QUANTIFYING CAUSES OF

DISCARD VARIABILITY

An indispensable assistance to discard estimation

and a paramount need for policy measures

25 August 2010

Fishery Dependent Information Conference

Jochen Depestele1,2 (presenter), Sofie Vandemaele1,3,

Willy Vanhee1, Hans Polet1, Els Torreele1, Herwig Leirs3, Magda Vincx2

Institute for Agricultural and Fisheries ResearchAnimal Sciences Unit

www.ilvo.vlaanderen.be

Agriculture and Fisheries Policy Area

2 31

Objective

Can we predict discards of commercial

fish species, based on gear selectivity?

Introduction

• Why?

Introduction

• Why?

• Prediction of fish discards based

on gear selectivity:

– Target species probably ok

– By-catch species probably

other

factors of

variability

Material and methods

• Beam trawl (80mm)

• Southern North Sea

• Landings (%)

BELGIAN DISCARDS OBSERVER PROGRAMME

Sole Plaice Cod Whiting

2006

2007

2008

21.3

24.3

23.1

22.7

19.0

20.5

6.3

6.2

8.6

1.5

0.8

1.6

(c) ILVO

Material and methods

• Observed discard rate:

• LFD high-grading?

No

ANALYSIS I

𝐷𝑂𝑏𝑠 = 𝑑𝑂𝑏𝑠

(𝑑𝑂𝑏𝑠 + 𝑘𝑂𝑏𝑠 )

𝐷𝑀𝐿𝑆 = 𝑑𝑀𝐿𝑆

(𝑑𝑀𝐿𝑆 + 𝑘𝑀𝐿𝑆)

Material and methods

• Observed discard rate:

• LFD high-grading?

No Yes

Discard variability factors?

ANALYSIS I

𝐷𝑂𝑏𝑠 = 𝑑𝑂𝑏𝑠

(𝑑𝑂𝑏𝑠 + 𝑘𝑂𝑏𝑠 )

Material and methods

Discard variability factors? (Rochet & Trenkel, 2005)

– Resource availability

– Fishing operation

– Catch and discards

– Market incentives

– Technical constraints

– Quota regulations

ANALYSIS II

𝑅 = 𝐷𝑂𝑏𝑠

𝐷𝑂𝑏𝑠 + 𝐷𝑀𝐿𝑆

Results

DISCARD RATES & LFD (SOLE & PLAICE)

DObs DMLS Difference

Sole

Plaice

0.13 (0.11)

0.27 (0.21)

0.11 (0.10)

0.25 (0.18)

0.02 (0.03)

0.03 (0.10)

Sole (Dobs = 0.13) Plaice (Dobs = 0.27) N

um

bers

Total length (cm)

0

1000

2000

3000

4000

5000

6000

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

0

500

1000

1500

2000

2500

15

18

21

24

27

30

33

36

39

42

45

48

51

54

58

Discussion

PREDICTION OF DISCARD RATES (SOLE & PLAICE)

• Sole

- “true” target (~50% landed value)

• Plaice as a target?

~12% value match landings and catch?

Redirecting fishing effort on micro-scale

Reduced fishing effort in springtime

Misreporting (no formal figures)

High-grading in NE-SE North Sea (Rijnsdorp et al. 2007)

fishery-specific!

Results

DISCARD RATES & LFD (COD & WHITING)

DObs DMLS Difference

Cod

Whiting

0.47 (0.31)

0.61 (0.33)

0.36 (0.31)

0.46 (0.27)

0.12 (0.24)

0.15 (0.16)

Cod (Dobs = 0.47) Whiting (Dobs = 0.61)

Num

bers

Total length (cm)

0

50

100

150

200

250

30015

21

27

33

39

45

51

57

63

69

75

81

92 0

300

600

900

1200

1500

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

Results & discussion

DISCARD VARIABILITY FACTORS? (COD)

Discard variability factors? (GAMM)

– Response:

– Explained by

Fish price

Catch composition

Quota

Te(t

rip q

uotu

m, 1

.49)

Trip quotum

Adj R² = 0.28

𝑅 = 𝐷𝑂𝑏𝑠

𝐷𝑂𝑏𝑠 + 𝐷𝑀𝐿𝑆

Results & discussion

PREDICTION OF DISCARD RATE? (COD)

Quotum

(kg / day)DObs DMLS Difference

<200

>=200

0.571 (0.268)

0.374 (0.332)

0.183 (0.200)

0.376 (0.331)

0.391 (0.285)

0.038 (0.082)

• Non-limiting quotum: predictions are ok

• Limiting quotum: NOT ok!

Resource availability

Results & discussion

DISCARD VARIABILITY FACTORS? (WHITING)

Discard variability factors? Non-significant

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

Re

sp

on

se

va

ria

ble

Correction factor?

- Fishing behaviour

- Resource availability, incl. catch composition

Conclusion

• Objective: Can we predict discards of

commercial fish species, based on gear

selectivity?

– Yes, for target species

Be fishery-specific!

– Unlikely for by-catch species

• High-value (e.g. cod): quota!

• Low-value (e.g.whiting): correction factor?

• Indication of management implications

Thank you for your attention

Contact: Jochen.Depestele@ilvo.vlaanderen.be

Institute for Agricultural and Fisheries ResearchAnimal Sciences Unit

www.ilvo.vlaanderen.be

Agriculture and Fisheries Policy Area

Financially supported by

2 31

Jochen Depestele1,2 (presenter), Sofie Vandemaele1,3,

Willy Vanhee1, Hans Polet1, Els Torreele1, Herwig Leirs3, Magda Vincx2