52 FISHES AND FISHERY METHODS 



the sea bed. The trawl (Figure 4.4D) is a conical-shaped net which has 

 a wide mouth and tapers to a sock-like end which is referred to as the 

 ''cod end." Trawls may be subdivided into several categories depending 

 on the method used to spread the net. In the United States two trawling 

 techniques are used, the beam trawl and the otter trawl. The beam trawl 

 which is used in the Alaska and California shrimp fisheries uses a long, 

 tapered wooden beam to spread the mouth of the net. To the ends of the 

 beam "U"- or ''D"-shaped runners are attached. The net is secured to 

 the beam and runners. Unlike most trawls, the net is not always tapered 

 to a cod end. 



The mouth of the otter trawl is opened by boards (doors) which are 

 kite-like devices which shear in a horizontal direction. In the United 

 States two different techniques are used in operating otter-trawl gear. 

 On the East Coast the towing davits are arranged on the same side of 

 the fishing vessel and the net is shot and hauled from the side of the 

 vessel. On the West Coast small multi-purpose vessels, when engaged in 

 trawling, have a davit on each side of the stern. The net and boards are 

 shot or set over the stern and retrieved over the starboard side aft. 



Bull trawling or two-boat trawling is another method used to tow large 

 trawls. One vessel has the towing warp attached to one wing of the net, 

 while the other vessel has the towing warp attached to the other wing. 

 This technique was used off California during the development of the 

 early trawl fishery, but it has now been replaced by otter trawls. Two- 

 boat trawling, however, is still extensively carried on by the Japanese. 



Trawling is conducted in the shallow seas and on the continental shelf 

 and slope to depths of about 500 fathoms. 



Midwater trawling was initiated in commercial fisheries after World 

 War II. An excellent review of midwater trawls has been made by 

 Parrish'^. The midwater trawl has the same sock-like shape as otter trawls. 

 The trawl is generally cut so that the net is square in shape; that is, the 

 side, top, and bottom of the net have similar shapes and dimensions. 

 Various techniques are employed to open the mouth of the net, including 

 two-boat or pair trawling, especially designed boards, depressors, and 

 kites. These nets are used to harvest fish when they school in "mid 

 depths" and have been effective for harvesting herring-like and cod-Hke 

 fishes. 



Driftnets, gillnets (Figure 4.4B), tanglenets, and trammel nets are merely 

 vertical walls of \veb})ing which are normally set out in a straight line. 

 The capture of fish is based on their fortuitous encounter with the nets. 

 Driftnets may be set at the surface or fished off the bottom (diver nets) 

 or at intermediate depths. The top of the net is buoyed with floats and 

 the bottom weighted with leads so as to keep the net in an upright posi- 



