baskets weighed = 253.8 / 10 Total count * average = 28 * 25.38 = 710.64 kg Actual = 713.7 kg

27.2

26.1

25.8

24.4

23.6

27.9

26.2

24.4

25.4

27.4

27.1

25.1

27.0

23.4

25.5

24.5

25.9

23.9

26.2

23.5

26.8

24.8

27.7

25.4

27.7

23.6

23.9

23.4

27.2

24.4

27.1

25.1

23.4

23.9

23.5

26.8

24.8

27.7

Weigh subset of catch – after sorting

Total catch = Σ retained + discard estimate Σ retained = 80.1 + 79.8 + ….18.9 = 482.6 kgFill discard baskets evenly – randomly select 4 Σ basket weights (orange) / # baskets weighed = 105.2 kg/4 = 26.3 kgDiscards = Total basket count * average = 9 * 26.3 kg = 236.7 kg Total Catch = 482.6 + 236.7 = 719.3 kg Actual = 713.7 kg

25.9

23.9

26.2

23.5

26.8

24.8

27.7

25.4

27.7

Spp A80.1 kgSpp A

79.8 kg

Spp A80.5 kg

25.9

25.4

27.7

26.2

Spp B21.0 kgSpp B

19.9 kg

Spp B20.2 kgSpp B

20.5 kg

Spp C20.8 kgSpp C

20.0 kg

Spp D5.7 kgSpp E14.3 kg

Spp F19.7 kgSpp F

20.5 kg

Spp F20.4 kgSpp G

18.9 kg

Spp A20.3 kg

DiscardRetained

Tow comes on board


catch size

<16 baskets


>16 & < 30 baskets

2B – Method #2



> 30 baskets

Yes

3A Done

2

3Sampling for

catch composition?

No

Observer On deck



Similar tows?


Yes

No

2C – Method #3 –Volumetric

estimatei. Measure dimensions of bin, deck or codend ii. Calculate catch volumeiii. Collect 4-6 baskets of unsorted catch for density sampleiv. Weigh & record basket weightsv. Calculate total catch weight by multiplying volume by densityGo to #3


Volumetric estimate - binMeasurable areas on deck or holding binsPrimary measurements: length, width &

heightProblems/issues:

Too much waterAccessibility

StepsDetermine appropriate shape Measure Calculate volumeMultiply volume by density

http://www.seafish.org/b2b/subject.asp?p=325

Volumetric estimate - bin

Length = 3.24 m Width =

1.05

m

Heights = 0.56 m, 0.43m, 0.48m, 0.3m, 0.35m, 0.27mH Average = 2.49 / 6 = 0.415 m

Volume (rectangular bin) = L * W * Haverage

V = 3.24m * 1.05m * 0.415 m = 1.41183 m3

Estimated weight = V * density

= 1.41183 m3 * 0.912554 mt/m3 = 1.28837111382 mt

Estimated weight = 1.29 mt

Volumetric estimate - bin

Wedge Volume = ½ (H * L * W)

V= ½ (1.2 * 4.32 * 1.47) = 3.81024 m3

H= 1.2 m

L= 4.32 m

W= 1.47 m

Volumetric estimate - codend

Ellipsoidal SolidV = π * short radius * long radius * lengthV = π * a * b * LV = 3.14 * .37 m * 1.53 m * 4.2 mV = 7.469513 m3

L= 4.2 m

b= 1.53 m

a= 0.37m

http://www.net-sys.com

Codend measurementsBe aware of moving nets in

trawl alleyMeasure dimensions using

actual measurements and/or reference points (e.g. premeasuring trawl alley width, length can save time)

Measure large codends in sections

http://www.photolib.noaa.gov

DensityDensity = Mass (mt) / volume (m3) = Σ basket weights / Σ basket

volumes Basket weights (all filled to bottom of handle): 24.3 kg, 20.7 kg, 21 kg, 22.9 kg, 22.7 kg , 23 kg

Total basket wt = 134.6 kg or 0.1346 mt

Basket volume = L * W * HV= 0.41 * 0.26 * 0.22V= 0.023452 m3

Density (ρ) = mass (mt) / V (m3)ρ = .1346 mt / (0.023452 m3 * 6)ρ = .1346 mt / (0.140712 m3)ρ = 0.95656376 mt/m3

L= 0.41 m

W= 0.26 m

H=

0.2

2 m

Total Weight CalculationsTotal weight estimate = volume *

densityObserver logbook Diagrams – if possible, make

measurements of trawl alley and/or bins before leaving portSpace for haul by haul calculationsRecord all original measurements and formulas used

Issues – removal of catch prior to sortingDangerous or other ‘protected species‘

may be removed prior to sorting. Inorganic debris & plant materialLarge fish

Record number, species, estimated weight and include the weight in the total catch estimate

Record on spp comp form if appropriate

Retained catch estimateSometimes you will be unable to access

discardsRetained based on # cartons retained *

average carton weightSpecies # cartons retained

Carton weight

Estimated retained weight per species

Cassava fish 3 25 kg 75 kgShrimp 12 20 kg 240 kgGrouper 1 25 kg 25 kgTOTAL 340 kg or 0.34

mt

Trawl Effort / Total Catch formMake an entry for EVERY DAY you are

on board (including non-fishing days)And EVERY HAUL

Trawl Effort / Total Catch form


Figure from Davies and Lesch (1998)

Activity / HomeworkWorking alone but you can discuss among

yourselves15 minutes then the rest is homework Turn in when class starts tomorrow Review answers after graded

SummaryWhat are the 2 types of volumetric catch

estimates? When should a volumetric estimate be used?

When should you weigh a subsample of the catch and extrapolate to the total basket count?

How do you estimate catch if you were asleep during a haulback?

True or false – Make an entry on the trawl effort and total catch form for sampled hauls only.

Activity - answers

.35

Activity - answersHaul #: 1Total catch WT: 1.00

Total Weight CalculationCaptain estimate

Density / Other Calculation

Haul #:2Total catch WT:0.39

Total Weight CalculationUsed hauls 3-5 for catch/effort ratio

(0.78 +0.70+0.34 mt) * 147min =(267+217+204 min)

(1.82 mt) * 147min = 0.38886627 mt(688 min)


Haul Total catch

Start End Duration

2 X 0730 0957 147 3 0.78 1023 1450 257 4 0.70 1525 1902 217 5 0.34 2117 0041 204

267

Activity - answersHaul #: 3Total catch WT: 0.78 mt

Total Weight CalculationWeigh subsample / count baskets

Basket weights (kg) 26.3 +23.2+ 24.6 + 24.1 + 23.6 + 25.8 + 24.9 + 23.8+ 24.1+ 23.3 = 243.7 kg

Average basket weight= 243.7 kg / 10 baskets = 24.37 kg / basket

Total baskets=32

Total catch = 32 baskets* 24.37 kg /basket = 779. 84 kg

Convert to metric tons (mt) = 779. 84 kg ÷ 1000 kg/mt = 0.77984 mt


Activity - answersHaul #:4Total catch WT:0.70 mt

Total Weight CalculationWeigh subsample / count baskets

Total retained: 347.3 kg

Total # discard baskets: 14

discard baskets weighed=26.3 +24.2+ 26.9 + 25.1 + 25.6 + 24.8 + 26.9 + 24.2+ 24.7+ 24.3 = 253.0 kg

Average discard basket weight253.0 kg / 10 baskets = 25.3 kg / basket

Discard wt = 14 baskets * 25.3 kg = 354.2 kg

Total = Retained + Discards = 347.3 kg +354.2kg =701.5 kg

Convert to metric tons701.5 kg ÷ 1000 kg/mt = 0.7015 mt


Activity - answersHaul #: 5Total catch WT: 0.34 mt

Total Weight CalculationWeigh everything

=24.3+22.2+24.6+25.1+25.6+24.8+23.9+22.8+24.1+22.3+25.2+25.4+25.7+23.5 = 339.5 kg


Activity - answersHaul #:6Total catch WT:6.41 mt

Total Weight CalculationCodend volume – ellipsoidal solidMeasurementsL=3.75, Width=2.15, H=1, 1.1, 0.8, 0.8m

Average height= (1+1.1+.8+.8) / 4 = 0.925m

Vol. = π*length * short radius * long radiusV=3.14 * 3.75 * (2.15/2) * (.925/2)V=5.8543828125 m3

Total catch = V * density = 5.85438 m3* 1.094789 mt/m3 = 6.40931 mt

Density / Other CalculationBasket wts: 25.6, 24.8, 26.8, 25.5 kg

Total basket weights=25.6 + 24.8 + 26.8 + 25.5 = 102.7 kg

Convert to mt= 102.7/1000 = .1027 mt

Basket volume (Note)0.023452 m3*4 = 0.093808 m3

density= mass / volume=0.1027 mt /0.093808 = 1.094789 mt/m3

Note: standard volume from manual

INCORRECT

References Davies, S., and H. Lesch. 1998. The Commercial

Sampling Programme, Grade One Observer Manual, Version 2.1 - CSP/03. Ministry of Fisheries and Marine Resources, Commercial Sampling Programme, Observer Section, Walvis Bay, Namibia.

Pauly, D. 1984. Some simple methods for the assessment of tropical fish stocks. FAO Fish. Tech. Paper 234, FAO, Rome.

Homework 2 – Effort/catch form

trawl – total effort & catch estimation joël prado - fao/fiit

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