Note: Descriptions are shown in the official language in which they were submitted.
W O 92/17999 PC~r/N 092/00071
2~7~
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DEVICE FOR T~AW~
The described invention concerns a device for trawls that
makes the codend of the trawl able to retaln flsh larger
than a certain size, while smaller ~sh are glven a po88i-
bilty to escape through the new device.
As a principle the trawl i8 a large funnel of net meshes
pulled through the water by a ves~el and fish that enters
the open part in front of the trawl are herded into the
trawl and will become the catch in the aft and closed part
of the trawl, the codend.
In order to give small and Juvenile fish a possibillty to
escape the trawl, the codend of thls flshing gesr has a
glven (legal) mesh size in order to obt~ln such release of
small fish.
The size selectivity of trawls are generally too poor, and
practical experience (experlments) show that a signlficant
proportion of æmall ish will be retained in the codend,
thus resulting into catches containing high numbers of
small and unwanted fish. This pattern of exploitation of
fish stocks may cause overfishing as ~uvenile fish are
caught, i.e. before they are able to reproduce.
As a result there ls a great demand for a trawl which ls
able to sort out a slgnlficant number of small fish durlng
the fishlng operation, thus resulting into a catch contain-
ing a higher proportion of fish larger than a certainminimum size. During a ICES WG Meeting (FTFB) in Rostock,
April 1990, a system for trawl allowing small fish to
escape the trawl was presented. The sorting effects
reported, however, were not satisfactory.
3S
The priciple of the known device, which will be mentioned
and described later on, is based on a construction where a
SUI~STITUTE SHEET
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WO92/17999 ; PCT/NO92/00071
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"window" was placed in the upper section o~ the trawl clo~e
to the codend. The window was covered by a rigid metal
grid, where the distance between the bars o~ the grid made
it possible for small fish to escape. The inven~ion under
consideration will thus be an improved version of the above
described device, which allows for a signlflcant lmprove-
ment wlth respect to size selectivity of fish and escape-
ment of juvenile fish.
; 10 The invention will be fully described with reference to
drawings attached, where
Fig. la and lb are respectively showing a priciple sketch
of a normal bottom trawl and the known device applicated to
lS this.
Fig. 2 show a sideview through the new device placed in a
trawl in a comparable way as the known device.
Fig. 3a, 3b and 3c are respectively showing a ground plan,
a side view and a perspective sketch of a typical sorting
grid used in the new device.
Fig. 4a, 4b and 4c are respectively showing a ground plan,
a side view and a perspective sketch of the second sorting
grid used in the new device.
Flg. 5a, Sb and 5c are respectively showing a ground plan,
a side view and a perspective sketch of the at stop sec-
tlon used in the new device.
Fig. 6 show a princlple sketch illustrating how the newdevice, when fitted into a trawl, does function.
Fig. 7-11 show selectivlty results rom experiments made
wlth trawls using the new device.
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W092/17999 ~ 7 ~ ~ 3 PCT/N092/00071
With respect to Fig. 1 it is shown a normal bottom trawl 1
made of largemeshed netmaterial with a codend 2, where the
catch will be collected.
S Fig. lb show the known device fitted into the trawl, i.e.
Fig. la. The known device consist of a tilted (oblique)
sorting grid 3 that stretches from the upper part of the
trawl 4 to the bottom (lower part) of the trawl 5. In an
oblique position to the sosting grid 3 there is arranged an
aft stop section 6 which may be made of a strong canvas or
- platelike material.
Close to the lower panel of the the trawl 5 there is made
an opening in the sorting grid allowing larger and wanted
fish to pass into the oodend of the trawl 2.
Fig. 2 show a sideview of the principle by the new sorting
device due to the excisting invention fitted to a trawl at
the same position as shown in Fig. la, where the corres-
ponding figures has the same meaning as in Fig. 2a. The newsorting device consists of a main sorting grid 3 fastened
lnslde and along the net from the upper panel of the trawl
and down to the midseam/half dlstance between the top of
the upper panel 4 and the bottom of the lower panel 5. At
location 7 the main sorting grld 3 is Jointed to the second
sorting grid 3'. The second grid 3' is at location 8
Jointed to a stop sectlon 6 which malnly is built up of a
"waterproof" canvas (pvc materlal). ~he function of this
stop section 18 to lead the water flow through the sorting
grids away from the net and thus guide escaping ~uvenile
~lsh away from the trawl.
The main sorting grld 3 has an angle of attack of 25 or
more lengthwlse to the trawl, whlle the second grld 3' runs
parallel or sllghtly angled to this direction as lndicated
ln Fig. 2. The stop sectlon 6 has an angle of attack of
approximately 20 lengthwi~e to the trawl.
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WO92/17999 PC~/NO~2/00071
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Fig. 3a-3c show a more detailed picture of the sorting grid
3 in Fig. 2. The sorting grid 3 i8 made of an outer frame
lO where a number of bars ll are welded to it. Both the
S frame and the bars are made of stainless steel ~l8.8) and
the distance should preferentially be no smaller than 55 mm
Sbased on regulations on minimum landing sizes for cod and
haddock caught in the Barents Sea). The bardistance, how-
ever, has to be adjusted to local regulations on landing
sizes of fish and ~izes of target species wanted. The
length of the sorting grid 3 could be l,5 m or more (or
less). In the aft part, the sorting grids are rounded
(R~00) as shown in Fig. 3b and 3c.
lS Fig. 4a-4c does in a corresponding way show the second
sorting grid 3' which is made of the same material and
built in a corresponding way as the main sorting grid 3,
but it may be shorter than the main sorting grid 3.
Fig. 5a-5c show the stop section 6 which is made of a
stainless steel (18.8) outer frame 50 whereto a solid pvc
canvas 5l is fastened by a thick twine (rope) 52. As an
alternative to the canvas, bars with very small distance
could be used. The stop section has approximately the same
dimensions as the sorting grids 3 and 3'.
As already mentioned the function of the stop section is to
guide away escaping fish (that passes out through the grids
3 and 3') up and away ~rom the latter section (codend) of
the trawl.
.
Fig. 6 show a perspective drawlng of how the sorting device
; do function.
,
Small, ~uvenile and not wanted fish that are caught by the
trawl when it is pulled ln the towing direction marked by
an arrow will have a possibility to ~scape between the bars
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WO92/17999 2 ~ ~ 7 ~ v ~ PCT/NO92/000~1
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of the sorting grids and be herded away from the trawl by
the stop section. Fish still retained by the trawl will end
up as the catch in the codend 2. The 50rting grid~ 3 and
3' do not act as passive filters, but their function 1
S based on natural behaviour of fish. Out of a number of
factors the sensitivity to light in fish is utilized. As
the fish approaches the grid area there will be a signifi-
cant change in light penetration through upper part of the
trawl body. Fish will seek towards the more bright area,
i.e. between the bars of the sorting grids 3 and 3'. Addi-
tionally, the waterflow through the sorting grids 3 and 3'
will be higher than in the adjacant areas, which is impor-
tant to guide fish towards the sortiny grids and between
the bars.
The distance between the bars of the sorting grids will
decide the rate of selectivity, i.e. sizes of fish that
escapes through the sorting grids 3 and 3' in relation to
sizes of fish retained by the trawl. A number of experi-
ments at sea are made to test the function of the newsorting device, and it is shown that a very good selectiv-
ity is achieved,significantly better than what is possible
to obtain by changes of the mesh size in the normal trawl
codends.
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; Experiments were made with the invented sorting device as
indlcated in Fig. la. To estimate the relations between
fish (of wanted slzes) retained by the trawl and ~ish that
escaped through the sorting grids 3 and 3', a smallmeshed
3G ¢over was used to collect all escaping fish. To exclude
bias in the selectlvity data the normal codend was blinded
simultaneously by a smallmeshed innernet. By length measur-
ing and counting fish by numbers from both the normal
codend (fish caught) and the cover (fish escaped), it is
possible to present by graphics the length-frequency dis-
tributions and the corresponding selection curves. The
ottaohed length-frequenoy d1otribut1ons anù thelr oorres-
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W092/17999 PCr/~092/00071
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ponding selection curves do clearly state a good sizeselectivity on fish, and that a conslderable proportion o
fish below the acceptable (legal) sizes are sorted out
through the sorting grids 3 and 3', thus excluded from the
S trawl catch. Special attention should be paid to Fig. 11
where a comparison on the species cod (Gad~ marhu~) and
haddock (Melano~rammus ae~ inus) is made between a trawl
with the new sorting device and a trawl with normal 135 mm
mesh codend.
Using the invented sorting device in trawls it will be
possible to avoid too hard exploitation on juvenile fish,
thus making it possible to avoid overfishing on fish
resources.
WO92/17999 2 ~ ~ 7 ~ ~ ~ PCT/NO~2/00071
6/1
Leqend to fiq. 7: Results for cod from tests with the
device in accordance with the invention (May 1990) repre-
sented in the form of length frequency distribution and
selection curve. The number of fish in the small meshed
cover and codend, respe.ctively, as well as mean selection
and selection interval are discloed in the figure.
Leaend to fiq. 8: Results for cod from tests with the
device in accordance with the invention (May l99o) repre-
sented in the form of length frequency distribution andselective curve. The number of fish in the small meshed
cover and the codend, as well as mean selection and
selection interval are disclosed in the figure.
15 Leq,end to fiq. 9: Results for haddock from tests with the
device in accordance with the invention (May l99O) repre-
sented in the form of length frequency distribution and
selection curve. The number of fish in the small meshed
cover and the codend, respectively, as well as mean
Zo selection and selection,interval are disclosed in the
; figure.
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', Leqend to fia. ,1,0: Results for cod from tests with the
,~, device in accordancewith the invention (May 1990) repre-
sented in the form of length frequency distribution and
selection curve. The number of fish in the small meshed
cover and the codend, respectively, as well as mean
selection and selection interval are disclosed in the
figure.
~e,~ gL~1s,~ Comparison between the selection
;,~ properties (selection curves) with respect to the species
cod and haddock between the device in accordance with the
invention and a commen 135 mm codend. The plotting of the
two curves are effected after the same method (non-smoothed
curves based on 3-point smoothed raw data).
