C.M. 1958
International Council for thc
Exploration ()f the Soa
Comparati ve Fishing Cornmi ttee·,,,.,,,'\#\,
No. 22
,/(."0
Escapement cf Haddook through a Wire Mesh Cod-End
by
A. von Brandt
According to the general definition the 50 percent. retention length 01'
50 peroent. escapement length is that length of aspodes of flsh at which
50 percent. cf the fish are retained by the geal:' and 50 percent. escape. In
co-ordination with every retention length there must be a fixed mesh-size. This
1s, however, known to change with the material applied.
Th1s must probably be due to the different propertios of various net materials.
Jansen (1949) is inclined to believe that the stiffness cf the twines used for
the nots has a considerable effect on mesh selection. Therefore, a stiffer not
material will retain more fish cf sizes which would possibly havo made them esoape
in a softer materiaL Later authors also hint at different flexibility as a
possib1e reason of different se1eotivity (Graham ot al" 1954; Luoas et a1. 1954;
Margetts, 1955; Parrish, 1956; von Brandt, 1957; C1eglewioz and Stryzewski, 195'7;
Clark, 1957; McCracken, 1957). With inoreasing flexibility the meshes more
readily fit the shape 01' the esoaping fish (McCraeken, 1957). So a mere general
definition was made, "Meshes of lightor, more flexible twines usually gi va higher
escapement than heavier and stiffer twines 01' the same s1ze". (Clark et a1.,1958).
In contrast to this there are the tests of Clark and Fritz (1957) for the
escapement of haddock and silver hake through a wire .mesh ood-end. 1'his material
has no flexibility as well aB H has no tension or elasticity compared with ether
net materials. One might have expected that thc 50 percont, retention length had
been smaller than in a eod-end ef manila with the sams mesh-81zs. The authorB
found, however, that the 50 percent. retention length cf the wire mash 1s about
2 cm, higher than that für manila.
During a cruise of FRV "JLnton Dohrn" te Buchan Doep in thi8 May!June there
Was a possibility of carrying out a similar experiment. Instead of thc God-end
a. cage in form of a semi-cylinder of 4 m. length and L 60 m. diameter spanned
with wire mesh was used. Bottom and end of the eage were closed vüth tighter
netting and cow hides. Thc wire used had a diameter ef 3 lThlL The interial single
bars of the meshes had a length of 40 111:.01. so that the average mesh-size was 80 mm.
The duration of haul was 1 hour. The number cf hauls was 3. No solection
oould be found for whiting. Most could oscaps because 01' the great SiZ8 of mesh.
All the following data refer '00 haddoek.
The wire mesh cod-end had been fastened to a herring trawl of manila with
180 feet ground rope like suoh cod-ends of manila$ Courlene, llPerlon H J and
Tre'Vira~
We used the coverod cod-end method. Tho results for the wire mesh ood~end and
that of cod-ends of other material oan be seen from the following table:
Cod-end material
1)
Average mesh-size
50% selection length
Selection factor
No. of fish in
seleotion range 3)
50% retention length
at 75 mm. mesh-8ize
Manila
64
21
3 .0c,
Courlene
'10
23
3.3
IlPorlo:n 1t
63
22
3.6 2 )
Trevira
77
28
3.6
Wire
80 rum.
30 cm~
3.8
2923
590
2784
5'7'1
1806
25
25
26
27
29
cm~
1) more ezact data on the net materials appHed will be pubHshed lateX' on,
2) lower than was expeoted.
:5) numher of f'iah between about 25 and 75 percent. retention length.
~
;;-
2 -
On principle thc experiment has affil"rrled tho rosult cf the montioned
tests by Clark and Fritz, The total escapement i8 greater in ths rigid
wirs mesh cod-end than in ood··ends of manila and eVen of tho tested synthetie
fibres. The missing flexibility 01' tension was not decisivs for the
selectivity of tho wire mesh ecru-end.
Reference
Clark, J.R,
1957
"Escapement of Haddock and Silver Hake through
a "vvire mesh cod-Gnd~ tl
IClJAF, S.24, Lisbon 1957.
Clark, J .R.,
Me Cracken, F.,
&, Templemann, W.
1958
"Summe.ry 01' goal" se1eotion infol"rrlation for
thc Commission Area".
ICNAF, No.33, Halifax, 1958.
MoCracken, F.D.
1957
"Sel,ection Factors for Cod and Haddock with
Cod-ends of different Materials')
ICNAF, S,13, Lisbon 1957.
& Fritz, R.L.
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