The following text is about the best explanation of how Dragging or “Trawling” works in layman’s terms that I’ve run across. It comes from Oregon State University and the credits to the writers will be included at the bottom of the post. Read the explanation of trawling and then look at the pictures I took this morning with the titles of each thing they are talking about so you can visualize what they are saying. Even though this is from Oregon, our fishermen fish the same way only for different species.
A trawler is a vessel that drags a funnel-shaped net through water to harvest fish or shrimp. The net is wide at the mouth and tapers back to a narrow cod end that collects the catch. The average bottom trawl opening is 40 to 60 feet wide and 8 to 10 feet tall. Bottom trawlers usually tow their nets at 1 to 2 knots on or above the ocean floor. Fishermen might tow midwater trawls faster to catch faster-swimming schooling fish.
Trawlers have a large metal trawl door that is attached to each side, or wing, on the front of the net. The water hits the doors and the pressure of the water passing over the door spreads the net open. The doors are flat, oval, or slightly v-shaped. A steel cable extends from the door to a winch just behind the pilot house. Most large trawlers have square sterns with inclined ramps and are referred to as stern trawlers. The nets are hauled aboard up the inclined Boom Bottom ramp on the stern. Older trawlers without inclined ramps haul their nets over the sides using a haul line and a block on an overhead boom to bring in the cod end of the net.
Bottom trawlers tow the net along the ocean floor to catch fish that live on or just off the bottom. These fish include rockfish, cod, sablefish (black cod), ocean perch, flounder, and sole. Trawls can be designed to catch particular groups of fish. A large mesh net (4 1/2 inches to 5 inches) is kept on a stern-mounted reel. The two doors are stored along the rails near the reel.
The net is set off the stern by unwinding the reel so that the cod end is put into the water first. The rest of the net is unrolled from the reel, and then the doors are placed in the water. Water pressure on the doors causes the doors to separate and open the net. Enough cable is then released to place the net at the desired depth. The upper lip of the net is lifted up by floats on the headrope while the lower lip of the net is pulled down by a weighted footrope. This action opens the net vertically.
Rubber discs may be attached to the net to hold it down. There are now restrictions on the size of the rubber discs that can be used on footropes when trawling on the Oregon continental shelf. These restrictions confine trawling to mostly smooth bottoms, such as sand and mud. Tow times can last from 30 minutes to several hours. Depths can range from 5 to 700 fathoms (a fathom equals six feet). Bottom trawlers typically fish from 1 to 40 miles offshore.
The crew hauls in the net by winching in the cables until the doors are back in place and most of the net is on the reel. Once the catch is on board, the net is reset for another tow. Then the fish are separated into deck bins (checkers) and put in the hold, where they are iced or refrigerated.
The Oregon State Research Report Can Be Found Here
Writers: Ginny Goblirsch and Steve Theberge
Consultant: Scott McMullen
Artist: Herb Goblirsch
Editor: Sandy Ridlington
Design: Rick Cooper