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BUJJ,ETIN No. 104
DFO
1ll~ll~fii/1~11ru·
10015415
A New Mid-Water Trawl
for Herring
BY
W. E. BARRACLOUGH and W. W. JOHNSON
Fisheries Research Board of Canada
Biological Station, Nanaimo, B.C.
PUBLISHED BY THE FISHERIES RESEARCH
BOARD OF CANADA UNDER THE CONTROL OF
THE HONOURABLE THE MINISTER OF FISHERIES
OTTAWA, 1956
;>....
31 l" i
BULLETIN No. 104
A New Mid-Water Trawl
for Herring
BY
W. E. BARRACLOUGH and W. W. JOHNSON
Fisheries Research Board of Canada
Biological Station, Nanaimo, B.C.
PUBLISHED BY THE FISHERIES RESEARCH
BOARD OF CANADA UNDER THE CONTROL OF
THE HONOURABLE THE MINISTER OF FISHERIES
OTTAWA, 1956
68412-1
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Bulletins of the Fisheries Research Board contain popular and scientific
information concerning the technology of catching, handling and preserving fish,
and concerning the biology of fish stocks.
In addition, the Board published the following:
An ANNUAL REPORT of the work carried on under the direction of
the Board.
The JOURNAL OF THE FISHERIES RESEARCH BOARD OF
CANADA, containing the results of scientific investigations.
ATLANTIC PROGRESS REPORTS, consisting of brief articles on
investigations by the Atlantic stations of the Board.
PACIFIC PROGRESS REPORTS, consisting of brief articles on investigations by the Pacific stations of the Board.
For a complete list of the Board's publications, write to:
Fisheries Research Board of Canada,
Publications Office,
Ottawa, Canada.
(iii)
68412-H
ABSTRACT
Plans and details of construction and operation are given for a one-boat two-warp midwater trawl designed for fishing herring at depths up to 50 fathoms, by boats of 150-175 h.p.
It is built of nylon netting to obtain maximum towing speed, the smallest size used being
1t inches stretched measure, in the cod end. The net is spread by specially designed curved
dual-fin otter boards, attached at the ends of "pennants" 10 fathoms long. The lower edge
is held down by depressors, and the upper edge is elevated by means of planing floats. Cost
of materials and fittings for the net, including netting, twine, lines, floats, otter boards and
depressors, was about $1,500. (This does not include the regular towing warps.) Labour costs
are additional.
Preliminary herring-fishing tests with the net show that it is most effective during the
early part of the night and just before daybreak. It took catches of 20 to 30 tons in hauls of
15 to 20 minutes each, at depths of 15 to 35 fathoms. These herring schools were located by
echo-sounder, and were 5 to 10 fathoms "thick". Much smaller catches were taken during the
later part of the night, in the upper 15 fathoms of water, and also during the day when the
herring were near bottom. However a few large daylight catches were obtained. The trawl is
designed to take up to 50 or 60 tons of fish, but with careful handling quantities up to 100
tons might be landed successfully.
(v)
INTRODUCTION
The success of trawling for catching groundfish has led to much speculation and experimentation toward applying similar methods to pelagic fishes.
Several models have been developed in Atlantic waters but, as of 1950, none
of them were well adapted to the needs of the British Columbia herring fishery.
Early in 1950, Mr. N. Nelson, skipper of the vessel Combat, built a successful
mid-water trawl mainly from used salmon seine netting, and caught a few
tons of herring in Gasboat Passage in northern British Columbia waters. In the
fall of 1951 mid-water trawling employing two boats was undertaken in the
Strait of Georgia. The vessels Combat and Violet Prince (Captain R. Prince)
fished together, while the Frank Winfield (Captain F. Smith) fished with the
Elwin S (Captain G. H. Smith). These vessels were moderately successful in
catching herring in the lower Strait of Georgia. However, the difficulty of
operating one large trawl from two vessels discouraged further attempts to
carry on fishing in this manner. Later in the same year the Combat used
a modified one-boat version of the Danish "Larsen" trawl and made successful catches of herring in Gasboat Passage. The spring of 1953 saw two
new vessels, the New Laurel (Captain A. Ritchie) and the Carolina Maria
(Captain F. Gale), operating the modified Larsen trawl successfully in Chismore
Passage, near Prince Rupert. In the fall of 1953 four more trawlers-Zenardi
(Captain C. Haan), Tor II (Captain D. Moore), Skeena Maid (Captain P.
Haan), and B.C. Producer (Captain F. Kohse)-fished with the type of net
used on the Combat. At that time some of the trawlers modified their nets,
constructed different types of 0tter-boards, and added kites of various designs
to spread the mouth of the net. However, the trawlers ·met with no success
in Meyers Passage and Chismore Passage in the autumn of 1953. Although
they had been successful in the spring, no further attempts were made to
pursue the fishery because of the complete failure of their efforts in the fall
months. The Violet Prince once more attempted to fish herring in the Strait
of Georgia region, but the catches were very small and further experimentation
was abandoned.
At this time a request for assistance in developing a better mid-water
trawl was made to the Fisheries Research Board of Canada. Through the
cooperation of the Chief Supervisor of Fisheries at Vancouver, trawl-net fishermen met with representatives of the Fisheries Research Board of Canada and
formed a committee to advise on a program. Financial support was obtained
from the Industrial Development Vote of the Federal Department of Fisheries,
and the work was carried out by the Board at its Biological Station, Nanaimo, B.C.
Mr. W.W. Johnson, a member of the fishermen's committee, was employed
by the Board to design and construct a new trawl. Many of the fishermen who
had been actively engaged in mid-water trawling were consulted. They recommended two principal improvements: a net of larger size, and some means
1
to prevent or minimize the scaring of herring schools by the otter-boards.
Accordingly, the new net was constructed of light-weight nylon; and curved
otter-boards, suspended from steel wire pennants, were developed to spread its
mouth (Fig. 1).
BRITISH
COLUMBIA
MIO-WATER
TRAWL
i
TOW POINT
110 torhomsl
OTTER-BOARD
-/-------PLANING
FLOATS - II
DEPRESSOR
JQ
FIG. 1.-Sketch of the British Columbia mid-water trawl in
operation.
2
DESCRIPTION OF THE NET
The essential requirement for commercial catches was a net of much larger
proportions than the types previously used, but one that could be handled
from a vessel of moderate power (150 to 175 h.p.). Hence it was necessary to
increase the water flow through the netting by decreasing the diameter of twine
and so decreasing the reeistance.
1. Nylon twine was the only material available that could be used with
smaller diameter and yet retain the strength of the #30 thread cotton twine
that had been used previously in the forward sections of the net and the #15
thread cotton twine that had been used in the cod-end. For example, #93 nylon
thread which has a diameter approximately equal to the diameter of #9 thread
in cotton has a breaking strain approximately equal to #42 thread in cotton
twine.
2. Because water absorption by nylon twine is much less than that of
cotton twine of an equal diameter, it does not swell as much. This reduces
the resistance of the net to the fl.ow of water.
3. Individual filaments of nylon run the full length of the twine without
a break, which gives the twine a smooth surface. There are no fuzzy fibre
ends sticking out of the twine to catch dirt or create an added resistance when
the net is in the water. The nylon net is thus much cleaner than a cotton net
while in use.
4. Single-knot nylon web was used in all meshes of the netting. The
double-knot was not deemed necessary because the meshes were hung so they
would tow in a long-diamond shape. In the fishing operation, the single weaver's
knots, treated with a bonding agent, did not slip or "turn over".
5. Concerning mesh size, it was expressed by some fishermen that an
increase in size of mesh from that used ·in former nets was desirable, particularly in the body and wing sections. An increase in the mesh size wherever
possible would decrease the resistance of the net in the water still further.
Accordingly, four sizes of mesh were used in the trawl-5-inch, 4j-inch, and
3!-inch in the wings and body and 1!-inch in the cod-end. (Sizes mentioned
are always stretched dry measure, centre of knot to centre of second knot
following.)
3
68412-2
CONSTRUCTION OF THE NET
Tables I and II show the materials required for the net and its accessories.
Their total cost was about $1,500, exclusive of labour for handing, etc.
The tapering box-shaped net is divided into three main parts: the body,
the cod-end with the "zipper" and the wings.
TABLE I.-Nylon netting required for the" mid-water trawl.
Thread
size
Mesh
size
No.
inches
Wings ...........................
93
5
100
Body ................... . .... · ...
93
5
100
Part
Cod-end .........................
Meshes
deep
Approx.
weight
Meshes
long
pounds
'
I
225
11
534
26·3
93
4!
100
93
3!
100
689
63
a
100
3,392
675
·-
31 ·4
24·8
34
TABLE IL-Other materials required for the trawl.
Item
No. 93 Nylon twine ...... -..... -.... .
No. 63 Nylon twine ............... .
Braided nylock twine No. 15, extra
soft .......................... .
5/16-inch braided nylon rope ....... .
7/16-inch braided nylon rope ....... .
!-inch braided nylon rope lead-line .. .
9 /16-inch Belflex rope ............. .
No. 27 Med. cotton twine .......... .
Jib rings 1i- by i-inch ... . ......... .
Galvanized seine swivels ........... .
Galvanized wire rope i-inch diam ... .
Galvanized wire rope !-inch diam ... .
Otter doors and depressors ......... .
Lead weights for otter doors .... .
Buoy keg (5-gallon size) ............ .
G-clips and receiver links ........... .
i-inch galvanized chain ............ .
Galvanized shackles-i-inch (34);
i-inch (6); !-inch (2) .......... .
Missing (split) chain links - i-inch
(12); 7/16-inch (6) ............ .
!-inch galvanized chain ............ .
Amount
Use
6 lb ....... ·. . . . . . . . . Hanging and mending
4 lb. . . . . . . . . . . . . . . . Hanging and mending
5 lb. . . . . . . . . . . . . . . .
120 fath. (24 lb.). . . .
14 fa th. ( 10 lb.). . . . .
50 feet (25 lb.)......
54 fath. (88 lb.).....
2 lb. . . . . . . . . . . . . . . .
34 rings. . . . . . . . . . . .
4 swivels. . . . . . . . . . .
768 feet. . . . . . . . . . . .
130 feet. . . . . . . . . . . .
Hanging
Rib-lines
Poke string and haul-back line
Lead-line
H~ad-rope
Jib rings
Zipper and cod-end
Wing corners
Sweep lines
Pennants
135 lb. each ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 keg. . . . . . . . . . . . . . . Brailing
. . . . . . . . . . . . . . . . . . . . Otter-boards
78 lb.... . . . . . . . . . . . 1 Otter-boards
42 shackles. . . . . . . . . Otter-boards and sweep-lines
18 links. . . . . . . . . . . . Otter-boards and sweep-lines
25 links (9 lb.). . . . . . Sweep-lines
4
.
... . .
BODY
The body of the net was the first part to be constructed. It was built in
three tapering sections, each of a different mesh.
Section 1, shown in Fig. 2, represents the widest part of the body and
·was built from four equal pieces of netting. Each piece measured 300 meshes
along the front edge (selvage edge) which gives a total of 1,200 meshes in
circumference. The sides were cut 50 meshes long with a 4-bar 1-mesh taper,
so that the back edge of each tapered piece was 234 meshes wide. All four
sides were laced together, commencing at the front or widest edge, by gathering in three meshes (four knots) on each side with the lacing to form a seam.
+ - - - - 300------+
234
234
300
\i
\510
r-1
MESH
5M
TAPER
BODY
FIG.
1068 • 50
4 BARS
I MESH
SECTION
I
2.-Detail of body, Section 1, showing number of meshes in
each piece.
Section 2 (Fig. 3) consists of four sides cut 275 meshes wide (making 1,100
in circumference) and 75 meshes deep, with the same 4-bar 1-mesh taper as
in the first section. This left the back edge of each of the four sides with 17 5
meshes. After the four sides were laced together in the same manner as the
first section, the second section of 4!-inch netting was joined to the first section
of 5-inch netting (Fig. 5). The two sections were joined mesh for mesh, but
with every fifth mesh of the 5-inch web it was necessary to pick up one mesh
of the 4!-inch netting (called 5 and 1 intake).
--275-
r
TLWT
\'. 751
1~"L
~
MESH
'15--.-175---.
I
\
4~2"
TAPER
BODY
1----~l
I
\\
I
!
900 a 75
4 BARS
I MESH
SECTION
2
FIG. 3.-Detail of body, Section 2.
Section 3 (Fig. 4) consists of four sides cut 212 meshes along the front
edge (848 in circumference), and 100 meshes along the side with the same 4-bar
1-mesh taper, leaving 78 meshes along the back edge. Once the four sides
5
68412-2!
were laced together, as in the first two sections, the third section was joined
to the second, mesh for mesh (Fig. 5). However, with every fifth mesh of the
4!-inch web, two meshes of the 3!-inch netting were taken in.
J.[Sj lQJ]d
!
212
~,,,_
78
212
78
1
78
~100-19-
l
M E SH
3 ~ •
TAPER
4 BARS
2
0
80 DY
689
1
100
I MESH
SECTION
3
FIG. 4.-Detail of body, Section 3.
Doubled nylon twine (#93 thread) was used in sewing each section together
in order to prevent the knots from sliding. In sewing the sections together any
odd meshes were gathered in along the seams.
300
5
11
MESH
PICK UP I EXTRA
EVERY
300
4
~2'
~2"
MESH
MESH
PICK UP
I EXTRA
EVERY
3 ~i'
4
FIFTH MESH
3 Y
2" MESH
FIFTH MESH
MESH
78
78
FIG.
5.-Design of body, showing sections and position.
Con-END (Fig. 6 and 7)
The cod-end, which is of box-type construction, was made of six sections
or strips of 1l-inch web, each strip 100 meshes long. The first four sections
were cut with a 4-mesh 1-bar taper, but the remaining two sections were built
without a taper. In the first section of the cod-end, each of the four sides was
cut 200 meshes wide on the front edge (making 800 meshes in circumference)
and 100 meshes long with a 4-mesh 1-bar taper, which left 178 meshes on the
back edge of each side. After lacing the four sides of the first section together
by gathering in three meshes from each side to form a seam, the remaining
6
r=~,~-=~~
200 meshes
I
:
I
I
I
200
JOIN
FIG. 6.-Construction of cod-end tapering sections showing method of attaching successive sections. Taper 4-mesh
1-bar. (Only forward edges of the cod-end need be counted.
Remaining strips are joined mesh for mesh, and the same
taper is used for the length of the tapering ~ , cod-end
sections.)
t+-200 meshes--!
't-112--+\
HI
\
f[ [
\ -
~
14------2496
-------TAPERED
i 11
I
i:
I
t+--448---448----+t
SECTIONS
- - - - - - - - - - - - - - - 3 3 9 2 meshes
NO
14'
I 1
TAPER~
mesh------->1
FrG. 7.-Diagram of complete cod-end assembly showing four tapering sections and two untapered sections.
7
three tapered sections were built independently in the same way as the first
section. In the construction of the remaining sections of the cod-end it was
not necessary to count the number of meshes along the front edges of the
sections because the same taper was used in the first four tapered sections
and each section was joined mesh for mesh. The remaining netting in the bale
of web was first laced mesh for mesh with the back edge (178 meshes) of the
first section (Fig. 6), and cut with a 4-mesh 1-bar taper to form each side.
After the front edges of the second section were laced to the back edges of the
first section, and the sides laced together, the third and fourth sections were
cut and laced together in the same manner. The number of meshes along the
front and back edges of each of the four cod-end sections is shown in Fig. 7.
The last two sections were cut from one piece of netting without a taper.
The netting was merely folded to form a box-like section (see Fig. 7) and then
sewn mesh for mesh to the fourth tapered section.
(Fig. 8)
The "zipper" began at the forward edge of the cod-end meshes on the
upper starboard corner and extended backward along the rib-line for 2! strips
of web or a distance of about 36 feet. The zipper was hung on two lines of
5/16-inch braided nylon rope (one of which forms part of the rib-line).
THE "ZIPPER"
"ZIPPER II
5/16"
PRAIDED NYLON
RIB-LINES
RIB-LINE
SEIZING
GALVANIZED METAL RINGS - I 1/4
SPACED
18
11
11
APART
4-------- 36 feet
(approx.I----------
FORWARD
ZIPPER IS LOCATED AT THE
AFT
UPPER
FORWARD STARBOARO
SEAM
FIG. 8.-Above: Detail of construction of the "zipper". Below: Location of
the "zipper" on the cod-end.
8
Galvanized metal rings 1! inches in diameter were lashed with #27 medium
cotton twine on each rib-line 18 inches apart and abreast of each other.
A 5/16-inch braided nylon rope laced through the rings was used to close the
zipper (Fig. 8). The length of the zipper normally should extend from the
rigging amidships backward to the stanchions.
WINGS
(Fig. 9)
The four wings were made from eight triangular pieces of 5-inch netting.
Each triangular piece was cut 75 meshes wide, 75 meshes long and tapered
with a bias cut. Each wing was made by lacing together two of these triangular
pieces down the sides, which were 75 meshes long. This formed a wing with
a base 150 meshes long and sides tapering up on the bias to the apex which
was 7 5 meshes above the base.
WINGS
5 inch MESH
-1s
ALL FOUR WINGS
N/J\I21l'
ARE EQUAL
75-+++-75-75 meshes-
-----------------300·------------------
~
ACTUAL LENGT
~
H
~
EYE
SPLICE
OF LINES WHEN HUNG
~
i_.
:--SPLICE
~
,j"""
v~
BEi.FLEX
ROPE
9/16"
16ft.
,._BEi.FLEX
ROPE
-RIB-LINE
ALL FOUR WING ENDS
THE SAME
'
~150meshes~
HANG
I 1/4
11
BODY
SECTION
I
FIG. 9.-Detail of construction of the wings and their attachment
to the body.
9
Each wing was laced mesh for mesh to the body section, commencing at
the junction of the centre of the base of the triangular shaped wings and the
corner seams of the body .section, and laced toward the square. The ·square
is the section of the net between two wings. In this net it was 16 feet long on
each of the four sides.
HANGING OF THE WINGS AND BODY TO THE RIB-LINES
The rib-lines of 5/16-inch braided nylon rope were first stretched 2! feet
for every 60 feet. The seams of the net were then "tacked" every 2! feet
along the stretched rib-lines commencing at the apex of the wings and continuing back along the 5-inch, 4!-inch and 3!-inch meshes of the body. Lacing
of the wings and body to the rib-lines was then completed. When the strain
on the rib-lines was released, the netting on the wings and body section hung
in small bights. It was necessary to hang the wings and body to the rib-lines
in this manner in order to prevent them from ripping apart when the net was
under tension in the water. When the net was fishing the rib-lines again took
the strain and the netting hung evenly along the entire length of the wings
and body.
Since this work was done, non-stretch braided nylon rope has become
available. Use of this material would eliminate the need for adjustments in
hanging as described above.
HANGING OF THE WINGS AND BODY TO THE HEAD-ROPES
A combination manila and six-strand wire rope (Belflex cable) of 9/16-inch
diameter was used for the head-ropes or head-lines. This rope has a breaking
strength of about 2,700 pounds. The wings and body were hung directly to
the head-ropes with braided nylon twine (#15 extra soft). When hanging the
wings to the head-rope, three meshes from the wings were g~thered in to form
a selvage edge. This shortened the wings to 72 meshes in length so that the
· taper on the wings (distance from the apex of the wing to the square) measured
29! feet. The 150 meshes of the body, which formed the square 16 feet long,
were hung or spaced 1i inches apart on the head-rope.
Great care must be taken in hanging the corners of the wings at the point
where the square terminates and the wing section begins, so that the corners
will not tear out when the net is towed. A 6-foot length of 5/16-inch braided
nylon rope, laced exactly from the corner down the meshes toward the cod-end,
usually overcomes this problem.
The "Belfi.ex" head-rope was continued for a length of 3 feet beyond the
wing tips and ended in an eye-splice (Fig. 9). This extra length was necessary
so that the shackles would not tear the netting, and would remain clear when
stacking the net on the stern of the vessel or when setting the gear.
HANGING OF THE LEAD-LINE TO THE WINGS AND BODY
The !-inch braided nylon lead-line was lashed to the "Belflex" lead-line
every 18 inches commencing about a point 1! fathoms from the tip of one
wing and extending along the square to the same position on the other wing.
It was approximately 50 feet long and weighed about 25 pounds.
10
..
HANGING THE COD-END TO THE RIB-LINES
The 5/16-inch braided nylon rib-lines were not stretched in the cod-end.
The four seams of the 1!-inch netting were hung evenly to the unstretched
rib-lines. A piece of 5/16-inch braided nylon line was hung around the selvage
at the end of the cod-end. This kept the end of the net in trim and evened the
web 'vhen tying the cod-end.
MID-WATER TRAWL NET
~
~
;
~
h
\
SECTIONAL VIEW
\
HANG DIRECT TO
75m~ GATHER
::' \
+=
IN
SQUARE
9/16" BELFLEX ROPE
3 MESHES FROM EACH WING
BETWEEN WINGS HUNG
I 114"
--r
'------BODY
TAPER 4 BAR I MESH
COD-END
WHEN
LACING
SEAMS
12 RINGS LACED TO
BRAIDED NYLON
RIB-LINES
FIG.
5116" x2 l/2fl.
LINES AND
56 MESHES APART
10.-General sectional view of the mid-water trawl.
POKE STRING AND COD-END HAUL-BACK LINE
At the end of the cod-end, eight lengths of 5/16-inch braided nylon line,
each 2! to 3 feet long were laced parallel to each other and 56 meshes apart
around the cod-end (Fig. 10). About 18 inches up from the end of the net,
twelve galvanized metal rings 1i inches in diameter were lashed separately
to the eight short nylon lines and to the four rib-lines with #27 medium cotton
11
twine. The poke string or cod-end rope and the cod-end haul-back line were
all one length of 7/16-inch braided nylon rope. In closing the cod-end, the
poke string, which passed through the metal rings, was given an extra turn
with the short end completely around the cod-end and then tied snugly with
several chain knots. The remainder of this line (haul-back line) was brought
forward along the upper starboard rib-line, where the zipper was located, and
made fast to the foremost zipper rings.
The completed sectional view of the wings, body and cod-end is shown
in Fig. 10.
FLOATS
Eleven aluminum alloy Phillips planing floats (Fig. 1) each 8 inches in
diameter, were lashed about 2 feet apart along the "square" of the head-rope.
These spherical "hat floats", as they are often called, are fitted with a lifting
plane around the lower portion of the sphere. They were used on the midwater trawl net because they increase the normal lifting ability to about five
times that of an aluminum float without a lifting plane at normal towing
speeds. When the net was towed about one fathom beneath the surface of
the water it was observed, from a power skiff, that the floats gave adequate
buoyancy and that the head-rope formed a broad upward arch.
12
GEAR REQUIRED FOR OPERATION OF THE NET
SWEEP-LINES
Each of the four sweep-lines which connected the four wings to the towing
warps were 30 fathoms in length and made from I-inch-diameter 9/16 galvanized wire rope (see Fig. 1 and 12). The two lower sweep-lines were lengthened
to 31 fathoms with one fathom of !-inch chain, so that the net would tow
horizontally through the water and give a maximum vertical opening to the
mouth of the trawl. The four sweep-lines were coupled to the corners of the
wings with swivels, and the upper and lower sweep-lines from each side were
shackled to the receiver-link, which was termed the tow-point.
Tow-POINTS
The tow-point (see Fig. 1) is the junction at the receiver-link of four
lines-the two sweep-lines, the single wire pennant to one of the otter-boards
and to the towing warp from one of the winches.
DEPRESSORiS
The depressors (Fig. 11, 12, 18) were designed to give a continuous downward thrust to the two lower wings thus assisting in producing not only a
maximum opening to the mouth of the trawl but also in depressing immediately the lower sweep-lines and lead-line when setting the gear. The two
depressors, each of which measured 15 by 23 inches, were cut from i-inch steel
plate and were shackled to the two lower sweep-lines just ahead of the two
lower corners of the wings. They were attached in this manner so that they
could slide freely on the sweep-lines and remain on deck when taking in the
MOORE
~"
FIG. 11.-Detail of the Moore depressor.
13
DEPRESSOR
STEEL
PLATE
first fleet of tl;ie wings. This type of depressor was first used by Mr. Dave
Moore on the trawler Tor II, and replaced the heavy and cumbersome weights
which had been used previously on mid-water trawls to depress the lead-line.
The angle of the depressor on the sweep-lines may be adjusted easily by the
addition of split chain links or shackles to the existing shackles which connect
the depressors to the sweep-lines.
•
~~oft\~
~
\ \o.... 011\
~"
\
DEPRESSOR
FIG. 12.-Attachment of sweep-lines to wing tips.
PENNANTS
The pennant, a single !-inch galvanized 6/19 wire rope, 10 fathoms long,
was shackled to the tow-point. To the other end of this rope was attached
the curved dual-fin otter-board (see Fig. 1 and 14).
DUAL-FIN OTTER-BOARDS
In the past many fishermen have expressed the opinion that the ordinary
otter-boards which were placed at the ends of the sweep-lines or bridles had
not sufficient spread, and tended to drive or scare the herring away from the
mouth of the trawl. The arrangement shown in Fig. 1, with the boards
suspended on pennants, was designed to minimize the disturbance immediately
in front of the net.
The dual-fin otter-boards (Fig. 1, 13, 14, 15, 16, 17) were designed by
Mr. Wesley \V. Johnson and built at Benson Shipyard in Vancouver, B.C.
Each board was constructed from two sheets of !-inch thick laminated plywood
and measured St feet long and 3 feet in height. The curvature of the boards
(Fig. 13) was held by two bent angle iron strips on the upper and lower edges.
Horizontal stability of the boards in the water was provided by two horizontal
upper and lower fins which were cut from !-inch plywood and bolted to the
curved iron strips. The two adjustable vertical fins which were made from
14
!-inch plywood·-and bolted to the upper and lower horizontal fins helped in
maintaining the correct angle of attack of the otter-boards when towed in
the water. All of the exposed plywood edges were bound by strips of halfround iron.
PORT
DUAL FIN
IN
POSITION
TOWING
OTTER-BOARD
POSITION
OF VERTICAL FIN
r
FIG.
GALVANIZED
CHAIN
13.-General appearance of the port otter-board in towing position.
The position of the fore and aft towing 1ugs is shown in Fig. 13. A bridle
was made from four pieces of i-inch galvanized chain. The upper chains had
16 links in the fore length and 26 links in the aft length; the lower chains had
17 links in the fore length and 27 links in the aft length. All four chains were
shackled to a steel seine ring 5 inches in diameter. The single !-inch wire
pennant was in turn shackled to this ring (Fig. 13, 16).
Each door was weighted and balanced with lead bars of different lengths,
totalling 135 pounds (Fig. 13, 14). One bar 3 by 1 by 66 inches was bolted
to the bottom edge parallel to the lower horizontal fin. Three smaller bars each
3 by 1 by 11 ! inches were bolted above e~ch other to the lower front edge of
the door arid a single bar 3 by 1 by 10 inches was bolted upright to the front
edge of the door. The four smaller bars gave the proper balance to the doors
during setting and towing.
15
DUAL-FIN
- -
f
OTTER-BOARD
'I
011
..
~a··~
12"
.._FINS
~·~r ~·PLYWOOD
ARE
7''
I
~"
7"
~ 4 or 8
~ -- -----': ___ ---- -_': _-- .-- ---~-l ------~- ----- -- -~ ---~
8
AFT LUG
PLATES
3 ..
e
36"
FORE LUGI"
PLATES o
0
plate steel
THIS
TWO
4'·
i
0
3' x I
11
LEAD o BALLAST
T'
,....._
0
NOSEo BARS
:I
9
0
0
9
0
t<-llf--~
PLATE
0
LEAD BALLAST
TOTAL WEIGHT
BORED
ALL HOLES
f
'j
0
3"x 1 o LEAD
~I:
f
~
11
I"
0
f
PLYWOOD
2
[3 1I·
0
I"
LAMINATIONS OF
14-5~
r
<I
0
14--9 '!-.+I
SECTION
D
0
TI"
l
1-k
1
135 pounds
IN DIAMETER
11
t----18 ---1'1
i------------5'· 6
11
OVERALL----------"1
l r1s
1.."
18
1
°"l'RoN
.i---12"--i--- 12 ~6 !....i
11
I1.
r
=======-==~=.:::~=~;;_j.:~
_,
---.
-
TWO
HORIZONTAL FINS
r r
OR
ADJUSTMENT
FIG.
SLOT
1
ALL
EDGES
MADE
l'
FROM
PLYWOOD
BOUND
WITH HALF ROUND IRON
14.-Details of construction of the otter-board.
When the otter-boards are being towed at full speed or at fishing speed
a number of factors in the adjustment of the boards must be accounted for:
(a) The weight of the boards. (b) The angle of attack of the boards in the
water. (c) The balance. (d) Placement of towing lugs. (e) Adjustment of
of the vertical fins (angle). (f) Length of each of four sections of chain in
the bridle.
The boards must be able to right themselves should they collapse or fall
over with any change in towing speed.
It was found that the vertical fins should be set at negative angle of
attack of about 35° from the towing axis in order to keep the aft section of
the boards closest to the vessel when in operation.
16
....
OPERATION OF THE TRAWL
HANDLING
Before handling or setting the trawl, it was particularly important that
all projections such as bolts, sharp corners and the stern railing be covered
with canvas, because the nylon netting has a tendency to catch very easily
on any small projection. Once these precautions were taken no difficulty was
experienced in stacking, fleeting, or setting the net.
FIG. 15.-The starboard otter-board.
FIG.
16.-The port otter-board being set beneath the surface of the water. Its upper fin may
be seen just breaking the surface.
FIG.
18.-The starboard depressor, which is used to pull one of the lower corners of the wings
down into the water.
FIG.
17.-Port otter-board hanging from the stanchion.
SETTING
When the school of herring was located with the echo sounder the vessel
proceeded to a distance beyond the school (just far enough to set the gear to
correct the depth), turned about, and reduced speed to slow ahead before
setting the gear (Fig. 19). The cod-end was dumped directly over the stern.
The body and wings which followed slid readily over the canvas covered stern
railing (Fig. 20). As the sweep-lines were paid out from the winches, the
effect of the depressors which were shackled to the lower sweep-lines at
the corners of the wings could be seen as they hit the water. The depressors
17
immediately submerged the lead-line and the two lower wings beneath the
surface of the water. After 20 fathoms of sweep-line had been paid out slowly
and all the planing floats were functioning, the mouth of the net opened almost
to its full extent (about 40 feet in height by 30 feet in width).
FIG. 19.-The mid-water trawl on deck ready to set.
FIG. 20.-The trawl being paid out over the stern.
FIG. 21.-"Fleeting" in the trawl.
FIG. 22.-A 20-ton catch of herring in the cod-end as it is surfaced and brought along the
starboard side of the vessel.
The otter-boards were shackled quickly to the wire pennants, as the latter
were unwound from the winches along with the sweep lines. Then the boards
were let out first at towing or fishing speed in order to clear the stern of the
vessel, then at full speed (Fig. 16). At this point the remainder of the pennants
and sweep-lines were unwound from the winches and passed smoothly over
the blocks on the stanchions. The speed of the trawler was now reduced to
the required fishing speed (about 3 knots). This was slightly less than the
full speed attainable with the net and gear in tow.
18
Opinions vary as to the best length of the pennants from the towing
point to the otter-doors. It was the suggestion of the investigators and crew
members that otter-doors attached to a long towing pennant, 10 fathoms or
more, tend to prevent the herring from being driven from the path of the
approaching net, and even may assist in driving a portion of the school toward
the mouth of the trawl. As yet it has not been determined with certainty
\vhether shorter pennants are as efficient or more efficient than the 10-fathom
pennant in opening the mouth of the trawl. However, it was proved in practice
that doors on pennants were far more efficient than doors attached directly
to the S\veep-lines.
\VIRE ANGLE ON THE TOWING \"\TARP
The depth of the tow-point in the water could be determined readily from
the angle of the towing warp over the stern of the vessel. A carpenter's bevel
protractor was used to measure the wire angle. The bottom of the net was
found to be fishing from 6 to 7 fathoms below the depth of the tow-point.
This depth was determined by sliding a bathythermograph (an instrument
which has a depth-recording mechanism) down a stainless steel trolling wire
which was attached to the centre of the lead-line. As the trawl gear was set
out to greater depths the trolling wire was paid out from a small winch over
the stern. The instrument was also passed down the towing warp to check
the calculated depth of the towing point from the wire angle. It was also
determined by this instrument that the top of the arch of the headrope was
about 40 to 45 feet above the lead-line.
After several days of experimentation, it was possible to establish a relationship between the observed wire angle and the depth at the bottom of the
trawl (lead-line) as recorded by the bathythermograph. For gear built exactly
to the specifications given above, the relationships indicated in Table III may
TABLE
Amount
of towing
warp out
Wire angle
on towing
warp
fathoms
III.-Fishing depths of the mid-water trawl.
Depth
at bottom
of trawl
Amount
of towing
warp out
fathoms
fathoms
Wire angle
on towing
warp
Depth
at bottom
of trawl
fathonis
25
25
25
25
25
25
12°
13°
14°
15°
16°
17°
11 ·2
11 ·6
12
12·5
13
13·3
100
100
100
100
100
100
12°
13°
14°
15°
16°
17°
27
28·5
30
32
33·5
35
50
50
12°
13°
14°
15°
16°
17°
16·5
17
18
19
20
20·5
125
125
125
125
125
125
12°
13°
14°
15°
16°
17°
32
34
36
38
40·5
42·5
75
75
75
75
75
75
12°
13°
14°
15°
16°
17°
21·5
23
24
25·5
26·5
28
150
150
150
150
150
150
12°
13°
14°
15°
16°
17°
37
39.5
42
45
47
50
50
50
50
50
19
serve as a rough guide in determining the depth of the trawl in the water.
The amount of towing warp does not include the length of the sweep lines
or the distance from the apex of the wing to the square on the head-rope.
SPEED OF Tow
The approximate
in strong tidal waters
to meet the required
vessel when towing in
towing speed was 3 knots, but this varied, particularly
in narrow passes. The r.p.m. of the engine was changed
wire angle of the towing warp over the stern of the
an ebb or flood tide.
HAULING OF GEAR
A note of caution is given here. The speed of the vessel should never be
decreased when hauling the gear in relatively shallow water. It was found
that the gear might sink as much as 12 fathoms when the speed is decreased
by approximately 1 knot during the hauling operation. Hence there is the
danger of tearing the net on the bottom. Both the sweep-lines and towing
warp were wound together on the drums of the winches. As soon as the otterboards were brought to the surface and were hung from the stanchions the
remainder of the sweep-lines were winched in on the drum.
"FLEETING" THE NET
The wings and body of the net were taken aboard (fleeted) with the winch
in the conventional manner used by trawlers rigged with Pacific coast or
double gear where stanchions are mounted on the stern quarters (Fig. 21).
The herring which were killed became bloated with the change in pressure
when the net was brought to the surface, and hence assisted in floating the
cod-end (Fig. 22). Once the net had surfaced, speed and efficient handling
of the net by the crew was required in preparing the cod-end for brailing.
The wings and body were piled quickly on the stern (Fig. 21) and re-stacked
after brailing before the gear was set out again. The vessel was reversed
slowly to bring the cod-end alongside the bow.
PREPARATION OF THE Con-END BEFORE BRAILING
Once the wings and body of the net were aboard, the vessel was swung
into a position so that the cod-end lay along the starboard side of the vessel
with the tip of the cod-end itself near the bow (Fig. 22). Immediately the
front section of the cod-end (where the ll-inch mesh joins the 3!-inch mesh)
was "choked off" by a rope strap, about 10 feet ahead of the starboard
stanchion. When the gear was being hauled the "Legaze" pole (Fig. 24, 25)
amidships and bow pole or fore-boom were lowered to about 45° over the
starboard side of the vessel. As soon as the front section of the cod-end was
pinched off, a crewman picked up the haul-back line which was attached for
convenience in handling to the forward end of the zipper, and proceeded with
the line to the bow of the trawler. The haul-back line was a continuation of
the same line that was used as a poke string. The last section of the cod-end
was lifted clear of water by the fore-boom so that the poke string could be
untied.
20
It was very important at this point to untie the poke string and re-close
the cod-end opening by a rope strap or sling which could be made fast to the
tackle on the fore-boom. Should the herring die quickly and start to sink,
the strap can be released and the cod-end passed overboard so that the herring
will escape, thus preventing the gear from being lost.
It took about 15 minutes to haul the gear to the surface, fleet the wings
and body of the net and bring the cod-end along the starboard side of the
vessel.
23.-The "zipper" open with the inner edge lashed to the
starboard rail and cod-end being "dried up" by the bow pole.
FIG.
FIG.
24.-A small brailer of herring being lifted from the "zipper"
opening.
21
PREPARATION OF THE "ZIPPER" FOR BRAILING
The purpose of the "zipper" is to provide an opening in the cod-end in
the shape of a pond, through which the herring can be brailed aboard the
trawler (Fig. 24). As soon as the zipper was laid on the starboard rail from
the stanchion to the rigging amidships, the braided nylon line which closes
the zipper was released, and the inner edge of the zipper was laced to the
railing of the vessel. The outer edge of the zipper was immediately laced to
the parallel pole or bar, beginning at the forward end near the rigging amidships and working aft. Three rings at the front end of the zipper were left
free. This permitted the front end of the zipper to spread away from the side
of the vessel. Lacing of the zipper to the railing and the parallel pole commenced with the fourth ring. The forward end of the parallel pole was made
fast to the rope which passed through the block at the end of the "Legaze"
pole, and supported the forward end of the parallel pole above the water.
Before lowering the forward end of the parallel pole over the side of the vessel
the outrigger pole was lashed at right angles across the after end of the parallel
pole. A 5-gallon buoy-keg was lashed underneath the junction of the poles
and its purpose was merely to assist in pushing out the outrigger pole perpendicular to the side of the vessel. The inner end of the outrigger pole was
BRAILING
11
FROM
ZIPPER
THE
11
-FORE BOOM
~
PARALLEL
I
I
/
I
,.,,
/'
I
I
,,., ,,"
/
/
,,,...-"
POLE
15 ff.
FIG. 25.-Mid-water trawl in position for brailing.
22
x 4 in.
POLE
13 ft. x 3 in.
butted against the side of the vessel and lashed through the scuppers and
over the starboard railing. A guy line from the junction of the parallel and
outrigger poles, which was made fast to the stanchion, held the aft end of the
parallel pole and outer end of the outrigger pole clear of the water.
From the time the cod-end lay alongside the starboard side of the vessel,
the complete operation of preparing the zipper ready for brailing (to this stage)
by a five man crew took between 4 and 5 minutes.
DRYING UP THE NET
"Drying up" is the term used in lifting the cod-end up on the fore deck
of the vessel with the fore boom or bow pole (Fig. 23, 25), so that the herring
can swim directly to the zipper opening where they can be brailed. If 25 to
30 tons of herring are caught, the net can be dried up solidly almost immediately.
With a larger catch a number of brails of herring should be removed at first,
before drying up the cod-end completely. About 65 to 75 tons of herring can
be handled successfully in this manner.
f
23
PRELIMINARY RESULTS WITH THE NEW TRAWL
Experimental fishing was conducted from the chartered vessel Sea Pride II,
skippered by Captain Norman Sigmund. This 62-foot trawler was equipped
with double stanchions or gallows on the stern quarters. The two trawling
winches and the brailing winch were hydraulically operated. The vessel was
powered with a 170-h.p. diesel engine.
The experimental mid-water trawling was conducted upon stocks of herring
in the Strait of Georgia off the lower east coast of Vancouver Island during
the months of January and February, 19SS. The gear was found to be very
effective in the early part of the night, just as the herring were rising off the
bottom, and also during the period just before daybreak as they were descending
to the bottom. Catches of 20 to 30 tons of herring were made in six different
tows in Trincomali Channel and in Satellite Channel at night. These drags
were 1S to 20 minutes in duration and were made on herring schools located
entirely off the bottom at depths ranging from 1S to 3S fathoms. One catch
estimated at 100 tons burst the cod-end shortly after the net was surfaced,
probably because of excessive towing speed while winching in the sweep lines.
The net was designed to catch between SO and 60 tons of herring. The actual
depth of the herring in the schools varied from S to 10 fathoms. When the
herring dispersed at night and became scattered in the upper 1S fathoms of
water, catches of about 2 tons were made in tows of 1S minutes duration.
Catches made in daylight were much lighter than those made during the
night, averaging 1 to 2 tons. During the day most of the herring schools were
very close to the bottom or concentrated right on the bottom. It was therefore
difficult to tow as successfully in the daylight as at night. However, two daylight drags on a large concentration of herring at depths between 4S and SO
fathoms in mid-water in Captain's Passage yielded 1S and 3S tons of herring.
All the herring caught appeared to be in good condition and very little loss
of scales was noticed.
The mid-water trawl developed here makes herring fishing possible for
boats down to about 60 feet long and having an engine of 1SO to 17S h.p.particularly if they are already rigged for bottom trawling. Smaller models
can no doubt be developed for use by somewhat smaller boats. The trawl
cannot take herring by the hundreds of tons, as purse-seines do, but it is much
less expensive than a seine and requires only half the crew. In addition, the
trawl has made successful catches at depths up to 20 fathoms greater than
existing seines can reach. We anticipate that similar nets, with different dimensions and different mesh sizes, would be successful in taking other commercial
species in British Columbia waters.
24
ACKNOWLEDGMENTS
The authors wish to thank Dr. K. S. Ketchen of the Biological Station,
Nanaimo, for his advice and assistance during many phases of the investigation. His constructive criticism in reading the manuscript is also gratefully
acknowledged.
The energetic cooperation shown by the members of the crew of the
Sea Pride JI-Captain Norman Sigmund, R. MacLeod, W. Sigmund, and
V. Cragg, expedited the experimental fishing with the mid-water trawl.
Thanks are also expressed to Mr. G. F. Hart for his attention to many
tedious administrative matters during the course of the work.
25