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|Herring Harvest: Mid-Water Trawling|
Mid-water (pelagic) trawling is the process of deploying and towing a net at a chosen depth in the water column to catch schooling fish such as herring and mackerel. This differs from "bottom" (benthic) trawling in which a net is dragged along the ocean bottom where fish such as cod, haddock, and flounders live.
Large mesh, typically 1.6m on the east coast of North America, is used at the front end of mid-water trawl nets. This allows for very large net openings compared to wide, but vertically narrow bottom trawls. The large front end of the net "herds" schooling fish toward the back end where they become trapped in the narrow "brailer." Independently swimming fish leave the trawl net at will.
To set the trawl, net is unrolled from a "net reel" until completely in the water. The net is held open along the bottom with 1000 to 5000 pounds of "wing-tip" weights and "foot chain" under the "foot rope" which connects the wing tips. The sides of the deployed net are spread horizontally with two large metal foils, called "doors," positioned in front of the net. As the trawler moves forward, the doors, and therefore the net, are forced outward. Alternatively, two vessels working together as "pair trawlers" can pull a single net between them, enabling the use of a large net due to reduced drag from not using doors to keep the net open.
Once off the reel, the net is attached by cable to winches on each side of the ship. Net depth and position are controlled using both speed of the boat and amount of wire released. A good analogy is flying a kite where windspeed and line released determine height. However, the captain of a trawler cannot see what's happening in the net hundreds of meters behind and below the vessel. They must rely on an array of sophisticated electronics, such as sonar units attached to the net, to relay information about the net's shape, how the fish are schooling, and how many fish have become entrapped in the net.
When the captain determines its time to "haul back" (as little as 10 minutes or as much as 8 hours after setting the net), the long tow cables are winched in and the net is spooled onto a large "net reel." A pump is attached to the end of the net and fish are pumped into a dewatering box and into holds below deck. A few trawlers bring the full net along one side then hoist small sections of the net for release on deck and into holds. This process is repeated until the net is emptied.
Mid-Water Trawl Photographs from the Gulf of Maine Fishery