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

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MSC 134 Fishing Gear Technology II Gill Nets: Concepts and Design Slide 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 1-inch square mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7 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.02(.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 Fishermans Workbook, Pg. 98 or by referring to the handout Entangling Nets, Gillnets and Seine Nets. Slide 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) OM = L (fish) / K OM = mesh opening (mm) 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 * Fishermans Workbook compiled by J. Prado, Fishing News Books, Oxford, England:1990. Pg. 98 Slide 4 OM = 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 from your text to decide on a suitable mesh size. Mesh Opening = 12 3.5 = 3.42 (3) Examples: Slide 5 Mesh selection factor based on known values obtained by local sampling experience and/or historical data. Mo = L / Km (L Km) If a gill net with the mesh size M 1 has caught fish of the length L 1 and a gill net with the mesh size M 2 catches fish of the length L 2, then: L 2 / L 1 = M 2 / M 1 (L 2 L 1 =M 2 M 1 ) Therefore, L 2 = L 1 x M 2 M 1 and M 2 = M 1 x L 2 L 1 The numerical value of the ratio L 1 M 1 = Km The Km is specific for each species of fish. Net selectivity data includes the hanging ratios, twine material and diameter, and the ballast and floatation specifications. File the net selectivity data and mesh selection factors for future reference. Slide 6 MO = L / Km L 2 / L 1 = M 2 / M 1 (L 2 L 1 =M 2 M 1 ) Therefore, L 2 = L 1 x M 2 M 1 and M 2 = M 1 x L 2 L 1 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 from your handout and refer to historical data from your sampling area to decide on a suitable mesh size and net design. 12.2 / 10.3 = 3.5 / 3 Therefore L 2 = L 1 x M 2 M 1 L 2 = 10.3 x 3.5 3 L 2 = 10.3 x 1.167 L 2 = 12.02 (12) Examples: OR MO = L / Km L 1 M 1 = Km 10.3 3 = 3.43 12.2 3.5 = 3.49 Therefore average Km = 3.46 So - MO = 12/3.46 MO = 3.47 (3) L 2 / L 1 = M 2 / M 1 12.2 / 10.3 = 3.5 / 3 Therefore M 2 = M 1 x L 2 L 1 M 2 = 3 x 12.2 10.3 M 2 = 3 x 1.185 M 2 = 3.56 (3) Slide 7 Examples: If asked to design and build a gillnet capable of catching a quantity of Bluefish ( Pomatomus saltatrix ) with an average length of: MO = L / Km 14 inches then MO = 14/3.46 so the mesh opening = 4.04 (4) 16 inches then MO = 16/3.46 so the mesh opening = 4.62 (4-4) 18 inches then MO = 18/3.46 so the mesh opening = 5.20 (5 -5 ) 10 inches then MO = 10/3.46 so the mesh opening = 2.89 (2-3) 8 inches then MO = 8/3.46 so the mesh opening = 2.31 (2) Remember: The Km is specific for each species of fish. Net selectivity data includes the hanging ratios, twine material and diameter, as well as the ballast and floatation specifications. File the net selectivity data and mesh selection factors for future reference. Slide 8 A spread-sheet designed to file your net selectivity data and mesh selection factors may look similar to the example shown below. Slide 9 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 1-inch square mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7 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.02(.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? Slide 10 Calculating the Twine Ratio When selecting a twine diameter for gillnets, the diameter should be proportional to the mesh size. The ratio should be between 0.0025 and 0.01..0025 for calm waters and/or low catches..01 for rough waters and/or bottom set nets. An average ratio is 0.005. * The twine ratio can be calculated as shown in the Fishermans Workbook, Pg. 99 or by referring to the handout Entangling Nets, Gillnets and Seine Nets. Ratio = Twine Diameter Stretch Mesh Size (always use the same units of measurement to compare the two variables) Twine Diameter =.40mm and.40mm =.015748 in. (1mm 25.4mm =.03937 so.40mm =.03937in. x.40mm =.015748 inches) Stretch Mesh Size = 3 in. and 3 in. = 88.9mm (1in. = 25.4mm so 3 in. = 3 in x 25.4mm =88.9mm).40mm 88.9mm =.0044994 =.005 Or.015748in. 3.5in. =.0044994 =.005 Slide 11 Exactly how much rope do I need to complete this net? 935 Feet (920 to 950) Slide 12 Exactly how much rope do I need to complete this net? How much Classic hollow braided polypropylene rope would you order? What would the rope cost? 935 Feet (920 to 950) One 1200 reel cost $24.70. 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? 1200 Feet $26.00 (Discretionary Decision Why?) Slide 13 How many meshes deep should I order the webbing? E 1 = L/Lo. E 1 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. E 2 = H/Ho. E 2 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 E 2 Slide 14 How many meshes deep should I order the webbing? 1 Mesh = 1 Square = 3 Stretch E 1 = 50% (.50) therefore E2 = 87% (.87)** Finished Gill Net = Approximately 7 feet deep. ** Refer to Fishermans Workbook, pg.38; the handout Introduction to Netting Theoryor use the formula E 2 = 1 E 1 E 2 = H Ho therefore H = E 2 x Ho and Ho = H E 2 The answer can be found by mathematically using the known variables and then referring to the materials available in our catalog. Slide 15 How many meshes deep should I order the webbing? 30 Meshes Deep 1 Mesh = 3 Stretch . E2 = 87% (.87)*. 7 feet deep. E 2 = H Ho therefore H = E 2 x Ho and Ho = H E 2 H =.87 x 3.5 therefore the hung height of one mesh = 3.045 and 7.5 ft. x 12 in. = 90 inches so 90 3.045 = 29.557 Meshes obviously we will look for 30 Meshes Deep in our catalog. Proof: 30 meshes x 3.5 = 105 str.in. H = E 2 x Ho so H =.87 x 105 = 91.35 12 = 7.61 ft. E 2 = H Ho so E 2 = 91.35 105 =.87 (87%) Ho = H E 2 so Ho = 91.35 .87 = 105 Slide 16 Slide 17 How many pounds of webbing will I need to complete this net? 7.2 Pounds E1 = L Lo 150 yd. Net x 3 ft. = 450 foot long Net 125 stretch feet per pound (Stk. #N221A) 125 str. ft.(Lo) x.50 (E 1 ) = 62.5 hung Ft./Lb. (L) 450 ft. net 62.5 hung Ft./Lb. = 7.2 Pounds Slide 18 What would the total cost of this webbing be? How many pounds of webbing would you order? 8 Pounds $70.80 5 thru 24 Lbs. = $8.85 Lb. therefore 8 Lb. x $8.85 = $70.80 Cant order partial poundage unless ordering a hank. The catalog presents a hank as the standard most people would base their gill net calculations with: A 100 yard net hung on a half would require 4.8 pounds of stk. # N221A webbing. 125 str.ft.(Lo) x.50 (E1) = 62.5 hung ft./lb. (L) 300 ft. net 62.5 hung ft./lb. = 4.8 Pounds How many feet of repair material would you have left if you order 8 pounds of webbing?.8 lbs. x 62.5 hung ft./lb. = 50 hung feet for repairs. Slide 19 When hanging the webbing to the headrope and footrope, how many meshes will you insert into every pickup? When hanging the webbing to the headrope and footrope, what will the distances of your pickups be? Options: 1 mesh = 3.5 str. E 1 =.50 or 50% L =.50 x 3.5 = 1.75 per mesh 2 meshes = 3.5 3 meshes = 5.25 4 meshes = 7 Refer to standards discuss in the handout Entangling Nets, Gillnets & Seine Nets Slide 20 Complete a schematic of the gill net showing a one unit section representing the distance and number of pickups between floats and the number of pickups between leads. *Complete the schematic in detail, all answers concerning floats and leads are based on this schematic. Unit Size? Pickup Size? Meshes per Pickup? Floatation per Unit? # of Leads per Unit? Ballast per Unit? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Label Pickup Size Label Unit Size Show Lead Placement Show Float Placement # of meshes Gill Net Schematics Slide 21 Complete a schematic of the gill net showing a one unit section representing the distance and number of pickups between floats and the number of pickups between leads. *Complete the schematic in detail, all answers concerning floats and leads are based on this schematic. Unit Size? Pickup Size? 7 Meshes per Pickup? 4 Floatation per Unit? # of Leads per Unit? 2 Ballast per Unit? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 42 Unit 7 Option 1: 4 meshes @ 7 inches Units: This net is designed to be 7 (90) deep. 50% of 90= 45 which we have defined a

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