poster_ICES_tromso - ILVO

BY-CATCH OF RAYS IN TRAWL FISHERIES FOR ATLANTIC SEABOB SHRIMP:
HOW EFFECTIVE ARE TEDs AND BRDs?
Tomas Willems1,2, Jochen Depestele1, Annelies De Backer1, Kris Hostens1 & Magda Vincx2
1Institute for Agricultural and Fisheries Research (ILVO) – Bio-environmental Research
2Ghent University, Department of Biology, Marine Biology Section
Contact: [email protected]
Background
Tropical shrimp trawling often causes unwanted bycatch of rays (Batoidea). Several ray species occur on
the IUCN Red List of Threathened Species.
Conclusions
• TEDs (not BRDs) cause an overall 36% reduction in catch rates of rays
• Reduction is size-dependent: larger rays escape better
• Reduction-at-size is species-dependent
• Small sized rays are most abundant in the population and remain being captured
Objective
Assess the effectiveness of commonly used gear
adaptations in reducing ray by-catch: Turtle Excluder
Device (TED) and By-catch Reduction Device (BRD)
Discussion
Advice to fishery: test alternative TED with reduced bar-spacing to protect
small sized rays
Data collection
• Suriname continental shelf
• Twin-rig bottom trawling for Atlantic seabob shrimp
Xiphopenaeus kroyeri
• 65 simultaneous catch-comparison hauls
• test-net: TED & BRD
• control-net: no TED, no BRD
• Rays sorted from catch, identified, measured (disk
width)
Results
14
Mean ray catch rate per species in test- and control-net
Catch rate (Indiv. h-1)
12
***
***
10
test-net
control-net
8
6
4
2
***
0
Dasyatis geijskesi
Dasyatis guttata
Gymnura micrura Rhinoptera bonasus
Urotrygon
microphthalmum
Mean (+SE) catch rate of all ray species in test and control nets. Significant differences in mean
catch rate are indicated with asterisks (***; paried t-test; p<0.001)
(a)
(a)
Body width (cm)
Number of individuals
Number of individuals
thetest-net
in the
Proportion
test-net
caught in
Proportioncaught
test/(test+control)
test/(test+control)
top view
thetest-net
inthe
Proportion
test-net
caught in
Proportion caught
test/(test+control)
test/(test+control)
side view
Codend in the test-net configuration with TED and BRD
(b)
width(cm)
(cm)
BodyBody
width
Body width
(cm)
Body width
(cm)
width(cm)
(cm)
BodyBody
width
Hans
Pictures and figures © Tomas Willems & Hillewaert/ILVO
(a) Pooled length-frequency distributions (solid line: control-net; dotted line: test-net) and the observed proportion (hollow dots) of the total
catch caught in the test-net; (b) GLMM modelled proportion of the total catches caught in the test-net. Interpretation of (b): A value of 0.5
(dashed line) indicates an even split between the two trawls, whereas a value of 0.2 indicates that 20% of all rays at that body width were
caught in the test-net and 80% were caught in the control-net.
Research funded by:
Proportion caught in the test-net
(b)
Smooth butterfly ray
Gymnura micrura
Proportiontest/(test+control)
caught in the test-net
test/(test+control)
TURTLE EXCLUDER DEVICE (TED)
• Downward excluding type
• 100 mm bar spacing
Body width
(cm)
Body width
(cm)
Body width (cm)
Proportion caught in the test-net
BY-CATCH REDUCTION DEVICE (BRD)
• Square mesh window panel
• 11 x 11 meshes
• 150 mm stretched mesh size
• Top of codend, behind TED
Longnoze stingray
Dasyatis guttata
Smooth butterfly ray
Gymnura micrura
Proportiontest/(test+control)
caught in the test-net
test/(test+control)
Number ofofindividuals
individuals
Number
Longnoze stingray
Dasyatis guttata

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