| Fig. 16.—Conical Dredge being hoisted in. |
Triangular and Conical Dredges.—Two other dredges are worthy of mention. The triangular dredge, much resembling Müller’s but with a triangular mouth, and hung by chains from its angles, is an old fashion now not in general use. It is, however, very useful for rocky ground. At the Plymouth marine laboratory was also devised the conical dredge (1901), the circular form being the suggestion of Garstang. This dredge (fig. 16) was intended for digging deeply. It is of wrought iron, and of the following dimensions: diameter of mouth 16 in., length 33 in., depth of ring at mouth 9 in. Its weight is 67 ℔. As at first used the spaces between the bars are closed by wire netting; if used for collecting bottom samples it is furnished with a lining of strong sail-cloth.
Its weight and the small length of edge in contact with the ground cause this dredge to dig well, and enable the user to obtain many objects which though quite common are of rare occurrence in an ordinary dredge. Thus on the Brown Ridges, a fishing-ground west of Holland, although Donax vittalus is known from examination of fish stomachs to be abundant, it is rarely taken except in the conical dredge: the same is true of Echinocyamus pusillus, which is in many parts of the North Sea abundant in bottom samples and in no ordinary dredgings. With the sail-cloth lining the conical dredge fills in about 10 minutes on most ground, and no material washing out of fine sediment occurs on hauling. In shallow seas such as the North Sea commercial beam and other trawls are now used as quantitative instruments in the estimation of the fish population, especially of the Pleuronectidae.
Use of Small Trawls for Dredging.—Although these trawls do not here concern us, certain adaptations of small beam trawls for biological exploration are of such identical use with the dredge, and differ from it so little in structure and size, that they may be here described.
A small beam trawl was first used from the “Challenger” (fig. 17). It was sent down in 600 fathoms off Cape St Vincent, the reason for its use being the frequency with which the dredge sank into the sea-bottom and there remained until hauling. The experiment was entirely successful. The sinking of the net was avoided, the net had a much greater spread than the dredge, and in addition to invertebrates it captured several fish. After this the trawl was frequently used instead of the dredge. Indeed tangle bar, dredge and trawl form a series which are fitted for use on the roughest, moderately rough and fairly firm, and the softest ground respectively, although the dredge can be used almost anywhere.
| From Sir Charles Wyville
Thomson’s Voyage of the
“Challenger.” By permission
of Macmillan & Co., Ltd. Fig. 17.—Trawl of the “Challenger.” |
The frame of the “Challenger” trawl consisted of a 15 ft. wooden beam which in use was drawn over the sea-bed on two runners resembling those of a sledge, by means of two ropes or bridles attached to eyes in the front of the runners or “trawl heads.” A net 30 ft. long was suspended by one side to the beam by half-a-dozen stops. The remainder of the net’s mouth was of much greater length than the beam, and was weighted with close-set rolls of sheet lead; it thus dragged along the bottom in a curve approximately to a semicircle, behind the beam. The net tapers towards the hinder end, and contains a second net with open bottom, which, reaching about three-quarters of the way down the main net, acts as a valve or pocket. Both heels (or hinder ends) of the trawl heads and the tail of the net were weighted to assist the net in digging sufficiently and to maintain its balance—an important point, since if the trawl lands on its beam the net’s mouth remains closed, and nothing is caught.
The main differences of this trawl from the dredge are the replacement of scraping lip by ground rope, the position of this ground rope and the greater size of the mouth. The absence of a lip makes it less effective for burrowing and sessile creatures, but the weighted ground rope nevertheless secures them to a very surprising extent. The position of the ground rope is an important feature, as any free swimming creature not disturbed until the arrival of the ground rope cannot escape by simply rising or “striking” up. This and the greater spread make the trawl especially suitable for the collection of fishes and other swiftly moving animals. The first haul of the “Challenger” trawl brought up fishes, and most of our knowledge of fish of the greatest depths is due to it.
A tendency to return to the use of the small beam trawl for deep-sea work has lately shown itself. That used by Tanner on the “Albatross” has runners more heart-shaped than the “Challenger’s” instrument; the net is fastened to the downward and backward sloping edge of the runner as well as to the beam, being thus fixed on three sides instead of one; and a Norwegian glass float is fastened in a network cover to that part of the net which is above and in front of the ground rope in use, to assist in keeping the opening clear. These floats can stand the pressure at great depths, and do not become waterlogged as do cork floats. The largest “Albatross” trawl has a beam 11 ft. long, runners 2 ft. 5 in. high, and its frame weighs 275 ℔.
Agassiz or Blake Trawl.—This is generally considered to possess advantages over the preceding, and is decidedly better for those not experts in trawling. Its frame (fig. 18) consists of two iron runners each the shape of a capital letter D, joined by iron rods or pipes which connect the middle of each stroke with the corresponding point on the other letter. The net is a tapering one, its mouth being a strong rope bound with finer rope for protection till the whole reaches a thickness of some 2 in. It is fastened to the frame at four points only, the ends of the curved rods, and thus has a rectangular opening.