This diagram shows how an otter trawl works, with towing lines (or warps) from the trawler going to the otter boards or doors, which hold the net open. The net drags along the bottom when catching ground fish, and the caught fish collect in the cod end of the net. The net is hauled back to the trawler every few hours and emptied.
Otter trawls can be used for midwater species, and sometimes two ship share the towing in what is called pair trawling.
Shore herring weir near Eastport, Maine. This is the common form of brush weir. Poles driven into the bottom and interlaced with brush guide the herring into a trap from which they can be netted. This image is from G. Brown Goode's The Fisheries and Fishery Industries of the United States, 1884-1887, Section V, Plate 128. The book can be found on line at: http://celebrating200years.noaa.gov/rarebooks/fisheries/welcome.html
Seine-boat and crew "pursing the seine." The net has been rowed around the mackerel school to entrap it, and now the fishermen are hauling it in. The dory on the left pulled the seine around the school while the net was fed out.
Once they have the seine pursed up, the schooner will come alongside, hoist the net further, and dip out the mackerel.
Diagram showing the different sections of a purse-seine. The seine is spread around the mackerel school by a boat. These seine boats were rowed before internal combustion engines, and are now powered. Purse seines are used for mackerel and herring.
Norwegian method of setting the nets at the bottom. 1, Nets. 2, Large stones used for moorings. 3, Buoy. 4, Buoy-line. 5, Glass floats attached to buoy-line. 6, Watch-floats. In 1885 this was new technology in the American ground fishery which was still based on hook and line fishing.
Dory and crew setting cod trawl-lines on the Bank. From the latter half of the 19th century to the 1920s, this was the most productive form of hook and line ground fishing, finally superseded by power vessels towing otter trawl nets.
Long lines are made up in tubs with baited hooks on short lines called snoods attached every six feet. The trawl line would be marked by buoys at each end and anchored to the bottom. They could be half a mile long. After setting trawls the dory fisherman would run down their lines taking off fish.
Trawl tub with line and two trawl anchors. When the line is stowed properly, the hooks hang over the top edge of the tub, so that they do not get entangled with the line. Trawl line is tarred cotton line, often about 1/4" in diameter. Gangings, or leaders, are spaced every fathom, each with a hook at the end. Tubs were often made from barrels cut in half.
Hand-line dory cod fishing on the Grand Bank. The fishing schooner would anchor and launch a fleet of dories, which let the fishermen spread out from the ship. A fisherman could tend several handlines.