Conditions of life in the sea : a short account of quantitative marine biological research / by James Johnstone.

  • Johnstone, James, 1870-1932.
Date:
1908
Catalogue details

Licence: In copyright

Credit: Conditions of life in the sea : a short account of quantitative marine biological research / by James Johnstone. Source: Wellcome Collection.

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    and the beam is about two feet from the ground. Behind the beam and fastened to it and the irons is the ''headline/’ and connecting the lower parts of the irons is the “ foot-rope,” a rope which is about three inches in diameter, which is often weighted with iron chain, and which drags on the sea bottom, forming a wide bight behind the beam. To the foot-rope and headline is “bent” the trawl net. This is a long conical bag of strong netting, the mesh of which varies with the size and nature of the animals which the trawl is designed to catch. The length of the net varies but it is always one and a half times or more than the length of the beam. About half-way between the foot-rope and the end, or “ cod end ” of the net, are two folds of netting attached to the back and belly of the trawl net and inclined backwards towards the middle line of the latter but not meeting. These are the “pockets,” and they form a funnel-shaped trap through which the animals caught by the net pass into the tail and through which they find difficulty in passing out again on account of the eddies caused within the net by the water which does not pass through the meshes. The cod end is open but is laced by a rope when the net is put down. From each of the irons a strong rope, the “ bridle,” passes forward and the ends of the bridles are “ shackled ” on to the trawl warp or rope. In the largest trawls used by fishing steamers the beam is discarded and the ends of the net are kept apart by two otter boards,” which are heavy wooden framed boards shod with iron and about the size of an ordinary house door. To these the headline and foot-rope are attached. The otter boards are so fastened to the two separate trawl warps, by means of which the trawl is towed, that they are inclined at an angle to the direction of tow of the apparatus, and thus the mouth of the net is kept open. In the largest otter trawls the length of the headline may be as much as 100 feet, the foot-rope being half as much again. For scientific fishing, that is fishing with a scientific object, a “ shrimp trawl ” is usually more suitable than the ordinary wide meshed fish trawl. In the latter the meshes are usually about one to one and a half inches from knot to knot, while in the shrimp net the distance is only about half an inch. Otherwise