The naturalist's dredge is an instrument consisting essentially of a net
or bag attached to a framework of iron which forms the mouth of the net.
When in use as the apparatus is drawn over the sea-bottom mouth
forwards, some part of the framework passes beneath objects which it
meets and so causes them to enter the net. It is intended for the
collection of animals and plants living on or near the sea-bottom, or
sometimes of specimens of the sea-bottom itself, for scientific
purposes.
Until the middle of the 18th century, naturalists who studied the marine
fauna and flora relied for their materials on shore collection and the
examination of the catches of fishing boats. Their knowledge of
creatures living below the level of low spring tides was thus gained
only from specimens cast up in storms, or caught by fishing gear
designed for the capture of certain edible species only. The first
effort made to free marine biology from these limitations was the use of
the dredge, which was built much on the plan of the oyster dredge.
[Illustration: FIG. 13.--Otho Frederick Muller's Dredge (1770).]
_The Oyster Dredge._--At first naturalists made use of the ordinary
oyster dredge, which is constructed as follows. The frame is an iron
triangle, the sides being the round iron "arms" of the dredge, the
base a flat bar called the shere or lip, which is sloped a little, not
perpendicular to the plane of the triangle; an iron bar parallel to
the base joins the arms. The net is fastened to the parallel bars and
the portion of the arms between them, and consists of two parts: that
attached to the shere is of round iron rings linked together by
smaller ones of wire lashings, that attached to the upper bar is of
ordinary network. Where these two portions of the bag meet a wooden
beam is fastened. In use the frame is towed forward by its apex: the
shere passes below oysters, &c., which pass back on to the iron
netting. The length of each side of the triangular frame is about 6
ft., the width of the shere 3 in. and the height of the mouth just
under a foot. The rings vary in size, but are usually some 2-1/2 in.
in diameter. The weight is about 60 lb. This dredge was soon
abandoned: its weight was prohibitive for small boats, from which the
naturalist usually worked, its wide rings allowed precious specimens
to fall through, and its shallow net favoured the washing out of light
objects on hauling through the moving water of the surface. Moreover,
it sometimes fell on its back and was then useless, although when the
apex or towing point was weighted no great skill is needed to avoid
this.
Otho Muller used a dredge (fig. 13) consisting of a net with a square
iron mouth, each of whose sides was furnished with a thin edge turned
slightly away from the dredge's centre. As any one of these everted
lips could act as a scraper it was a matter of indifference which
struck the bottom when the dredge was lowered. The chief defect of the
instrument was the ease with which light objects could be washed out
on hauling, owing to the size of the mouth. However, with this
instrument Muller obtained from the often stormy Scandinavian seas all
the material for his celebrated _Zoologia Danica_, a description of
the marine fauna of Denmark and Norway which was published with
excellent coloured plates in 1778; and historical interest attaches to
the dredge as the first made specially for scientific work.
_Ball's Dredge._--About 1838 a dredge devised by Dr Ball of Dublin was
introduced. It has been used all over the world, and is so apt for its
purpose that it has suffered very little modification during its 70
years of life. It is known as Ball's dredge or more generally simply
"the dredge."
[Illustration: FIG. 14.--Ball's Naturalist's Dredge.]
Ball's dredge (fig. 14) consists of a rectangular net attached to a
rectangular frame much longer than high, and furnished with rods
stretching from the four corners to meet at a point where they are
attached to the dredge rope. It differs from Muller's dredge in the
slit-like shape of the opening, which prevents much of the "washing
out" suffered by the earlier pattern, and in the edges. The long edges
only are fashioned as scrapers, being wider and heavier than Muller's,
especially in later dredges. The short edges are of round iron bar.
Like Muller's form, Ball's dredge will act whichever side touches the
bottom first, as its frame will not remain on its short edge, and
either of the long edges acts as a scraper. The scraping lips thicken
gradually from free edge to net; they are set at 110 deg. to the plane
of the mouth, and in some later patterns curve outwards instead of
merely sloping. All dredge frames are of wrought iron.
The thick inner edges of the scrapers are perforated by round holes at
distances of about an inch, and through these strong iron rings about
an inch in diameter are passed, and two or three similar rings run on
the short rods which form the ends of the dredge-frame. A light iron
rod, bent to the form of the dredge opening, usually runs through
these rings, and to this rod and to the rings the mouth of the
dredge-bag is securely attached by stout cord or strong copper wire.
Various materials have been used for the bag, the chief of which are
hide, canvas and netting. The hide was recommended by its strength,
but it is now abandoned. Canvas bags fill quickly with mud or sand and
then cease to operate: on the other hand wide mesh net fails to retain
small specimens. Probably the most suitable material is hand-made
netting of very strong twine, the meshes half an inch to the side, the
inter-spaces contracting to a third of an inch across when the twine
is thoroughly soaked, with an open canvas or "bread-bag" lining to the
last 6 in. of the net. A return to canvas covering has latterly
occurred in the small dredge called the mud-bag, trailed behind the
trawl of the "Albatross" for obtaining a sample of the bottom, and in
the conical dredge.
The dimensions of the first dredges were as follows: Frame about 12
in. by about 4 in.; scraping lips about 2 in. wide; all other iron
parts of round iron bar 5/8 in. diameter; bag rather more than 1 ft.
long. These small dredges were used from rowing boats. Larger dredges
were subsequently made for use from yawls or cutters. The mouth of
these was 18 by 5 in., the scraping lips about 2 in. wide and bag 2
ft. deep; such a dredge weighs about 20 lb. The dredge of the
"Challenger" had a frame 4 ft. 6 in. by 1 ft. 3 in. and the bag had a
length of 4 ft. 6 in.; the "Porcupine" used a dredge of the same size
weighing 225 lb. Doubtless the size of Ball's dredge would have
grown still more had it not been proved by the "Challenger" expedition
that for many purposes trawls could be used advantageously instead of
dredges.
_Operation of the Dredge from Small Vessels._ For work round the coasts
of Europe, at depths attainable from a row-boat or yawl, probably the
best kind of line is bolt-rope of the best Russian hemp, not less than
1-1/2 in. in circumference, containing 18 to 20 yarns in 3 strands. Each
yarn should be nearly a hundredweight, so that the breaking strain of
such a rope ought to be about a ton. Of course it is never voluntarily
exposed to such a strain, but in shallow water the dredge is often
caught among rocks or coral, and the rope should be strong enough in
such a case to bring up the boat, even if there were some little way on.
It is always well, when dredging, to ascertain the approximate depth
with the lead before casting the dredge; and the lead ought always to be
accompanied by a registering thermometer, for the subsequent haul of the
dredge will gain greatly in value as an observation in geographical
distribution, if it be accompanied by an accurate note of the bottom
temperature. For depths under 100 fathoms the amount of rope paid out
should be at least double the depth; under 30 fathoms, where one usually
works more rapidly, it should be more nearly three times; this gives a
good deal of slack before the dredge if the boat be moving very slowly,
and keeps the lip of the dredge well down. When there is anything of a
current, from whatever cause, it is usually convenient to attach a
weight, varying from 14 lb. to half a hundredweight, to the rope 3 or 4
fathoms in front of the dredge. This prevents in some degree the lifting
of the mouth of the dredge; if the weight be attached nearer the dredge
it is apt to injure delicate objects passing in.
In dredging in sand or mud, the dredge-rope may simply be passed through
the double eye formed by the ends of the two arms of the dredge-frame;
but in rocky or unknown ground it is better to fasten the rope to the
eye of one of the arms only, and to tie the two eyes together with three
or four turns of rope-yarn. This stop breaks much more readily than the
dredge-rope, so that if the dredge get caught it is the first thing to
give way under the strain, and in doing so it often alters the position
of the dredge so as to allow of its extrication.
The dredge is slipped gently over the side, either from the bow or from
the stern--in a small boat more usually the latter--while there is a
little way on, and the direction which the rope takes indicates roughly
whether the dredge is going down properly. When it reaches the ground
and begins to scrape, an experienced hand upon the rope can usually
detect at once a tremor given to the dredge by the scraper passing over
the irregularities of the bottom. The due amount of rope is then paid
out, and the rope hitched to a bench or rowlock-pin. The boat should
move very slowly, probably not faster than a mile an hour. In still
water or with a very slight current the dredge of course anchors the
boat, and oars or sails are necessary; but if the boat be moving at all
it is all that is required. It is perhaps most pleasant to dredge with
a close-reefed sail before a light wind, with weights, against a very
slight tide or current; but these are conditions which cannot be
commanded. The dredge may remain down from a quarter of an hour to
twenty minutes, by which time, if things go well, it ought to be fairly
filled. In dredging from a small boat the simplest plan is for two or
three men to haul in, hand over hand, and coil in the bottom of the
boat. For a large yawl or yacht, and for depths over 50 fathoms, a winch
is a great assistance. The rope takes a couple of turns round the winch,
which is worked by two men, while a third hand takes it from the winch
and coils it down.
It is easier to operate a dredge from a steam vessel than a sailing
boat, but if the steamer is of any size great care should be taken that
the dredge does not move too rapidly.
Two ingenious cases of dredging under unusual conditions are worthy of
mention, one case from shore, one from ice. In the Trondligem Fjord,
Canon A. M. Norman in 1890 worked by hauling the dredge up the
precipitous shores of the fjord. The dredge was shot from a boat close
to the shore, to which after paying out some hundreds of fathoms of line
it returned. The dredge was then hauled from the top of the cliffs up
whose side it scraped. Hitches against projecting rocks were frequent
and were overcome by suddenly paying out line for a time. The dredge was
lifted into a boat when it reached the surface of the sea. The other
case occurred during the Antarctic expedition of the "Discovery."
Hodgson dropped loops of line along cracks which occasionally formed in
the ice. The ice always joined up again, but with the line below it; and
a hole being cleared at each place at which the end of the line emerged,
the dredge could be worked between them.
The dredge comes up variously freighted according to the locality, and
the next step is to examine its contents and to store the objects of
search for future use. In a regularly organized dredging expedition a
frame or platform is often erected with a ledge round it to receive the
contents of the dredge, but it does well enough to capsize it on an old
piece of tarpaulin. There are two ways of emptying the dredge; we may
either turn it up and pour out its contents by the mouth, or we may have
a contrivance by which the bottom of the bag is made to unlace. The
first plan is the simpler and the one more usually adopted; the second
has the advantage of letting the mass slide out more smoothly and
easily, but the lacing introduces rather a damaging complication, as it
is apt to loosen or give way. Any objects visible on the surface of the
heap are now carefully removed, and placed for identification in jars or
tubs of sea-water, of which there should be a number secured in some
form of bottle basket, standing ready. The heap should not be much
disturbed, for the delicate objects contained in it have already been
unavoidably subjected to a good deal of rough usage, and the less
friction among the stones the better.
_Examination of the Catch. Sifting._--The sorting of the catch is
facilitated by sifting. The sieves used in early English expeditions
were of various sizes and meshes, each sieve having a finer mesh than
the sieve smaller than itself. In use the whole were put together in the
form of a nest, the smallest one with the coarsest mesh being on top. A
little of the dredge's contents were then put in the top sieve, and the
whole set moved gently up and down in a tub of sea water by handles
attached to the bottom one. Objects of different sizes are thus left in
different sieves. A simple but effective plan is to let the sieves of
various sized mesh fit accurately on each other like lids, the coarsest
on top, and to pour water upon material placed on the top one. In the
United States Bureau of Fisheries ship "Albatross" these sieves are
raised to form a table and the water is led on them from a hose: the
very finest objects or sediments are retained by the waste water
escaping from a catchment tub by muslin bags let into its sides. Any of
these methods are preferable to sifting by the agitation of a sieve hung
over the side, as in the last anything passing through the sieve is gone
past recall.
_Preservation of Specimens._--The preservation of specimens will of
course depend on the purpose for which they are intended. For
microscopic observation formaldehyde has some advantages. It can be
stored in 40% solution and used in 2%, thus saving space, and it
preserves many animals in their colours for a time: formalin
preparations do not, however, last as well as do those in spirit. The
suitable fluids for various histological inquiries are beyond the scope
of the present article; but for general marine histology Bles' fluid is
useful, being simple to prepare and not necessitating the removal of the
specimen to another fluid. It is composed of 70% alcohol 90 parts,
glacial acetic acid 7 parts, 4% formaldehyde 7 parts.
The scientific value of a dredging depends mainly upon two things, the
care with which the objects procured are preserved and labelled for
future identification and reference, and the accuracy with which all the
circumstances of the dredging--the position, the depth, the nature of
the ground, the date, the bottom-temperature, &c.--are recorded. In the
British Marine Biological Association's work in the North Sea, a
separate sheet of a printed book with carbon paper and duplicate sheets
(which remain always on the ship) is used for the record of the
particulars of each haul; depth, gear, &c., being filled into spaces
indicated in the form. This use of previously prepared forms has been
found to be a great saving of time and avoids risk of omission. Whether
labelled externally or not, all bottles should contain parchment or good
paper labels written with a soft pencil. These cannot be lost. The more
fully details of reference number of station, gear, date, &c., are given
the better, as should a mistake be made in one particular it can
frequently be traced and rectified by means of the rest.
_Growth of Scope of Operations._--At the Birmingham meeting of the
British Association in 1839 an important committee was appointed "for
researches with the dredge with a view to the investigation of the
marine zoology of Great Britain, the illustration of the geographical
distribution of marine animals, and the more accurate determination of
the fossils of the Pliocene period." Of this committee Edward Forbes was
the ruling spirit, and under the genial influence of his contagious
enthusiasm great progress was made during the next decade in the
knowledge of the fauna of the British seas, and many wonderfully
pleasant days were spent by the original committee and by many others
who from year to year were "added to their number." Every annual report
of the British Association contains communications from the English, the
Scottish, or the Irish branches of the committee; and in 1850 Edward
Forbes submitted its first general report on British marine zoology.
This report, as might have been anticipated from the eminent
qualifications of the reporter, was of the highest value; and, taken
along with his remarkable memoirs previously published, "On the
Distribution of the Mollusca and Radiata of the Aegean Sea," and "On the
Zoological Relations of the existing Fauna and Flora of the British
Isles," may be said to mark an era in the progress of human thought.
The dredging operations of the British Association committee were
carried on generally under the idea that at the 100-fathom line, by
which amateur work in small boats was practically limited, the zero of
animal life was approached--a notion which was destined to be gradually
undermined, and finally overthrown. From time to time, however, there
were not wanting men of great skill and experience to maintain, with Sir
James Clark Ross, that "from however great a depth we may be enabled to
bring up mud and stones of the bed of the ocean we shall find them
teeming with animal life." Samples of the sea-bottom procured with great
difficulty and in small quantity from the first deep soundings in the
Atlantic, chiefly by the use of Brooke's sounding machine, an instrument
which by a neat contrivance disengaged its weights when it reached the
bottom, and thus allowed a tube, so arranged as to get filled with a
sample of the bottom, to be recovered by the sounding line, were eagerly
examined by microscopists; and the singular fact was established that
these samples consisted over a large part of the bed of the Atlantic of
the entire or broken shells of certain foraminifera. Dr Wallich, the
naturalist to the "Bulldog" sounding expedition under Sir Leopold
M'Clintock, reported that star-fishes, with their stomachs full of the
deep-sea foraminifera, had come up from a depth of 1200 fathoms on a
sounding line; and doubts began to be entertained whether the bottom of
the sea was in truth a desert, or whether it might not present a new
zoological region open to investigation and discovery, and peopled by a
peculiar fauna suited to its special conditions.
In the year 1867, while the question was still undecided, two testing
investigations were undertaken independently. In America Count L. F. de
Pourtales (1824-1880), an officer employed in the United States Coast
Survey under Benjamin Peirce, commenced a series of deep dredgings
across the Gulf Stream off the coast of Florida, which were continued in
the following year, and were productive of most valuable results; and in
Great Britain the Admiralty, on the representation of the Royal Society,
placed the "Lightning," a small gun-vessel, at the disposal of a small
committee to sound and dredge in the North Atlantic between Shetland and
the Farue Islands.
In the "Lightning," with the help of a donkey-engine for winding in,
dredging was carried on with comparative ease at a depth of 600 fathoms,
and at that depth animal life was found to be still abundant. The
results of the "Lightning's" dredgings were regarded of so great
importance to science that the Royal Society pressed upon the Admiralty
the advantage of continuing the researches, and accordingly, during the
years 1869 and 1870, the gun-boat "Porcupine" was put under the orders
of a committee consisting of Dr W. B. Carpenter, Dr Gwyn Jeffreys, and
Professor (afterwards Sir Charles) Wyville Thomson, one or other of whom
superintended the scientific work of a series of dredging trips in the
North Atlantic to the north and west of the British Islands, which
occupied two summers.
In the "Porcupine," in the summer of 1869, dredging was carried down
successfully to a depth of 2435 fathoms, upwards of two miles and a
half, in the Bay of Biscay, and the dredge brought up well-developed
representatives of all the classes of marine invertebrates. During the
cruises of the "Porcupine" the fauna of the deep water off the western
coasts of Great Britain and of Spain and Portugal was tolerably well
ascertained, and it was found to differ greatly from the fauna of
shallow water in the same region, to possess very special characters,
and to show a very marked relation to the faunae of the earlier Tertiary
and the later Cretaceous periods.
In the winter of 1872, as a sequel to the preliminary cruises of the
"Lightning" and "Porcupine," by far the most considerable expedition in
which systematic dredging had ever been made a special object left Great
Britain. H.M.S. "Challenger," a corvette of 2306 tons, with auxiliary
steam working to 1234 h.p., was despatched to investigate the physical
and biological conditions of the great ocean basins.
The "Challenger" was provided with a most complete and liberal
organization for the purpose; she had powerful deck engines for hauling
in the dredge, workrooms, laboratories and libraries for investigating
the results on the spot, and a staff of competent naturalists to
undertake such investigations and to superintend the packing and
preservation of the specimens reserved for future study. Since the
"Challenger" expedition the use of wire rope has enabled far smaller
vessels to undertake deep sea work. The "Challenger," however, may be
said to have established the practicability of dredging at any known
depth.
_Operating Dredges and Trawls in deep Seas._--Dredging operations from
large vessels in deep seas present numerous difficulties. The great
weight of the ship makes her motion, whether of progress or rolling,
irresistible to the dredge. The latter tends to jump, therefore, which
both lowers its efficiency and causes it to exert a sudden strain on the
dredge rope.
The efficiency or evenness of dredging was secured, therefore, by the
special device of fastening a heavy weight some 200 or 300 fathoms from
the dredge end of the dredge rope. This was either lowered with the
dredge or sent down after by means of a "messenger," a ring of rope
fixed round, but running freely on, the dredge rope. The latter plan was
used on the "Challenger"; the weights were six 28 lb. leads in canvas
covers: their descent was arrested by a toggle or wooden cross-bar
previously attached to the rope at the desired point. When, however, the
rope used is of wire this front weight is unnecessary.
The possibility of sudden strain necessitates a constant watching of
the dredge rope, as the ship's engines may at any moment be needed to
ease the tension by stopping the vessel's way, and the hauling engines
by paying out more rope. The use of accumulators both renders the
strain more gradual and gives warning of an increase or decrease;
indeed they can be calibrated and used as dynamometers to measure the
strain. One of the best forms of accumulator consists of a pile of
perforated rubber disks, which receive the strain and become
compressed in doing so. The arrangement is in essence as follows. The
disks form a column resting on a cross-bar or base, from which two
rods pass up one on each side of the column. Another cross-bar rests
on the top disk, and from it a rod passes freely down the centre
perforation of disks and base. Eyes are attached to the lower end of
this rod and to a yoke connecting the side rods at the top: a pull
exerted on these eyes is thus modified by the elasticity of the
dredge. In the "Porcupine" and other early expeditions the accumulator
was hung from the main yard arm, and the block through which the
dredge rope ran suspended from it. In more recent ships a special
derrick boom is rigged for this block, and a second accumulator is
sometimes inserted between the topping lift by which this is raised
and the end of the boom.
The margin of safety of steel wire rope is much larger than is that of
hempen rope, a fact of importance both in towing in a rough sea and in
hauling. Galvanized steel wire with a hempen core was first used by
Agassiz on the "Blake." He states that his wire weighed one pound per
fathom, against two pounds per fathom of hempen rope, and had a
breaking strain nearly twice that of hempen rope, which bore two tons.
Thus in hauling the wire rope has both greater capability and less
actual strain. It has also the advantages of occupying a mere fraction
(1/9) of the storage space needed for rope, of lasting much longer,
and its vibrations transmit much more rapid and minute indications of
the conduct of the dredge.
Wire rope is kept wound on reels supplied with efficient brakes to
check or stop its progress, and an engine is often fitted for winding
it in and veering it out. From the reel it passes to the drum of the
hauling engine, round which it takes some few turns; care is taken by
watching or by the use of an automatic regulator (Tanner) that it is
taken at a rate equal to that at which it is moving over the side.
From the hauling engine it passes over leading wheels (one of which
should preferably be a registering wheel and indicate the amount of
rope which has passed it), and so it reaches the end of the derrick
boom.
The dredge is lowered from the derrick boom, which has been previously
trained over to windward so that its end is well clear of the ship,
while the ship is slowly moving forward. The rope is checked until the
net is seen to be towing clear, and then lowered rapidly. Where a weight
is used in front of the trawl Captain Calver successfully adopted the
plan of backing after sufficient line had been paid out: the part of the
rope from weight to surface thus became more vertical, while the shorter
remainder, previously in line with it, sank to the bottom without change
of relative position of weight and dredge. The ship was then ready for
towing. When no front weight is used the manoeuvre is unnecessary.
There should be a relation maintained between speed of vessel onward and
of rope downward, or a foul haul may result owing to the gear capsizing
(in the case of a trawl), or getting the net over the mouth (in a
dredge). The most satisfactory method of ensuring this relation seems to
be so to manage the two speeds that the angle made by the dredge rope is
fairly constant. This angle can be observed with a simple clinometer.
The following table abridged from Tanner most usefully brings together
the requisite angles with other useful quantities.
+----------+----------------+---------+---------------+---------------+
| Depth of | Speed of ship |Length of|Angle of dredge|Angle of dredge|
| water. | while shooting | rope | rope while | rope while |
| |dredge or trawl.|required.|lowering trawl.|dragging trawl.|
+----------+----------------+---------+---------------+---------------+
| Fathoms. | Knots. | Fathoms.| | |
| 100 | 3 | 200 | 60 | 55 |
| 200 | 3 | 400 | 60 | 55 |
| 400 | 3 | 700 | 60 | 52 |
| 600 | 2-3/4 | 1000 | 55 | 50 |
| 800 | 2-1/2 | 1200 | 50 | 44 |
| 1000 | 2-1/2 | 1500 | 50 | 40 |
| 1500 | 2-1/4 | 2166 | 50 | 40 |
| 2000 | 2 | 2670 | 45 | 35 |
| 3000 | 2 | 4000 | 40 | 35 |
+----------+----------------+---------+---------------+---------------+
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.
_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.
[Illustration: FIG. 15.--Deep-sea Dredge, with Tangle Bar.]
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 lb. 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.
_Triangular and Conical Dredges._--Two other dredges are worthy of
mention. The triangular dredge, much resembling Muller'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
lb. 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.
[Illustration: FIG. 16.--Conical Dredge being hoisted in.]
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.
[Illustration: From Sir Charles Wyville Thomson's _Voyage of the
"Challenger."_ By permission of Macmillan & Co., Ltd