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Resource Assessment and Conservation Engineering

Field Videos

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FIELD GEAR

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Nets and Codends

Video clips:   1 |  2 |  3 |  4 |  5 |  6

 

The first four clips here show the Poly Nor'eastern trawl net, a standard net that is commonly used in RACE field operations. Clips 1 and 2 show net deployment, while clips 3 and 4 show net retrieval with a rather small catch. The high-opening Poly Nor'eastern is used in Gulf of Alaska, Aleutian Islands, and Pacific West Coast shelf surveys. The net is constructed with polyethylene mesh and is outfitted with different footrope types specific to seafloor characteristics. The net is built and rigged with a 27.2 meter headrope and a 37.4 meter footrope. Floats along the headrope hold the net open vertically. The net has 127mm stretched mesh (which is orange in these clips) and the codend is 89mm stretched mesh with a 32mm stretched mesh codend liner (which is usually black or green). The codend and its liner, shown in clips 5 and 6, is essentially the business end of the net proper. Fish collect in the codend and are emptied on a sorting table, or on the trawl alley of the vessel, after a quick undoing of the zipper knot.

 

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Trawl Doors

Video clips:   1 |  2 |  3

 

These three clips are footage of trawl doors. Clips 1 and 2 show the doors during net deployment, and clip 3 is underwater footage of a door planing through abyssal waters. Trawl doors are like heavy underwater kites that spread the trawl net open during the tow. There are two sizes of steel V-doors used in groundfish surveys. These clips show the standard 6 X 9 foot steel V-door used on the upper continental slope survey. This door has a 4-point bridle on its backside made with 13mm link chain, having 33 links forward and 22 links aft in both the top and bottom, and is fished at 2 knots. The Gulf of Alaska and Aleutian Islands surveys also use the 6 X 9 foot doors but require a 2-point bridle because the nets are towed at a faster speed (3 knots). The second size trawl door, the 5 X 7 foot door, is used only on the West Coast shelf survey.

 

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Ship Winches

Video clips:   1 |  2 |  3

 

These clips provide footage of the winches used to deploy and retrieve trawl nets. Winches are powerful hydraulic spools, fixed to the vessel's deck, and wound with 2,200 meters of cable for trawling to depths of 1,300 meters, accounting for trawl angle. Over 1 mile of cable can be let out for deep tows. Tension on this system during a trawl is tremendous due to the 14,000 pounds of cable each winch holds, plus the drag of a potentially full net and trawl doors.

 
» Trawl Sweep Modifications Video: All clips
  Scientists with the AFSC's Midwater Assessment and Conservation Engineering (MACE) program compared the effects of conventional and modified sweeps (herding cables ahead of the trawl net) on sessile invertebrates. These videos were taken at four study sites on the eastern Bering Sea shelf. A seafloor sled with both sonar and video sensors was then towed across the parallel trawl tracks at several points to compare the condition of seafloor animals in areas affected by these different gears.

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