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THIS PAPER NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHORS
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C.M. 1980/B :21
Fish Capture Commi~tee
International' Council for the
Exploration of,theS~a
Preliminary results of selectivity experiments with beam trawls
11
R. De Clerck and G. Vanden Broucke
,Fisheries Resear~h'Station, Ankerstr~at, 1,
Ostend, Belgium.
ABSTRACT.
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Experi~ents
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on a beam traloJler wi th ulow" horsepower showed no
differences
in selcctivity withresearch carriedout in the
mnrked
60's~ Research on beam' trawlers with "high" hors'epower is pl~nned.
RESUME~
•
Des experienccs sur un chalutier ng~ules',d'une puissnnce motrice '
peu elevee'; ont dcmontre da d'ifferonces peu signific'ati ves im 'ce qui
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con~erne la selecti vi te avec( les recherches executees' dans les anncos' '60.
Des ctudes similaires sur des chalutiers a gaules d'une plus forte puissance
motrice soriten cours.
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INTRODUCTION.
' . A riational p~ogramme has been set'up to investigate the rel~tionship
between selectivity and horsepower in the Belgian beam trawl fisheryfor
soles.
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These selectivity experim~nts will b~ ca~ried'out onthree types of
vesselswith a hor~epower in theo~der of 250,500 and 120Ö. This paper
is dealing with, the resl;llts on the'trawler with the lowest,horsepower.
The other, experiments
will take place during the last,quarter
of this year.
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HATERIAL AND r-IETHODS •
Two cruises were made on the commercial vessel Z 12 "SABRINA" of
49,96 -, gross tonnage" of' 285 hp and- equipl'ed for beam trawling wi th
beums,of 4 metres. ~o tickler chains were used. The main fishing area is
sho.....:n on figure 1 . ' ,
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Apart from the different cod-ends no alteration was made to the normal,
co~~ercial operation 6f the vessel~ In thc cod-end four different ~e6h'sizes
were used alternativ~ly, vize 75 mrn,' 80 mm, 85 mm and 90 mm. The cod-end cove!
. (whole cover) hada mesh size of 63 mm.
The physical properties of t~e netting used -for cod-ends and cover are,
shown in table 1. Mesh,sizes were measured regularly during the experiments
\'rith an ICES, spring-loaded gauge, with,an operating pressure of 4'kg. N6sig-nificant differences in,the mes~ size due to'ihe oper~tion of the net ,were ._
found. ,Some hauls were carried out without acover to ensure that no masking
occurred.
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, Throughout ,the experiments thc two beam trawls werc shot 'land haulcd
sicultaneously.,'"
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Thc catch of soles of each haul was measured to thc nearest;co and,
.. rough
cstimates
on thc, by-catch were made.
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RESULTS.
Thc resul ts 'of the first cruise are gi ve,n in figurcs 2,. 3 and 6.
During thc first 19 haule wi th 80 mm ,on Dtarboard, (figure 2) thc 50 %l'ength
value ,of 80 mm c~d-end was 26~8 cm andthc"one of 90 mm cod-eniwas28.5 c~~
During ,thc second part'ofthe cruisc when changing the_80 mm':gear to the port
thc corresponding valucs were somcwhntlower, viZe 25.8 cm 'and,27.2 cm (figur
3). Itmust, be,mentioned that during the B~c~nd half of the'cruis~-weather'
condi tions b~ca~e worseand the'catch of s'oles 'decreased substantiall'y mainly
inthe icngth ciass~s '23 'cm,t6 26 c~.,
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\ : '.rhe com~ined ~e~ect'ion oßi ven,of the whol~ trip, shows' -€liD.:-E 'tbc 50 %-~'
'retention length for the 80 mr:! cod-cnd reached 26.3, co end 'for the ,90 mm " r
, c6d-end ,28.1 co (figure 6).". The ,.80 r.1r.1 value is sioilar to the one uscd •.;n
the Coop~ Res. Rep. Sera A.9, 1969' (26 co)'but thc 90 mm cod-end'value
(30cm) is.lo'wer i~ 'this s~udy~ " '_ ~ : ' "
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. 'Th~ selection facto~s: were3.2 "for the 80 mm cod-en'd- and 3.1 for the
90' mm cod-end. Iri'th~ Coop. Res.Re~. Se~.A.9 and in the Report ~f the j"
,North SeaFlatfish \.,iorking' Group 1974 a. sele'ction fact'or of 3.3 was used~
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:,The secondcruise concerned the selectivity' of the 75 mm'and85mm
cod-end. As in" the', first experiment the ge'ar was' ch~ng~d from, starboard '
to port half' way, the trip but,:this did "not substantinlly, alter the results
.(figu~es 4 ~nd 5).' The50 % retention,lengt~ values were'24.5 ~m and24.6
cm, for ,the 75 mm and '~7.3'cm and 27~4 cm for the 85 mm cod-~nd. Thc overall
6electiv~ty - ogives of·the complete survey1resulted!n 24.5'cm as
%'~ength
value,for 75 ':nm co'd-end and in 27.5cmfor the 85 mm cod':'end (figure 7).
Those ,two values are similar to ~~c o~~s u~~d in th~'Coop.Res. Rep~ Sera
A.9. with values of25:cmfor 75 m~,cod-end and 28 cm for85 mm' cod~end.
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The selection~actor5 were in' the 'same order asthe ones' obtained !
in, the firstsurvey, viz.: 3.1 for the' 75 tim cod-end ,and 3,.15 ,for,t'he.'85 _
cod';;end.
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I,t became obvious as, a general c;nclusio~. ,of these 't\.:o experir.lent s
'that the selection wi th a bcnm trD,\'ller wi th "lowuhorsepower was similar ,
'to the resuits published in the Coop.' Res'.' Rep.' Ser. A.9. ' As mentfoned in:
. the introduction, the results of thc further >experiments 'on,vcGsels wi th. tlhigh'!
horsepower 'may gi ve a, darinite ansuer to' the problemof h~rsepower_in bea:::l' '
\' trawl selectivi ty~
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Table 1 - Properties of the netting used for cod-ends and cover.
Codend
Material'
R tex
Twine construction
Braiding
Mesh size (mm)
Mean
Range'
N° of measurements
Type of mesh gauge
Cover
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Material
Polyethylene monofilament
R tex
1'2 200 '
Twine construction
Braided
Braiding
Single twine
Mesh size (mm)
63
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'Figure 1. _ Main' fis hing grounds.
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