During fishing the cod end is closed by the cod line, but at the conclusion of the trawl the net is hoisted aboard, mouth upwards, and the contents are discharged upon the deck by drawing the cod line. The otter trawl that is used by modern steam trawlers is from seventy to one hundred and twenty feet wide across the mouth, according to the character of the fishing, and a hundred and ten feet long from the mouth to the cod end. The otter trawl is shown in Fig. 17. It differs from the beam trawl in that its mouth is kept open, not by being attached to a beam, but by otter boards, which are attached one to each side of the mouth of the net. These are attached to the net and to the warps by which the net is towed in such a way that the pressure of the water upon them causes them to diverge, thus keeping the mouth of the net open. The size of a beam trawl is necessarily limited by the length of beam obtainable. The size of the otter trawl, however, is obviously only limited by the power of the steam trawler. The otter boards measure 11 ft. by 4 ft. 6 ins., are shod with iron, and weigh 15 cwts. each. The warps, as the ropes are called which attach the otter boards to the ship, are from three hundred to a thousand fathoms long—generally a little over three times as long as the depth of the water in which the trawl is to be used. Each board is attached to the steamer by a separate warp. The upper edge of the mouth of the net is attached to a strong rope, called the “head” rope. The lower edge of the mouth of the net is also attached to a strong rope, called the “foot” rope. As in the beam trawl, the foot rope is considerably longer than the head line, and forms a bosom. Traps and pockets also are inserted in the sides of the net. When trawling on rough ground, the foot rope is furnished with large, heavy, wooden rollers, called the “bobbins.” Trawl fishing, until quite recently, was almost entirely confined to demersal fish, such as cod, plaice, haddock and halibut. In recent years, however, considerable quantities of herring have been caught by trawlers. [Illustration: FIG. 10 DRIFTING (_circa_ 1750)] =Drifting.= The drift net is essentially a completely submerged, vertical curtain of netting, one end of which is attached to a boat called a drifter. The net extends in a straight line from the boat, and may be as much as three miles long. Unlike the trawl net, the drift net generally catches one kind of fish only—either herring or mackerel—drift net fishing being carried on at a time when these fish come together in shoals near the surface for the purpose of spawning. The trawl obviously only captures fish living at the bottom. At the same time, of course, it captures all the fish at the bottom, whether immature, or useless star fish, etc. The drift net, on the other hand, is generally used for a particular kind of fish—herring, mackerel, sprat—and only catches fish above a certain size.