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Fishing gear
and operations

PURSE SEINES

Purse seine : example of plan and rigging

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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.

Purse seines : minimum dimensions, mesh sizes, twine sizes

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).

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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 :

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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

Weight of ballast*, buoyancy of floats, weight of netting

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.

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Buoyancy = 1.3 to 1.6 x (weight of netting in water + weight of ballast in water)

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(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).

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Buoyancy = 1.3 to 1.6 (weight of netting in water + weight of ballast in water)

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In summary, the procedure of choosing weight of ballast and buoyancy*** required is to calculate :

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* 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

Hanging, leadline, tow line, purse line, depth, volume on board, performance

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)

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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.

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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 SEINES

Types of beach seine, bridles, ropes

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

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Beach seine with bag

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Hauling points

For a rather nigh small seine with bridle, handled by one man alone

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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

Beach seine : materials and hanging

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 headrope

The 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.

BOTTOM SEINES

Bottom seines : types of bottom seines and method of setting

Construction, rigging :

very similar to bottom trawls

Bottom seine

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Bottom seine with high headline

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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)

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Bottom seines : dimensions and properties of net

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)

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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
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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.

Bottom seines : ropes

Durabilty, resistance to abrasion, and weight are essential qualities of seine ropes.

Materials

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Anchor seining
(Danish sening) :

combination rope
18-20

Fly dragging
Scottish seining) :

PE or PP, Ø 20-32 (3 strands with lead core in each strand)

Fly dragging
(Japan, Korea]

small boats :
manila mid-sized boats : PVA

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

Bottom seines : Operations

Operating with an anchor (Denmark)

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Fly-dragging (Scotland)

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Fly-dragging (bull trawling) (Japan, Korea)

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Operations of 2 boats (pair seining, Canada)

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TRAWLS

Plan of 2-panel bottom trawl

This example, from FAO, is for a 50-70 hp vessel. See table below for terms.

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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)

Plan and rigging of a 4-panel midwater trawl

This example is a midwater pair trawl used by French vessels of 120-150 hp, for herring and mackerel

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Trawls : relationship between mesh size and twine size for bottom trawls

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

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* 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

Relationship between mesh size and twine size for Midwater trawls

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

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* Brake horsepower (BHP} or Apparent Nominal Power(APN), see page 95.
Power in Hp = 1 36 x (power in kW)

Choosing the right size trawl for the power of the vessel

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.

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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

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* Brake Horsepower (BHP) or Apparent Nominal Power (ANP), see page 95
Power in (HP) = 1.36
x Power in (kW)

Opening of bottom trawls

Bottom trawl with low vertical opening (VO)

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High-opening bottom trawl

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Shrimp trawl (flat or semi-balloon)

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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

Opening of bottom trawls and mid-water trawls

High-opening, 4-panel bottom trawl

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Single-boat mid-water trawl

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Mid-water pair trawl

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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)

Rigging of bottom trawl for one boat

Principal types, adjustments, relative length

Bottom trawls with low headline height

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Bottom trawls with high headline heights (OV): sweeps and bridles

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Adjustments

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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'**

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* 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

Rigging of bottom and midwater trawls for single-boat operation

High-opening bottom trawls : fork rigging

The length of warps equals 3 to 4.5 times the depth of water

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Single-boat midwater trawl

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* For power to use in calculation, see page 95
Power in (HP) = 1.36 x Power in (kW)

Rigging of pair trawling

Bottom trawls

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Midwater trawls

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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)

Estimating the depth of a midwater pair trawl

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

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The warp angle may be measured with a protractor or other device

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Depth of the trawl is estimated as follows :

  1. Measure the worp angle A
  2. On the horizontal scale of the graph below, find the warp length
  3. Follow the warp length down to the angle A
  4. Read the estimated trawl depth from the vertical scale at the left

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Another method without using a pro­tractor is shown below

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  1. Mark the warp 1 m aft of block
  2. Drop o vertica line from the block
  3. Measure the distance D
  4. Find the trawl depth in the table on the right
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

Shrimp (prawn) trawls and thier rigging

Gulf of Mexico type

Example:

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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

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(2)  Special rigging

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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.

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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)

Rigging between different parts of trawls

Bottom trawls

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Midwater trawls for 1 boat

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Midwater pair trawls

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Headline buoyancy and groundrope weight recommended for trawls

Real horsepower' hp

 

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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)

Examples of groundropes

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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

Spread of otter boards and trawl

Estimating the spread of otter boards (doors)

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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 :

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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 :

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Otter boards : proportions, angles of attack

Proportions of different types of otter boards

Flat rectangular otter boards

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Shrimp otter boards

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Angles of attack

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Rectangular V section otter boards

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Suberkrub pelagic otter boards

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Otter boards : angle of attack, adjustments

Angle of attack

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Adjustment of angle of attack

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Adjustment of orientation

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Problem

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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)

Otter boards : properties of the principal types, choice depending on the trawler's power

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)

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* Broke horsepower (BHP) or Apparent. Nominoi Power (ANP), see page 95
Power in HP = 1.36 x Power .n (kW)

Kites

Example, for a 25.5/34 trawl

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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

Warps : diameter and length

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.

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* Brake horsepower (BHP; or Apparent Nominal Power (ANP), .see page 95
Power in (HP) = 1.36 x Power in (kW)

Trawling speed

Main species groups

 Average trawling speed (knots)

shrimp, small bottom species, flat fish
very small trawlers
mid-sized and large trawlers

1.5-2
2.5-3.5

mid-sized bottom species, small pelagic fish
small trawlers
mid-sized to large trawlers

3-4
4-5

cephalopods (squid, cuttlefish)

3.5-4.5

mid-sized pelagic fish

>5

Power of trawlers

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).

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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 :

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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

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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.

Pulling power of trawlers

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),

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Choosing the appropriate engine speeds (RPM) for 2 boats of different characteristics for pair trawling

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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.

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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

ENTANGLING NETS

Plan and rigging of a gillent : example

Gillnet Vessel
bottom set for spider crabs
Brittany, France
length 5-15 m
HP 15-20

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 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

Choosing the meshsize of gillnets*

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).

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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

Choosing twine type for gillnets

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

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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                

Rigging or hanging gillnets

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

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On the footrope with sinkers attached

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Plan and rigging of trammel net

Trammel net*

Bottom set or drifting, for shrimp
Sri Lanka

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* For clarification of symbols used in drawing of entangling net, see page 97

Trammel nets : mesh sizes and rigging

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 :

FISHERMAN'S WORKBOOK

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:

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Average bouyancy (B) and ballast (W) of gillnets and trammel nets

Floating gillnets and trammel nets

 

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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 <
Length of floatline

(smaller or equal)

B1 ~ Wf + W1
Wf = weight of netting in water
 

Bottom set gillnets and trammel nets

 

FISHERMAN'S WORKBOOK

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 <
length of floatline

(greater or equal)

Note : These weights do not include anchors, etc.

Rigging of entangling nets : some examples

Set gillnets and trammel nets

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Drifting gillnets

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TRAPS AND POTS

Plan and rigging of pots : an example

Crab trap Vessel
Hokkaido, Japan
Nova Scotia, Canada
Length 12 - 15 m
hp 40 - 100

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Dimensions of pots and traps

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

Making fish traps and pots

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.

Entrances : shape and position

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)

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Traps for crustaceans : entrance(s) on the side(s) or on the top

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Entrances : dimensions

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

Examples of pots or traps

For fish or cephalopods

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For crustaceans

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LINE FISHING

Vertical line fishing : examples, breaking strength

A : Mainline

B . Branchline (also called snood, leader, gangion, drop line)

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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
(kg, wet, knotted)

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

Troling methods

Trolling speeds vary from 2 to 7 knots, depending on target species

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S : shock absorber or snubber
DP
 : depressor or diving boord
Pb
 : 'cannonball' weight

Trolling lines : rigging equipment

Shock absorber or snubber

Absorbs the shock load on the line when the fish strikes

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Depressor or diving board to troll deeper

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Shearing depressor or diving board

May be adjusted to dive and also shear horizontally to spread lines

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LONGLINES

Plan and rigging of bottom longlines : an example

Longline for dogfish, rays, conger, ling Boat
Channel, France Length 14-15 m
  TJB 2C - 30
  hp 150

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Longline components

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

FISHERMAN'S WORKBOOK

But, if one expects to catch individual fish weighing 10 kg, it s necessary to calculate

FISHERMAN'S WORKBOOK

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.

Set longlines : various rigs in use

Semi-pelagic longlines

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Bottom longlines

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Drifting lonlines : various ring in use

Some examples :

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Longlines : automation of operations

TYPES Longline with fixed snoods

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Longline with detachable snoods

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STOWAGE ON BOARD

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Mainline

Snoods for (or books)

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SHOOTING EQUIPMENT Baiting machine 

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HAULING EQUIPMENT

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NETS, TRAPS, LINES

Marking bouys and anchors; for nets, traps and lines

On the surface

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On the bottom

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Some types of anchors

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DREDGES

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

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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

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