2011 depestele fdi_ijms_quantifying-causes-of-discard-variability
1 like205 views
This document summarizes a study that aimed to predict fish discard rates based on gear selectivity. The study analyzed discard data from Belgian beam trawl fisheries from 2006-2008. For sole and plaice, which are target species, observed and predicted discard rates matched well. However, for bycatch species like cod and whiting, other factors like resource availability and quotas influenced discard variability and rates could not be accurately predicted based on selectivity alone. The study concluded that discard prediction is possible for target species if the fishery is well understood, but other correction factors are needed to predict discards of low-value bycatch species.
2011 depestele fdi_ijms_quantifying-causes-of-discard-variability
- 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?
- 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? No = ( + )
- 7. Material and methods ANALYSIS I • 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