1. QUANTIFYING CAUSES OF
DISCARD VARIABILITY
An indispensable assistance to discard estimation
and a paramount need for policy measures
Jochen Depestele1,2 (presenter), Sofie Vandemaele1,3,
Willy Vanhee1, Hans Polet1, Els Torreele1, Herwig Leirs3, Magda Vincx2
25 August 2010
Fishery Dependent Information Conference
Institute for Agricultural and Fisheries Research
Animal Sciences Unit
1 2 3
www.ilvo.vlaanderen.be
Agriculture and Fisheries Policy Area

2. Objective
Can we predict discards of commercial
fish species, based on gear selectivity?

3. Introduction
• Why?

4. Introduction
• Why?
• Prediction of fish discards based
on gear selectivity:
– Target species probably ok
– By-catch species probably
other
factors of
variability

5. Material and methods
BELGIAN DISCARDS OBSERVER PROGRAMME
• Beam trawl (80mm)
• Southern North Sea
• Landings (%)
Sole Plaice Cod Whiting
2006 21.3 22.7 6.3 1.5
2007 24.3 19.0 6.2 0.8
2008 23.1 20.5 8.6 1.6
(c) ILVO

6. Material and methods
ANALYSIS I
������������������������
• Observed discard rate: ������������������������ =
(������������������������ + ������������������������ )
• LFD high-grading?
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No ������������������������ =
(������������������������ + ������������������������ )

7. Material and methods
ANALYSIS I
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• Observed discard rate: ������������������������ =
(������������������������ + ������������������������ )
• LFD high-grading?
No Yes
Discard variability factors?

8. Material and methods
ANALYSIS II
Discard variability factors? (Rochet & Trenkel, 2005)
– Resource availability
– Fishing operation
– Catch and discards
– Market incentives ������������������������
– Technical constraints ������ =
������������������������ + ������������������������
– Quota regulations

9. Results
DISCARD RATES & LFD (SOLE & PLAICE)
Sole (Dobs = 0.13) Plaice (Dobs = 0.27)
6000 2500
5000 2000
Numbers
4000
1500
3000
1000
2000
1000 500
0 0
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
15
18
21
24
27
30
33
36
39
42
45
48
51
54
58
Total length (cm)
DObs DMLS Difference
Sole 0.13 (0.11) 0.11 (0.10) 0.02 (0.03)
Plaice 0.27 (0.21) 0.25 (0.18) 0.03 (0.10)

10. 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!

11. Results
DISCARD RATES & LFD (COD & WHITING)
Cod (Dobs = 0.47) Whiting (Dobs = 0.61)
300 1500
250
1200
200
Numbers
900
150
100 600
50 300
0
0
57
63
69
15
21
27
33
39
45
51
75
81
92
17
27
15
19
21
23
25
29
31
33
35
37
39
41
43
45
Total length (cm)
DObs DMLS Difference
Cod 0.47 (0.31) 0.36 (0.31) 0.12 (0.24)
Whiting 0.61 (0.33) 0.46 (0.27) 0.15 (0.16)

12. Results & discussion
DISCARD VARIABILITY FACTORS? (COD)
Discard variability factors? (GAMM)
– Response: Adj R² = 0.28
Te(trip quotum, 1.49)
������������������������
������ =
������������������������ + ������������������������
– Explained by
 Fish price
 Catch composition
 Quota Trip quotum

13. Results & discussion
PREDICTION OF DISCARD RATE? (COD)
Quotum
DObs DMLS Difference
(kg / day)
<200 0.571 (0.268) 0.183 (0.200) 0.391 (0.285)
>=200 0.374 (0.332) 0.376 (0.331) 0.038 (0.082)
• Non-limiting quotum: predictions are ok
• Limiting quotum: NOT ok!
Resource availability

14. Results & discussion
DISCARD VARIABILITY FACTORS? (WHITING)
Discard variability factors? Non-significant
1
0,9
0,8
Response variable
0,7
0,6
0,5
0,4
0,3
Correction factor?
0,2
- Fishing behaviour
0,1
- Resource availability, incl. catch composition
0

15. 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

16. Thank you for your attention
Jochen Depestele1,2 (presenter), Sofie Vandemaele1,3,
Willy Vanhee1, Hans Polet1, Els Torreele1, Herwig Leirs3, Magda Vincx2
Contact: Jochen.Depestele@ilvo.vlaanderen.be
Financially supported by
Institute for Agricultural and Fisheries Research
Animal Sciences Unit
1 2 3
www.ilvo.vlaanderen.be
Agriculture and Fisheries Policy Area