MSC 134 Fishing Gear Technology II. Gill Nets: Concepts and Design

Transcription

1 MSC 134 Fishing Gear Technology II Gill Nets: Concepts and Design

2 Gill Net Design: Class Example We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch square mesh. We would like to fish the net on the bottom and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a Sponge SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately 0.02(0.0163) ounces per foot of rope. We will also consider the weight of the webbing in our design calculations. How did we decide what mesh size to use? *The mesh size of gillnets can be calculated as shown in the Fisherman s Workbook, Pg. 98 or by referring to the handout Entangling Nets, Gillnets and Seine Nets.

3 There is a ratio between the body girth or the length of a fish one wants to catch, and the gillnet meshsize which will be effective for that fish. * Choice of Meshsize (Textbook, Pg. 98) MO = L (fish) / K MO = mesh opening (mm) - stretch L (fish) = average length (mm) of fish one wants to catch K = coefficient, according to species 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 Can you think of examples of fish for each K-factor? *Fisherman s Workbook compiled by J. Prado, Fishing News Books, Oxford, England:1990. Pg. 98

4 Example: MO = L (fish) / K 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 If asked to design and build a gillnet capable of catching a quantity of Bluefish (Pomatomus saltatrix) with an average length of 12 inches, use the formula to decide on a suitable mesh size. Mesh Opening = = 3.42 (3½ ) Alternatively, a mesh selection factor may be based on known values obtained by local sampling experience and/or historical data (see reading).

5 A spread-sheet designed to file your net selectivity data and mesh selection factors may look similar to the example shown below. Mesh Selection Factors Species Average Km Twine Hanging Stretch Gear Net Size (inches) Diameter Ratio Mesh Code Depth (ft.) Gear Descriptions Atlantic Menhaden 10.40" (6) mm 61% 3" 210 6' 10.56" (6) mm 60% 3" 213 7' 6.17" (2) mm 67% ' Atlantic Sharpnose Shark 17.4" (1) mm 60% 3.5" 213 7' 20.6" (2) mm 60% 4" 213 7' 16.10" (1) mm 60% 3.5" 213 7' Gill Net Unclassified:Length, mesh size,depth ' floating,drifting,mono. gill net with 3 panelsof varying mesh size ' floating, anchored, mono. Gill net with 3 panels of varying mesh size. 212-mono. Gill net, floating, anchored with variable length panels of variable mesh size. 213-experimental gill net. 220-monofilament gill net, sink, anchored 222-multifilament gill net, sink,anchored 230-monofilament gill net, floating, drifting 240-monofilament gill net, sink, drifting 241-monofilament gill net, sinking, run-around 242-multifilament gill net, sink, drifting 245- monofilament gill net, floating, anchored Bluefish 12.2" (4) mm 61% 3.5" 213 7' 11.6" (3) mm 50% 3" ' 10.3" (3) mm 61% 2.875" ' 11.44" (2) mm 59% ' Harvestfish 4.96" (1) mm 59% ' Ladyfish 18.31" (2) mm 60% 4" 213 7'

6 Mesh Selection Factors Species Average Km Twine Hanging Stretch Gear Net Size (inches) Diameter Ratio Mesh Code Depth (ft.) Atlantic Menhaden 10.40" (6) mm 61% 3" 210 6' 10.56" (6) mm 60% 3" 213 7' 6.17" (2) mm 67% ' Atlantic Sharpnose Shark 17.4" (1) mm 60% 3.5" 213 7' 20.6" (2) mm 60% 4" 213 7' 16.10" (1) mm 60% 3.5" 213 7' Bluefish 12.2" (4) mm 61% 3.5" 213 7' 11.6" (3) mm 50% 3" ' 10.3" (3) mm 61% 2.875" ' 11.44" (2) mm 59% ' Harvestfish 4.96" (1) mm 59% ' Ladyfish 18.31" (2) mm 60% 4" 213 7'

7 Gill Net Design: Class Example We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch square mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a Sponge SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately 0.02(0.0163) ounces per foot of rope. We will also consider the weight of the webbing in our design calculations. What is the twine ratio selected for this gillnet? (Why are we using #6 monofilament (0.40mm)?)

8 Calculating Twine Diameter & Twine Ratio When selecting a twine diameter for gillnets, the diameter should be proportional to the mesh size. The ratio should be between and for calm waters and/or low catches for rough waters and/or bottom set nets. Ratio = Twine Diameter Stretch Mesh Size Always use the same units of measurement to compare the two variables (twine diameter & mesh size) An good average ratio is *The twine ratio can be calculated as shown in the Fisherman s Workbook, Pg. 99 or by referring to the handout Entangling Nets, Gillnets and Seine Nets.

9 Using Twine Ratio to Calculate Twine Size Ratio = Twine Diameter Stretch Mesh Size When selecting a twine diameter for gillnets, the diameter should be proportional to the mesh size. Stretch Mesh Size = 3½ in. and 3½ in. = 88.9mm (1in. = 25.4mm, so: 3½ in. = 3½ in x 25.4mm =88.9mm) Twine Ratio = (average ratio) The ratio should be between and for calm waters and/or low catches for rough waters and/or bottom set nets. An average ratio is = Twine Diameter (mm) 88.9mm Twine Dia. (mm) = 88.9mm = 0.4mm (#6) OR Twine Ratio = 0.01 (bottom set net) 0.01 = Twine Diameter (mm) 88.9mm Twine Dia. (mm) = 88.9mm 0.01 = 0.9mm (#30) *The twine ratio can be calculated as shown in the Fisherman s Workbook, Pg. 99 or by referring to the handout Entangling Nets, Gillnets and Seine Nets.

10 Alternatively Calculating Twine Ratio Ratio = Twine Diameter Stretch Mesh Size (always use the same units of measurement to compare the two variables) If you have a complete net, but don t know the twine ratio for the net 0.40mm 88.9mm = = Or in. 3.5in. = = Twine Diameter = 0.40mm and 0.40mm = in. (1inch = 25.4mm, so: 0.40mm 25.4 = inches) Stretch Mesh Size = 3½ in. and 3½ in. = 88.9mm (1in. = 25.4mm, so: 3½ in. = 3½ in x 25.4mm =88.9mm) *The twine ratio can be calculated as shown in the Fisherman s Workbook, Pg. 99 or by referring to the handout Entangling Nets, Gillnets and Seine Nets.

11 Gill Net Design: Class Example We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a Sponge SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately 0.02(0.0163) ounces per foot of rope. We will also consider the weight of the webbing in our design calculations. How much rope is needed to construct this net? (And how much will you order?)

12 Exactly how much rope do I need to complete this net? 935 Feet Headrope = 450 feet + 5 feet tag (left) + 5 feet tag (right) = 460 feet Footrope = 450 feet + 5 feet tag (left) + 5 feet tag (right) = 460 feet Left Pull = 7.5 feet Right Pull = 7.5 feet

13 How much Classic hollow braided polypropylene rope would you order (3/16 )? 1200 Feet One 1200 reel cost $ Two 600 reels cost $26.00 ($13.00 ea.) Since we need two lengths of rope - is it worth $1.30 not to separate and measure the ropes, a job that would take approximately 30 to 60 minutes?

14 Gill Net Design: Class Example We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a Sponge SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately 0.02(0.0163) ounces per foot of rope. We will also consider the weight of the webbing in our design calculations. How much webbing should we order?

15 Gill Net Design: Class Example We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a Sponge SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately 0.02(0.0163) ounces per foot of rope. We will also consider the weight of the webbing in our design calculations. How much webbing should we order? How many meshes deep should the webbing be? AND How many pounds of webbing do we need?

16 How many meshes deep should I order the webbing? E1 = L/Lo. E1 is the Primary Hanging Ratio. L is the length of rope on which a net panel is hung. Lo is the length of stretched netting before hanging. E2 = H/Ho. E2 is the Secondary Hanging Ratio. H is the actual height of a hung net. Ho is the height of the stretched netting before hanging. Meshes Deep is a function of E2

17 How many meshes deep should I order the webbing? The answer can be found by mathematically using the known variables and then referring to the materials available in our catalog. KNOWN: 1 Mesh = 1¾ Square = 3½ Stretch E1 = 50% (0.50), therefore, E2 = 87% (0.87)** Finished Gill Net = Approximately 7½ feet deep. E2 = H Ho therefore H = E2 x Ho and Ho = H E2 **Refer to Fisherman s Workbook, pg.38; the handout Introduction to Netting Theory or use the formula E2 = 1 E1²

18 How many meshes deep should I order the webbing? 30 Meshes Deep 1 Mesh = 3½ Stretch. E2 = 87% (0.87)*. 7½ feet deep. E2 = H Ho therefore H = E2 x Ho and Ho = H E2 H = 0.87 x 3.5 therefore, the hung height of one mesh = and 7.5 ft. x 12 in. = 90 inches so, = Meshes. We will look for 30 Meshes Deep in our catalog. Proof: 30 meshes x 3.5 = 105 str.in. H = E2 x Ho so H = 0.87 x 105 = = 7.61 ft. E2 = H Ho so E2 = = 0.87 (87%) Ho = H E2 so Ho = = 105

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