Purse seine for sardine and other small pelagic species for a boat of 10 m LOA (PAJOT FAO)
* Note: With small purse seines where the purse line is not coiled on a drum, the purse line may be lashed to the buoy line.
■ Minimum length and depth of the purse seine, size of the bunt*
— Minimum length depends on the length of seiner : length of purse seine ≥ 15 x length of seiner
— Minimum depth : 10% of the length of seine
— Minimum length and depth of bunt = length of vessel
■ Choice of mesh size is a function of the target species. It is necessary to avoid enmeshing or gilling the fish (with respect for regulations on minimum mesh size).
where:
OM |
= mesh opening (mm) in the bunt |
L |
= length (mm) of target species |
K |
= coefficient, a function of the target species |
K |
= 5 for fish that are long and narrow |
K |
= 3.5 for average shaped fish |
K |
= 2.5 for flat, deep-bodied, or wide fish |
Some examples
Species |
Stretched meshsize (mm) |
Size of twine (Rtex) |
small anchovy, n'dagala, kapenta (East Africa) | 12 | 75-100 |
anchovies, small sardine | 16 | 75-150 |
sardine, sardinella | 18-20 | 100-150 |
large sardinella, bonga, flying fish, small mackerel and Spanish mackerel | 25-30 | 150-300 |
mackerel, mullet, tilapia, Spanish mackerel, small bonito | 50-70 | 300-390 |
Bonito, tuna, wahoo, Scomberomorus sp. | 50-70 (min) | 450-550 |
■ Relationship between the diameter of the twine and mesh size in different parts of the purse seine :
Some examples
Body of the purse seine | Bunt of the purse seine | |
Small Pelagic Fish | 0.01 to 0.04 | 0.01 to 0.05 North Sea 0.04 to 0.07 |
Large Pelagic Fish | 0.005 to 0.03 | 0.01 to 0.06 |
* In purse seines, as in many types of fishing gear, the 'bunt' refers to the section of net which is hauled last, or the section in which the catch may be concentrated
■ Ratio of ballast to weight of netting (in air)
The weight (in air) of the ballast normally ranges between 1 /3 and 2/3 the weight of the netting (in air).** The weight (in air) of the ballast per metre of seine footrope is often between 1 and 3 kg (although more is used for small mesh purse seines used to catch deep-swimming small pelagic fish and up to 8 kg/m is used in large tuna seines).
■ Ratio of buoyancy to total weight of the seine
The rigging of floats on a purse seine must take into account not only the buoyancy needed to balance the total weight of the gear in water, but also additional buoyancy.*** This additional buoyancy should be of the order of 30% for calm waters, and up to 50-60% in areas of strong currents, to compensate for rough sea conditions and other factors related to handling of the gear. Buoyancy should be greater in the area of the bunt (which has heavier twine) and mid-way along the seine (where pulling forces are greater during pursing).
In practical terms, the buoyancy of the floats should be equal to about 1.5 to 2 times the weight of the ballast along the bottom of the seine.
Examples
(a) If a large purse seine has relatively heavy netting (as is common), ballast may be relatively light, and the buoyancy needed is a bit more than half the weight (in air) of the netting.
Buoyancy = 1.3 to 1.6 x (weight of netting in water + weight of ballast in water)
(b) If a smaller purse seine has relatively light netting (as is common), the ballast should be relatively heavy, and the buoyancy may be equal to or slightly greater than the weight of the netting (in air).
Buoyancy = 1.3 to 1.6 (weight of netting in water + weight of ballast in water)
In summary, the procedure of choosing weight of ballast and buoyancy*** required is to calculate :
* Ballast in this case is
considered to include the sinkers on the leadline, purse rings, chain and any other
lead or iron rigging along the bottom of the seine
**
Weight of netting, see page 35
***
Buoyancy of purse seine floats, see pages 47-49
The leadline of a purse seine is usually longer than the floatline by up to 10%; however in some types, the two lines are equal in length.
The hanging ratio (E), is usually greater on the leadline than on the floatline. Hanging ratios generally range from 0.50 to 0.90, depending on the type of net. The hanging ratio may also vary along the floatline or leadline, usually being lower in the bunt. For more on hanging ratios and methods of hanging, see pages 38, 39, and 42.
The tow line is normally about 25% of the length of the purse seine.
The purse line is generally 1.1 to 1.75 times the length of the leadline, usually about 1.5 times the length of the purse seine. The purse line must have good resistance to abrasion and good breaking strength. As a general guideline, the breaking strength (R) of the purse line should be as follows :
R > 3 x (combined weight of netting, leadline, leads and purse rings)
Volume (on board) occupied by the seine when rigged
V(m3) = 5 x weight (tons) of the seine (in air)
Depth in water of the seine (see also pages 39 and 40). As an approximation, the actual depth or height (AD) can be considered equal to roughly 50% of the stretched depth (SD, or stretched meshsize x number of meshes) of the seine at its extremities, and 60% near the centre of the net.
Sinking speed of a purse seine — for different seines, sinking speed has been measured in a range from 2.4 to 16.0 m/min, with an average of 9.0 m/min.
■ Beach seine without bag
A single
panel of netting
— no particular rules
concerning heigh and length
or
Special meshsize and/or twinesize in
the central part
■ Beach seine with bag
■ Hauling points
For a rather nigh small seine with bridle, handled by one man alone
■ Ropes for hauling beach seines
Natural fibre rope or nylon, poly- ethylene, polypropylene
Seine length (m) | diameter synthetic fibre bridle (mm) |
50- 100 | 6 |
200 - 500 | 14-16 |
8001-500 | 18 |
■ Mesh size and twine thickness
In the wings, the mesh size and twine thickness may be the same as, or different from, those of the central section or bunt.
Examples of specifications for bunts of beach seines
target species | stretched mesh (mm) | twine thickness (R tex) |
sardine | 5-12 | 150-250 |
sardinella | 30 | 800-1200 |
tilapia | 25 | 100 |
tropical shrimp/prawn | 18 | 450 |
diverse large species | 40-50 | 150-300 |
The headrope and footrope (float line and lead line) are usually of the same material (PA or PE) and diameter.
Hanging ratios (E) are usually the same on headrope and footrope. For central sections, E = 0.5 or slightly greater (0.5-0.7). In the wings the hanging ratio is usually the same as in the bunt, but it is sometimes slightly greater (E = 0.7-0.9).
■ Floats on the headropeThe number of floats required increases with the height of the seine. The following are examples of buoyancy observed in the central part of seines :
height (m) of seine | buoyancy (g/m of hung net) |
3-4 | 50 |
7 | 150 |
10 | 350-400 |
15 | 500-600 |
20 | 1000 |
The floats are either evenly spaced along the headrope, or placed closer together in the bunt, and spaced increasingly farther apart toward the ends of the seine.
■ Sinkers on the footrope
The quantity and type of sinkers varies according to the intended use (to 'dig' more, or 'dig' less). Sinkers may be spaced evenly along the footrope, or concentrated more near the bunt.
■ Ratio of buoyancy/weight
In the bunt, the ratio of buoyancy/ weight of sinkers is around 1.5-2.0, but sometimes, to make the net 'dig' more, a net is rigged with more weight than buoyancy. In the wings, the ratio of buoyancy/weight of sinkers is equal to, or slightly less than, 1.
■ Construction, rigging :
very similar to bottom trawls
Bottom seine
Bottom seine with high headline
Bridles | Headline |
20-25 m | 35 m |
45-55 m | 45 m |
■ Track of the boat for shooting the anchor seine or Danish seine
Example : Shooting 12 'coils' or 2640 m (1 coil = 220 m)
■ Size of nets
|
Boat | Net | ||
Length (m) | Power (hp)* | Mouth** Opening (m) | Headline (m) | |
Bottom seine (Japan) | 10-15 | 30 | 50 | |
Bottom seine (Europe) | 15-20 | 100-200 | 20-30 | 55-65 |
Bottom seine (high op.)
|
10-20 | 100 | 35-45 | 25-35 |
20 | 200 | 45-65 | 35-45 | |
20-25 | 300-400 | ~100 | 45-55 | |
25 + | 500 | 55-65 |
■ Vertical opening (estimation)
■ Mesh size, twine size
stretched mesn (mm) |
Rtex |
110 150 |
1100-1400 |
90-110 |
1000 1100 |
70-90 |
700-1000 |
40-70 |
600 800 |
* Power in
(hp)=136 x Power in (kW)
** The mouth
opening is measured along the forward edge of the bellies, and is equal to
Howere, there
are local differences in how this term is used, (in some places it refers to
stretched mehsize x number of meshes), so caution in interpretation is
necessary.
Durabilty, resistance to abrasion, and weight are essential qualities of seine ropes.
Materials
Anchor seining |
combination rope |
Fly dragging |
PE or PP, Ø 20-32 (3 strands with lead core in each strand) |
Fly dragging |
small boats : |
Diameter
Rope | |
Ø | weight (kg/100 m) |
PP 20 | 35 |
24 | 43 |
26 | 55 |
28 | 61 |
30 | 69 |
Often the diameter changes along a single rope, from 24-36 mm (for mid-sized boats. Weights are often attached along the rope
Length is expressed in coils of 200-220 m total length usually 1000-3000 m.
Method | Fishing grounds | Rope length |
Scottish technique | shallow waters (50-70 m) or small areas of soft bottom surrounded by rocky areas | less than 2000 m |
medium depths (80-260) or large smooth bottom areas | 3000 m or longer | |
Japanese technique | for depths as great as 300-500 m or soft, regular bottam | 8 to 15 times depth of water |
■ Operating with an anchor (Denmark)
■ Fly-dragging (Scotland)
■ Fly-dragging (bull trawling) (Japan, Korea)
■ Operations of 2 boats (pair seining, Canada)
This example, from FAO, is for a 50-70 hp vessel. See table below for terms.
Terms used in net plan
MAT |
= twine material (see pages 6-8) |
Rtex |
= Resultant tex (twine size, see page 10) |
a (mm) |
= stretched meshsize (see pages 29-30) |
n |
= depth of panel in number of meshes (N direction) |
The numbers appearing along the front and aft edges of panels represent number of meshes.
Numbers and letters along inside edges of net represent cutting rates;
for example, 1N2B means 1 sideknot, 2 bars (see pages 32-33).
Ratios presented along inside edges represent numbers of meshes taken
up when joining the corresponding panels (see page 41).
Lengths of
lines are presented in metres (11.00, etc)
This example is a midwater pair trawl used by French vessels of 120-150 hp, for herring and mackerel
■ Bottom Trawls
Power 30 to 100hp* | |
Stretched mesh (mm) | Size of twine(Rtex) |
100 | 950-1 170 |
80 | 650-950 |
60 | 650 |
40 | 650 |
Power 100 to 300 hp* | |
Stretched mesh (mm) | Size of twine(Rtex) |
200 | 1 660-2500 |
160 | 1 300 |
120 | 1 300-2 000 |
80 | 950-1 550 |
60 | 850-1 190 |
40 | 850-1 190 |
Power 300 to 600 hp* |
|
Stretched mesh (mm) | Size of twine(Rtex) |
200 | 2 500-3 570 |
160 | 1 230-2 000 |
120 | 1 230-2 000 |
80 | 1 600 |
60 | 950-1 190 |
40 | 950-1 190 |
■ Shrimp trawls, American type, semi-balloon
try-net (see pg. 84) | |
Stretched mesh (mm) | Size of twine(Rtex) |
39.6 |
645 |
Power 150 to 300 hp* | |
Stretched mesh (mm) | Size of twine(Rtex) |
44 | 940-1190 |
39.6 | 1 190 |
Power 300 to 600 hp* | |
Stretched mesh (mm) | Size of twine(Rtex) |
47.6 | 1 190 |
39.6 | 1 540 |
* brake horsepower (BHP) or Apparent Nominal Power (ANP), see pg. 95 Power in HP= 1.36 x (power in kW)
■ High-opening bottom trawls
Power 75 to 150 hp* | |
Stretched mesh (mmW) | Size of twine (Rtex) |
120 | 950 |
80 | 650-950 |
60 | 650-950 |
40 | 650-950 |
Power 150 to 300 hp* | |
Stretched mesh (mm) | Size of twine (Rtex) |
200 | 1 660-2 500 |
160 | 1 300-1 550 |
120 | 1 300-2 000 |
80 | 950-1 550 |
60 | 850-1 190 |
40 | 850-1 020 |
Power 300 to 800 hp* | |
Stretched mesh (mm) | Size of twine (Rtex) |
800 |
5 550 |
400 |
3 570 |
200 |
2 500-3 030 |
160 |
1 660-2 500 |
120 |
1 550-2 500 |
80 |
1 300-2 500 |
60 |
1 190-1 540 |
40 |
940-1 200 |
■ Midwater trawls (for single vessel)
Power 150 to 200 hp* |
|
Stretched mesh (mm) | Size of twine (Rtex) |
400 | 2 500 |
200 | 1 190-1 310 |
160 | 950-1 190 |
120 | 650-950 |
80 | 650-950 |
40 | 450 |
40 | 950-1 310 |
Power 400 to 500 hp* | |
Stretched mesh (mm) | Size of twine (Rtex) |
800 |
3 700 |
400 |
2 500 |
200 |
1 310-1 660 |
160 |
1 190-1 310 |
120 |
950 |
80 |
650-950 |
40 |
650-950 |
40 |
1 660 |
Power 700 hp* | |
Stretched mesh (mm) |
Size of twine (Rtex) |
800 |
7 140-9 090 |
400 |
3 700-5 550 |
200 |
2 500-3 700 |
160 |
2 500 |
120 |
1 660 |
80 |
1 660 |
40 |
1 660 |
40 | 2 500 |
■ Midwater pair trawls
Power 2 x 100-300 hp* | |
Stretched mesh (mm) | Size of twine (Rtex) |
800 |
3 030-4 000 |
400 |
1 190-2 280 |
200 |
1 190-1 540 |
120 |
950 |
80 |
650-950 |
40 | 450-950 |
Power 2 x 300-500 hp* | |
Stretched mesh (mm) | Size of twine (Rtex) |
800 |
5 550 |
400 |
2 280 |
200 |
1 540 |
120 |
950-1 190 |
80 |
950-1 190 |
40 |
950-1 190 |
* Brake
horsepower (BHP} or Apparent Nominal Power(APN), see page 95.
Power in Hp = 1 36 x (power in kW)
■ Selection according to the calculated twine surface area of the net (see page 37 for twine surface area)
Given the essel horsepower, and the type of trawling intended, the best results will be obtained by choosing a net of which the twine surface area falls within a particular range.
Given the vessel horsepower and trawl type, the twine surface area may vary according to several factors, for example : real delivered horsepower, rate of utilisation of the motor, type of rigging, meshsize, type of bottom, strength of currents, etc.
For pair trawling, the twine surface areas (m2) indicated above should be multiplied by the factors shown in the table:
trawl type |
factor |
two-panel bottom trawls: |
2.4 |
four-panel bottom trawls: |
2.2 |
single-boat mid-water trawls (stretched mesh in wings up to 200 mm) : |
2 |
single-boat mid-water trawls (wing meshes larger than200 mm): | 2 |
■ Choice by comparison with a trawl of the same type used by a vessel in the same horsepower range
Let us say you know the dimensions of a particular trawl (T) used by a particular trawler which has horsepower P1. In order to calculate the right net size for another vessel of horsepower P2, the length and width of each panel of P1 are multiplied by
* Brake
Horsepower (BHP) or Apparent Nominal Power
(ANP), see page 95
Power in (HP) = 1.36 x Power in (kW)
■ Bottom trawl with low vertical opening (VO)
■ High-opening bottom trawl
■ Shrimp trawl (flat or semi-balloon)
N or n = width in number of meshes of front edge of belly (seams not included)
a = meshsize, length in metres of one stretched mesh at the part of net considered
VO
= approximate vertical opening of net mouth (metres)
S = approximate horizontal spread between ends of wings (metres)
HR = length in metres of headrope
■ High-opening, 4-panel bottom trawl
■ Single-boat mid-water trawl
■ Mid-water pair trawl
n = width in number of meshes of front edge of belly (seams not included)
nv
= width in number of meshes of oft edge of belly (seams not included)
HR = length of headrope in metres (not including free ends)
a = meshsize (length in metres of one stretched mesh at the part of the
net being considerea)
VO = approximate vertical opening of net mouth (metres)
S = approximate horizontal spread between ends of wings (metres)
Principal types, adjustments, relative length
■ Bottom trawls with low headline height
■ Bottom trawls with high headline heights (OV): sweeps and bridles
■ Adjustments
N.B. the adjustments made are extremely small, measured in single chain links
■ Relative lengths of different parts of the trawl gear
F about 2.2 times the depth for deep water
about 10
times the death for shallow water
As a general rule
B = F/3 to F/8
F = trawl warps (m)
B = length of
sweeps or sweeps + bridles or 'forks'**
* Broke horsepower (BHP) cr Apparent Nominal Power (ANP), see page 95
Power
in (HP) = 1.36 x Power in (kW)
** Fork rig, see page 81
■ High-opening bottom trawls : fork rigging
The length of warps equals 3 to 4.5 times the depth of water
■ Single-boat midwater trawl
* For power
to use in calculation, see page 95
Power in (HP) = 1.36 x Power in (kW)
■ Bottom trawls
■ Midwater trawls
P = power of the trawler
L = distance trawl — trawler
G =
weights in front of the trawl
d =
distance between the trawlers
* Brake horsepower (BHP) or Apparent Nominal Power (ANP), see page 95
Power
in {HP) = 1.36 X Power in (kW)
It is necessary to estimate the vertical angle of the warps. (In other words the inclination, or angle between the warps and the horizontal plane.)
Note : These methods give only very rough approximations. They should be used only when you have no netsounder to give more accurate information. Be careful to keep the net away from the bottom
The warp angle may be measured with a protractor or other device
Depth of the trawl is estimated as follows :
Another method without using a protractor is shown below
Distance measured D cm | WARP LENGTH (M) | ||||
100 | 200 | 300 | 400 | 500 | |
99 |
14 |
27 |
42 |
56 |
70 |
98 |
21 |
42 |
62 |
83 |
103 |
97 |
25 |
49 |
72 |
94 |
116 |
96 |
28 |
57 |
82 |
106 |
130 |
95 |
31 |
62 |
92 |
123 |
153 |
94 | 34 | 68 | 103 |
138 |
174 |
■ Gulf of Mexico type
Example:
Examples of mesh sizes
Stretched
mesh (in mm) French Guyana : 45 West Africa : 4C-50 Persian
Gulf : 30-40/ 43-45.
Madagascar : 33-40 India: 50-100 Australia : 44
In tropical zones the catch rate is proportional to the horizontal spread of the trawl. In order to obtain the greatest Horizontal opening, special types of trawl are used, ard also special rigging.
(1) Special types of trawl
(2) Special rigging
■ Rigging of booms
This rigging allows an increase in shrimp catch rate of 15-30% over that of a sing e trawi. Towing speed is 2.5 to 3 knots.
Power of engine |
Lengths (m) | ||
Headline | Bridles | Booms | |
150 to 200 |
12-14 |
33 |
9 |
200 to 150 |
15-17 |
35 |
9 |
250 to 300 |
17-20 |
40 |
9 |
300 to 400 |
20 |
45 |
10 |
500 |
24 |
50 |
12 |
Depth (m) |
Warp length (m) |
20 |
110 |
20 to 3D |
145 |
30 to 35 |
180 |
35 to 40 |
220 |
* Brake horsepower (BHP) or Apparent Nominal
Power (ANP). see page 95
Power in (HP) = 1.36 X Power in (KW)
■ Bottom trawls
■ Midwater trawls for 1 boat
■ Midwater pair trawls
Real horsepower' hp
|
|
|||||
B1 (kgf) P (hp)* |
W1 (kg air) P (hp)* |
B2 (kgf) P (hp)* |
W2 (kg air) P (hp)* |
B3 (kgf) P (hp)* |
W3 (kg air) P (hp)* |
|
50 | B1=Px... |
W1=Px ... |
B2 = Px ... |
W2 = Px ... |
B3 = Px... |
W3 = P x .. |
100 | 0.20 | 0.28 | 0.27 | 0.29 | 0.28 | 0.33 |
200 | 0.20 | 0.25 | 0.24 | 0.27 | 0.25 | 0.31 |
400 | 0.20 | 0.22 | 0.22 | 0.24 | 0.22 | 0.28 |
600 | 0.20 | 0.22 | 0.21 | 0.23 | 0.21 | 0.27 |
800 | 0.18 | 0.20 | 0.19 | 0.22 | 0.19 | 0.26 |
— For buoyancy, the indicated values correspond to net made of polyamido (nylon), a synthetic fibre with negative puoyancy (it sinks), for nets made of floating materials, the floats may be decreased by 10-15%.
— The weights presented are estimated, with a 5-10% margin of error They may vary accoraing to the trawling speed, typo of bottom buoyancy of the net and floors, target species, etc. These weignts have been calculated assuming that steel chain will be used for ballast. If another material is used, its density trust be taken into account. For example, in oraer to get the same sinking force in water, a length of chain weighing 1 kg in air must be replaced by a quantity of rubber rollers which weighs 3 - 3.5 kg in air.
* Brake horspower (BHP) or Apparent Nominal Power (ANPi,
see page 95
Pewer in (HP) - 1.36 x Power in
(KW)
|
■ Midwater trawls (maximum vertical opening) : joining lines  of braided  PP. Groundrope of leaded rope |
■ High-opening bottom trawls : joininq lines of braided PP. Groundrope of chain | ■ Shrimp trawls, smooth bottom : Grassrope with lead rings (chain groun drope is also common) |
■ High-opening bottom trawl
with 2 bridles : groundrope of rubber rings
For use or rougher bottom : groundrope of rubber bobbins or rollers with rubber disc spacers and chain joining lines |
|
■ Fish or shrimp trawls, hard bottom : groundrope of rubber rings and hard plastic spheres | |
■ Fish or shrimp trawls for soft or muddy bottom : split wooden rollers which can be added or removed without running groundrope through centre |
■ Estimating the spread of otter boards (doors)
Example : On the vessel above,
if :
A =
4.00
B =
4.18
F = 200
then
D = [(4.18 - 4.00) x 200] + 4 =
40 m spread at otter boards
■ Estimating the spread of the trawl
To estimate the horizontal spread between the winq ends :
Example : given a traw of 25 m in length (without bag) rigged with sweeps of 50 m and otter board spread of 40 m, then spread of trawl wing ends :
Proportions of different types of otter boards
■ Flat rectangular otter boards
■ Shrimp otter boards
■ Angles of attack
■ Rectangular V section otter boards
■ Suberkrub pelagic otter boards
■ Angle of attack
■ Adjustment of angle of attack
■ Adjustment of orientation
Problem
|
Recommended adjustment Raise the towing brackers a Iittle if possible |
Lower the towing brackets a little if possible or add weight to the keel | |
Lengthen the upper backstrop (a) or shorten the lower backstrop (b), keeping in mind that a little upward till is good for certen bottom condition. | |
Lengthen the lower backstrop (b) or shorten the upper backstrop (a) |
■ Rectangular and oval curved
The weights indicated below (for single board) are the maximum values used. For a given horsepower, the Surface area listed below is often used, but with a lighter material which may make a board as much as 50% lighter.
Power* (hp) |
Rectangular flat otter boards | Oval Curved Other boards | Weight (kg) |
||||
Dimension | Surface | Dimension | Surface | ||||
L (m) |
h (m) |
m2 |
L (m) |
h (m) |
m2 |
||
50-70 | 1.30 | 0.65 | 0.85 | 45 | |||
100 | 1.50 | 0.75 | 1.12 | 1.40 | 0.85 | 0.93 | 100-120 |
200 | 2.00 | 1.00 | 2.00 | 1.75 | 1.05 | 1.45 | 190-220 |
300 | 2.20 | 1.10 | 2.42 | 1.90 | 1.10 | 1.65 | 300-320 |
400 | 2.40 | 1.20 | 2.88 | 2.20 | 1.25 | 2.15 | 400-420 |
500 | 2.50 | 1.25 | 3.12 | 2.40 | 1.40 | 2.65 | 500-520 |
600 | 2.60 | 1.30 | 3.38 | 2.60 | 1.50 | 3.05 | 600-620 |
700-800 | 2.80 | 1.40 | 3.92 | 2.90 | 1.60 | 3.65 | 800-900 |
■ V otter boards
Power* (hp) | Surface m2 | Weight kg |
100 | 1.40 | 240 |
200 | 2.10 | 400 |
300 | 2.50 | 580 |
400 | 2.90 | 720 |
500 | 3.30 | 890 |
600 | 3.60 | 1 000 |
700 | 3.90 | 1 100 |
800 | 4.20 | 1 200 |
■ Shrimp otter boards (double rig)
Power (hp)* | Dimensions m | Weight kg |
100-150 | 1.8 x 0.8-2.4 x 0.9 | 60-90 |
150-200 | 2 x 0.9 - 2.45 x 1 | 90-100 |
200-250 | 2.4 x 1 - 2.45 x 1 | 120 |
250-300 | 2.5 x 1 - 2.7 x 1.1 | 160 |
300-450 | 3 x 1.1 -3 x 1.2 | 220 |
450-600 | 3.3 x 1.1 -3.3 x 1.3 | 300 |
■ Midwater, Suberkrub
Power* (hp) |
Dimensions | Surface (m2) |
Weight (kg) |
|
H(m) | L(m) | |||
150 | 1.88 | 0.80 | 1.50 | 90-100 |
200 | 2.05 | 0.87 | 1.80 | 110-120 |
250 | 2.12 | 0.94 | 2.00 | 150-160 |
300 | 2.28 | 0.97 | 2.20 | 170-180 |
350 | 2.32 | 1.03 | 2.40 | 220-240 |
400 | 2.42 | 1.07 | 2.60 | 240-260 |
450 | 2.51 | 1.12 | 2.80 | 260-280 |
500 | 2.68 | 1.14 | 3.00 | 280-300 |
600 | 2.86 | 1.22 | 3.50 | 320-350 |
700-800 | 3.00 | 1.33 | 4.00 | 400-430 |
Example of the relationship between the twine surface area (see page 37) of a pelagic trawl (Sf in m2) and the surface area of a Superkrub offer board used by the boat (Sp in m2)
* Broke horsepower (BHP) or Apparent. Nominoi Power (ANP), see page 95
Power in HP = 1.36 x Power .n
(kW)
■ Example, for a 25.5/34 trawl
Power (hp)* |
L x 1 |
150-250 |
0.55 x 0.45 m |
250-350 |
0.60 x 0.45 m |
350-500 |
0.65 x 0.50 m |
500-800 |
0.80 x 0.60 m |
Many types of kites exist and are being tested, the simplest being a piece of sail cloth mounted on the headline and patched to the inside netting
* Brake horsepower (BHP) or Apparent Nominal
■ Characteristics of steel trawl warps, according to power of trawler
hp* |
Ø (mm) |
kg/m | R kgf |
100 | 10.5 | 0.410 | 5 400 |
200 | 12.0 | 0.530 | 7 000 |
300 | 13.5 | 0.670 | 8 800 |
400 | 15.0 | 0.830 | 11 000 |
500 | 16.5 | 1.000 | 13 200 |
700 | 18.0 | 1.200 | 15 800 |
900 | 19.5 | 1.400 | 18 400 |
1 200 | 22.5 | 1.870 | 24 500 |
R= breaking strength |
■ Length of warps according to depth of water (for bottom trawling)
(for shallow water less than 20 m the length should not be less than 120 m)
This curve gives only estimates; the captain should decide warp length according to the type of bottom, sea conditions, current, etc.
* Brake horsepower (BHP; or
Apparent Nominal Power (ANP), .see page 95
Power in (HP) = 1.36 x Power in
(kW)
Main species groups |
Average trawling speed (knots) |
shrimp, small bottom species, flat fish |
1.5-2 2.5-3.5 |
mid-sized bottom species, small
pelagic fish |
3-4 4-5 |
cephalopods (squid, cuttlefish) |
3.5-4.5 |
mid-sized pelagic fish |
>5 |
■ The choice of fishing gear depends on the power of the trawler
For trawlers with a fixed propeller, reduction gear between 2 : 1 and 4 : 1, and no nozzle, the tables in this book are intended for use with the Brake Horsepower (BHP).
This is the figure given most offer by manufacturers as the horsepower or rated power of an engine. It is expressed in horsepower (HP) or in kilowatts (kW).
If a trawler has a variable pitch propeller and/or a nozzle. Apparent Nominal Power (ANP), should be used in the tables of this book.
It may be calculated as follows :
Example : A trawler, with a variable pitch propeller and a nozzle, has an engine rated at 400 BHP, and the bollard pull is 6000 kg
Thus, the fishing gear should be chosen from the tables according to on Apparent Nominal Power of 540 HP, and not 400 HP.
Power available for trawling (p), is usually 15 to 20% of the BHP on ANP. This power is used to pull the gear, and may be calculated as follows :
In calm waters, p = 0.75 x k x (BHP or ANP)
type of propeller and engine | k | |
fixed propeller | high RPM engine | 0.20
0.25 - 0.28 |
slow turning engine | ||
variable pitch propeller | 0.28-0.30 |
In rough weather, o is reduced by 1/3.
■ Bollard pull BP of a trawler at fixed point (speed = 0)
BP (kg)
= 10 to 12 kg per BHP* (with fixed propeller)
13 to 16 kg per HP of Apparent Nominal Power* (with a variable pitch propeller or nozzle)
■ Bollard pull BP (when fishing)
If you have calculated the engine power (p) available for towing (page 95),
Choosing the appropriate engine speeds (RPM) for 2 boats of different characteristics for pair trawling
Vessel A pulls vessel B, engine in neutral, at the chosen speed, for example 2 knots. Then vessel B engine is engaged and the revs progressively increased until vessel B holds vessel A stationary.
The engine RPM of both vessels A and B are noted, for the chosen speed of 2 knots. The same operations are repeated for other speeds until the range of normal trawling speeds is covered.
■ Trawl: pull of trawler
Revs |
Vessel A | Vessel B |
Speed |
||
2 knots | – | – |
2.5 | – | – |
3 | – | – |
3.5 | – | – |
Gillnet | Vessel |
bottom set for spider crabs Brittany, France |
length 5-15 m HP 15-20 |
This drawing shows the following information about the net: |
for more details |
|
Stretched meshslze | : 320 mm |
pages 29-30 |
Length | : 313 meshes | |
Height | : 5 /2 meshes | |
Hanging ratio (E) | : 0.50 | paqes 38-39 |
Floats | : 32 plastic floats, each with buoyancy of 50 gf | pages 47-49 |
Sinkers | : 156 leads, each weighing 50 g | |
Twine | : material — polyamide; size — R 1666 tex | pages 7-10 |
Floatline | : polypropylene/polyamide, diameter 6 mm, length 50 m | pages 7-8 |
Leadline | : polypropylene/polyamide, diameter 6 mm, length 50 m | pages 7-8 |
■ Choice of meshsize according to fish species
There is a ratio between the body girth or length of a fish one wants to catch, and the gillnet meshsize which will be effective for that fish (Fridman formula).
where
OM |
= mesh opening (mm) |
L(fish) |
= average length (mm) offish one wants to catch |
K |
= coefficient, according to species |
and
K |
=5 for long, thin fish |
K |
= 3.5 for average-shaped fish (neither very thick nor thin) |
K |
= 2.5 for very thick, wide or high (shaped) fish |
A few examples of stretched meshsizes (mm) adapted for particular species
Demersal tropical species | |
threadfin (Polynemidae) |
50 |
small catfish |
75 |
grunt (Pomadasidae) |
50 |
mullet |
110-120 |
maigre (Sciaenidae) |
120-140 |
croaker (Sciaenidae) |
160-200 |
seabream (Sparidae) |
140-160 |
barracuda | 120 |
* For clarification of terms stretched meshsize and mesh opening see page 29
Temperate demersal species | |
cod | 150-170 |
pollack | 150-190 |
Pacific pollack |
90 |
sole | 110-115 |
hake | 130-135 |
red mullet (Mugilidae) |
25 |
halibut (Greenland) |
250 |
turbot, monk, anglerfish | 240 |
Crustaceans | |
shrimp (India) |
36 |
shrimp (El Salvador) |
63-82 |
green spiny lobster |
160 |
red spiny lobster |
200-220 |
spider crab |
320 |
king crab |
450 |
Small pelagic species | |
sprat | 22-25 |
herring | 50-60 |
anchovy |
28 |
sardine | 30-43 |
sardinella | 45-60 |
shad (Ethmalosa) |
60-80 |
small mackerel |
50 |
large mackerel | 75 |
Spanish mackerel | 100-110 |
Large pelagic species | |
mackerel, bonito, | |
skipjack | 80-100 |
marlin, flying fish | 120-160 |
bonito, jacks | 125 |
Atlantic bluefin | |
tuna | 240 |
sharks | 170-250 |
swordfish | 300-330 |
salmon | 120-200 |
The twine should be relatively thin, but not so fine that it damages entangled fish. Good breaking Strength is important, especially for bottom set gillnets, taking nto ac-count the size of the fish and the meshsize. The twine should have low visibility, either clear (mono or monofilament monofilament) or of a colour wh:cn blends -n with the environment. It should also be flexible.
Note : A length of twine may stretch 20 40% before breaking
■ Choosing twine diameter for gillnets
Twine aiameter should oe proporional to meshsize. The ratio
should be between 0.0025, for calm waters and ow catches, and 0.01, for rough waters or bottom set. An average ratio is 0.005.
■ Examples of twine sizes used with certain types of gillnets and meshsizes
stretchec meshsize | inland waters, lakes, rivers | coastal waters | open ocean | |||||
mm | multifill m/kg | monofil. Ømm | multifill m/kg | monofil. Ømm | multimono. n x Ømm |
multifill m/kg |
monofil. Ømm | multimono. n x Ømm |
30 | 20 000 | 0.2 | 10 000 | 0.4 | ||||
6 660 | ||||||||
50 | 20 000 | 13 400 | 0.2 | 6 660 | ||||
60 | 13 400 | 0.2 | 10 000 | 4 440 | ||||
80 | 10 000 | 6 660 | 4 x 0.15 | 4 440 | 0.28-0.30 | 6 à 8 x 0.15 | ||
100 | 6 660 | 4 440 | 0.3 | 3 330 | 0.5 | 6 x 0.15 | ||
120 | 6 660 | 4 440 | 0.35-0.40 | 3 330 | 0.6 | |||
140 | 4 440 | 3 330 | 0.33-0.35 | 6 x 0.15 | 2 220 | 8 x 0.15 | ||
160 | 3 330 | 3 330 | 0.35 | 8 à 10 x 0.15 | 2 220 | 0.6-0.7 | ||
200 | 2 220 | 2 220 | 1 550 | 0.09 | 10 x 0.15 | |||
240 | 1 550 | 1 550 | 1 100 | 0.09 | ||||
500 | 1 615-2 220 | |||||||
600 | 3 330 | 1 615-2 220 | ||||||
700 |
■ Effect of the hanging ratio on the catching efficiency of the net
Generally the horizontal hanging ratio is about 0.5 for gillnets (see page 38).
— If E is smaller than 0.5 the net will tend to tangle fish, and will capture a variety of different species. This is the case with most set nets.
— If E is greater than 0.5 the net will tend to gill the fish and be more selective than in the preceding case. This is the case with most driftnets.
■ Examples of rigging
On the headrope with floats attached
On the footrope with sinkers attached
Trammel net*
Bottom set
or drifting, for shrimp
Sri Lanka
* For clarification of symbols used in drawing of entangling net, see page 97
■ Choosing the mesh sizes according to the size of target species*
— Central panel : The meshsize should be small enough to catch the smallest fish wanted, by bagging. A rough estimate of the required meshsize is given by the Fridman formula for net bags:
OM should be smaller than :
where
OM (mm) |
= mesh opening of the central net |
L (mm) |
= length of the smallest fish wanted |
K |
= coefficient dependent on the target species |
K |
= 5 for long and narrow fish |
K |
= 3.5 for average fish |
K |
= 2.5 for flat, thick or large fish |
— External panels : the mesh size should be 4 to 7 times larger than that of the central netting.
* For clarification of terms stretched meshsize and mesh opening see page 29
The stretched height of the central net panel should be 1 5 to 2 times the stretched height of me external netting.
The actual height in the water of the trammel net depends on the height of the external netting. The central net panel should be very slack.
■ Hanging ratios of the net panels
The horizontal hanging ratios are often close to the following values:
■ Floating gillnets and trammel nets
B(gf/m) | 100-160 | B2 = 50- 120 | 600-1 500 |
B1 = 50 - 80 | |||
W (g/m) | 50-60 | W1 = 30-80 | 300-1 000 |
W2 = 25-60 | |||
B/W | 2 | B2/W2 ~ 2-2.5 | 1.5-2 |
Length of leadline < 1 |
B1 ~ Wf + W1 Wf = weight of netting in water |
Bottom set gillnets and trammel nets
B (gf/m) | 40-80 | 100-200 |
W (g/m) | 120-250 | 250-400 |
B/W | 1/3-1/5 | 1/2-1/2.5 |
length of leadline < 1 length of floatline (greater or equal) |
Note : These weights do not include anchors, etc.
■ Set gillnets and trammel nets
■ Drifting gillnets
Crab trap | Vessel |
Hokkaido, Japan Nova Scotia, Canada |
Length 12 - 15 m hp 40 - 100 |
These gears, which can be used for catching fish, crustaceans, molluscs, and cephalopods (squid, octopus, etc.), are made in a wide variety of shapes and sizes, using many different materials. They may be used on the bottom or in mid-water, with or without bait.
■ Choosing the size of a pot or trap
If a pot gets too crowded with captured fish inside, it will stop catching. The interior volume of c pot must be large enough to avoid this situation. On the other hand, in some cases an interior volume which is too large may lead to cannibalism (some captives eating others!. Some types of pots appear to be effective because their shape and size make them attractive shelters for certain species.
A few examples :
Species |
Country | Volume (cubic decimeters - see p. 157) |
octopus | 6 | |
small shrimp | 40-70 | |
small crabs | Japan | 70-90 |
crabs | Canada | 450 |
King crab, snow crab | USA | 2500-4500 |
spiny lobster | Europe | 60-130 |
lobster | USA | 200 |
spiny lobster | Caribbean | 300-800 |
spiny lobster | Australia | 2500 |
sea bream | Morocco | 150-200 |
mixed reef fish | Caribbean | 500-700 (up to 2000) |
torsk, wolf fish | Norway | 1300 |
grouper | India | 1400 |
black cod | USA, Alaska | 1800 |
Choice of materials must consider such factors as durability, resistance to immersion, corrosion, and fouling by marine growth.
Spacing of bars or laths; or size of meshes has a direct relation to the size of the target species.
A few examples (measurements in mm) :
Species | bar of
mesh (diamond shape) |
small
shrimp (Europe) |
8-10 |
small
crabs (Japan) |
12 |
rock crab (Europe) |
30 |
crab (Canada, USA) |
50 |
King crab (Alaska) |
127 |
spiny
lobster (France, Morocco) |
30-40 |
lobster | 25-35 |
torsk,
wolffish (Norway) |
18 |
sea bream | (see Alternatives) |
grouper (India) |
40 |
reef fish (Caribbean) |
15-20 |
black cod (USA) |
(see Alternatives) |
threadfin (Australia) |
(see Alternatives) |
Alternatives
— For lobster pots :
Triangular meshes ∆\ 60-80 mm side
Rectangular meshes 25 x 50 mm
Parallel wooden strips or laths, spaced 25-38 mm apart
— For fish pots :
For sea bream, triangular meshes ∆\ 35-40 mm on a side
For black cod, USA west coast, square meshes 51 x 51 mm For threadfin, Australia, hexagonal meshes 25-40 mm across
Ballast in traps is very variable, from 10 to 70 kg per trap, according to the type and size of trap, the type of bottom, and strength of currents.
The shape is usually that of a cone or truncated pyramid, straight or curved.
■ The position : examples
Traps for fish and cephalopods : entrance(s) at the side(s)
Traps for crustaceans : entrance(s) on the side(s) or on the top
The diameter of a pot entrance is directly related to the size and characteristics of the target species.
A few examples:
Species | Country | Entrance diameter (mm) |
small shrimp |
40-60 |
|
small and medium crabs | Japan, USA |
140-170 |
snow crab | Canada |
360 |
King crab | USA Alaska |
350-480 |
spiny lobster, crayfish | Europe |
100-200 |
spiny lobster | Australia, Caribbean |
230 |
lobster | Europe |
100-150 |
sea bream | Morocco |
70-100 |
torsk, wolffish | Norway |
100 |
grouper | India |
210 |
black cod | USA, W. coast |
250 |
threadfin |
Australia |
250-310 |
snapper |
Caribbean |
230 |
■ For fish or cephalopods
■ For crustaceans
A : Mainline
B . Branchline (also called snood, leader, gangion, drop line)
The breaking strength of the mainline should be greater than or equal to the maximum weight of an individual fish to be caught (even if there are several branchlines).
Examples of mainline breaking strength in common use for certain species
Species |
Breaking strength |
sea bream, snapper |
7-15 |
meagre, conger, dogfish |
15-30 |
weakfish, grouper, cod, moray |
30-40 |
snapper, grouper |
100 |
yellowfin tuna |
150-200 |
Note : Some vessels equipped with hydraulic or electric reels for catching snapper and grouper in depths greater than 180 m, use stainless steel or monel mainlines with breaking strength of the order of 400 kg
The breaking strength of branchlines is usually 50-100% of the breaking strength of the mainline.
for hooks and lures sec pages 43-45
Trolling speeds vary from 2 to 7 knots, depending on target species
S : shock
absorber or snubber
DP : depressor or diving boord
Pb : 'cannonball' weight
■ Shock absorber or snubber
Absorbs the shock load on the line when the fish strikes
Depressor or diving board to troll deeper
■ Shearing depressor or diving board
May be adjusted to dive and also shear horizontally to spread lines
Longline for dogfish, rays, conger, ling | Boat |
Channel, France | Length 14-15 m |
TJB 2C - 30 | |
hp 150 |
A longline consists of a main fire, to which a number of branchines (also called snoods or gangions) are attached. A hook is attached to the end of each branchline.
The material and diameter of the mainline will depend on the target species, the type of longline (bottom or mid-water), and gear-handling methods (manual or mechanical hauling). The diameter and breaking strength must take into account not only the weight of the fish, but also the displacement (and therefore, inertia) of the vessel.
As a general rule, one can choose a mainline whose breaking strength (dry, unknotted, in kg) is :
— both greater than 10 times the tonnage of the vessel, and greater than the square of the vessel's length (in metres).
— at least 10 times the weight of the largest fish one expects to catch.
For example :
What would be the minimum breaking strength for the main line of a longline used by a 9 m, 4 t vessel, catching sea bream and gurnards?
Breaking strength must be greater
But, if one expects to catch individual fish weighing 10 kg, it s necessary to calculate
Therefore, the line could be twisted or braided nylon (PA), 2 mm diameter (breaking strength 130-160 kg); or nylon monofilament 170/100 (breaking strength 1 10 kg); or polyethylene (PE) 3 mm diameter (breaking strength 135 kg).
Branchlines (snoods or gangions) should be as close as possible to invisible 'n water, but sometimes of steel (for example, in some tuna and shark fisheries).
Breaking strength of branchlines (wet, with knots) should be at least equal to twice the weight of the fish one expects to catch. (The breaking strength of the main line should equal 3 to 10 times that of the branchlines.
The length of a branchline is usually less than half the distance between branchlines, in order to avoid tangling.
Hooks are usually chosen by experience, according to the size and behaviour of the target species; hooked fish should stay alive (for species which can live when hooked), but should not come unhooked.
■ Semi-pelagic longlines
■ Bottom longlines
Some examples :
TYPES | ■ Longline with fixed snoods | ■ Longline with detachable snoods | |
STOWAGE ON BOARD |
|
Mainline |
Snoods for (or books) |
SHOOTING EQUIPMENT | Baiting machine | ||
HAULING EQUIPMENT |
|
■ On the surface
■ On the bottom
■ Some types of anchors
■ Characteristics
Rigid fishing gear 'or polling over the bottom (types for soft bottom, types for very hard bottom)
Sizes Usually the width is less than 2 m, exceptionally up to 5 m) Height is always less than 0.5 m Heavy (to scrape the bottom)
■ Examples of different types
■ Power required
1 hp per 2 kg of dredge
■ Towing cable
(one)
■ Amount of warp depends on the depth of water and the speed
(The warp paid out will need to be increased with the speed). In general, 3 to 3.5 x depth (at 2-2.5 knots)
■ Speed of dredging:
2 to 2.5 knots
■ Rigging, some examples