The speed of towing, always slow, may be assumed to be approximately correct if the appropriate angle is maintained. Hauling should at first be slow from great depths, but may increase in speed as the gear rises.
For further details of deep-sea dredging, especially of the hauling machinery and management of the gear, the special reports of the various expeditions must be consulted. Commander Tanner, U.S.N., has given in Deep Sea Exploration (1897) a very full and good account of the equipment of an exploring ship; and to this book the present article is much indebted.
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| Fig. 15.—Deep-sea Dredge, with Tangle Bar. |
Modifications and Additions to the Dredge.—From 1818, when Sir John Ross brought up a fine Astrophyton from over 800 fathoms on a sounding line in Baffin’s Bay, instances gradually accumulated of specimens being obtained from great depths without nets or traps. The naturalists of the “Porcupine” and other expeditions found that echinoderms, corals and sponges were often carried up adhering to the outer surface of the dredge and the last few fathoms of dredge rope. In order to increase the effectiveness of this method of capture a bar was fastened to the bottom of the dredge, to which bunches of teased-out hemp were tied. In this way specimens of the greatest interest, and frequently of equal importance with those in the dredge bag, were obtained. The tangle bar was at first attached to the back of the net. From the “Challenger” expedition onward it has been fixed behind the net by iron bars stretching back from the short sides of the dredge frame which pass through eyes in their first ends (fig. 15). The swabs are thus unable to fold over the mouth of the dredge. Rope lashings to the lips of the dredge are sometimes added, and a weight is tied to the larger bar to keep it down.
Occasionally the tangle bar is used alone (Agassiz), and one form (Tanner) has two bars, stretching back like the side strokes of the letter A from a strong steel spring in the form of an almost complete circle. The whole is pulled forward from a spherical sinker fastened in front of the spring apex; and should the apex enter a crevice between rock masses, the side bars are closed by the pressure instead of catching and bringing up. This is said to be a very useful instrument among corals.
The Blake Dredge.—In the soft ooze which forms the bottom of deep seas the common dredge sinks and digs much too deeply for its ordinary purpose, owing partly to its chief weight bearing on the frame only, partly to its everted lips. To obviate these defects Lieutenant Commander Sigsbee of the “Blake” devised the Blake dredge. Its novel features were the frame and lips. The former was in the form of a skeleton box; that is, a rectangle of iron bars was placed at the back as well as the front or mouth of the net and four more iron bars connected the two rectangles. The lips instead of being everted were in parallel planes—those, namely, of the top and bottom of the net. The effect of this was to minimize digging and somewhat spread the incidences of the weight. Another advantage was that the net being constantly distended by its frame, and, moreover, protected top and bottom by an external shield of canvas, quite delicate specimens reached the surface uninjured. The dredge weighed 80 ℔ and was 4 ft. square and 9 in. deep.
Rake Dredges.—These are devices for collecting burrowing creatures without filling the dredge with the soil in which they live. Holt used, at Plymouth, a dredge whose side bars and lower lip were of iron, the latter armed with forward and downward pointing teeth which stirred up the sand and its denizens in front of the dredge mouth. The upper lip of the dredge was replaced by a bar of wood. The bag was of cheese-cloth or light open canvas, and the whole was of light construction. The apparatus was very useful in capturing small burrowing crustacea. The Chester rake dredge is a Blake dredge in front of which is secured a heavy iron rectangle with teeth placed almost at right angles to its long sides and in the plane of the rectangle. Each of these instruments has a width along the scraping edge of about 3 ft.
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| 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.
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| From Sir Charles Wyville Thomson’s Voyage of the “Challenger.” By permission of Macmillan & Co., Ltd. |
| Fig. 17.—Trawl of the “Challenger.” |
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.
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