Collection of Nekton
In commercial fishing, diverse methods involving nets, trawls, traps, hooks, and harpoons are used, depending upon the animals sought, but we shall here consider only the trawl, which has been much used in collecting deep-water animals for scientific research. The beam trawl (fig. 89c) is constructed somewhat like the dredge, but the frame does not form digging and scraping edge and it may have a much larger opening, up to 15 m or more; since it is not designed to dig into the bottom, it may be towed at greater speed and thus catch the faster moving animals—shrimps, fishes, and so forth—that live on or near the bottom.
For pelagic trawling the otter trawl is used mostly. The opening to the web sack is kept distended, not by a rigid beam, as in the beam trawl, but by means of otter boards attached to opposite sides of the opening. Upon being towed, these boards are forced apart by the resistance of the water (fig. 89d). The span of the opening may be 20 to 26 m and the net may be 40 m long. A small otter trawl was employed successfully by the Michael Sars in fishing at depths as great as 5160 m.
The ring trawl is essentially a large, relatively coarse plankton net attached to a strong ring of large diameter and provided with a towing bridle (see below).