MEI‘lIODS OF SAMPLING THE BENTHOS
183
satisfactory sainplitig of rocks a i d firmly-packed sediments respectively
may be traced through these variations.
On rocky bottoms the rectaiigular dredge is widely used, and on such
grounds, within its limits, it functions well. For rock-dredging under
severe conditions the net is made up of a chain-mail of interlacing steel
rings or grommets of wire rope (Fig. 6), and the frame may be variously
modified according to whether it is designed to break off and collect pieces
of rock, or to work among the rocks, scraping off encrusting growths
(MacGinitie, 1048, Fig. 2 ; Zoutrtrdyk, 1964). It is particularly necessary
to have a weak link to free thv dredge should it become caught up,
and this is usually achieved by shackling one arm to the towing cable,
the other being attached by several turns of twine (Fig. 2) which will
part and release the dredge if nwessary. This precaution is advisable
a t all times, but particularly so on rocks. Other methods of avoiding
excessive strains when dredging are mentioned on pp. 223-4.
Nalwalk et al. (1962) have illustrated a dredge especially designed
for collectiiig stones in deep water where the bottom may be of boulders
or massive rocky outcrops. The bag is of chain mail, and the frame
free to pivot to some extent to clear rocks. A weak link and safety
chain are incorporated in the design, and by the use of acoustic pulses
from a pinger attached to the warp (see p. 230) it is possible to dredge
in deep water with wire cqual to the depth plus only 10% paid out.
The development, of a dredge designcd to dig deeply into the
sediment) dates back to Robertson’s (1868) bucket dredge (Fig. 3).
This is, however, rather a small instrument and not very effective,
although successfully used for a recent survey of shallow-water faunas
in the Clyde by Clark and Milne (1955). Any departure from a straight
cutting edge as on the rectangular dredge no doubt increases the power
of digging, and a t Plymouth we use a dredge in which the sides are well
bowed out to give the frame an almost oval shape (Fig. 5). A much
larger and heavier ring dredge, successful in capturing deeper burrowing Crustacea, is described by Custafson (1 934).
At one time the conical dredge (Fig. 6) was a popular instrument
in Britain for obtaining samples of the sediment. It consists of a
cone-shaped inetad frame with a circular cutting cdge, inside which is
mounted a conical canvas bag. The dredge is towed by two or, more
usually, three eliaiiis, and a full description is given by Borley (1923).
The coiiicrtl dredge was extensivcly used by Borley for his survey of
North Sea sediments, and it is clear that other things being equal it
was preferred t o the grab even by Davis (1925) for obtaining a satisfactory sample of deposit. Smith (1032) surveyed the Eddystone
gravels with the conical dredge, Z’oweII (1937) used this instrument
* \ I H - - 9
c
183
satisfactory sainplitig of rocks a i d firmly-packed sediments respectively
may be traced through these variations.
On rocky bottoms the rectaiigular dredge is widely used, and on such
grounds, within its limits, it functions well. For rock-dredging under
severe conditions the net is made up of a chain-mail of interlacing steel
rings or grommets of wire rope (Fig. 6), and the frame may be variously
modified according to whether it is designed to break off and collect pieces
of rock, or to work among the rocks, scraping off encrusting growths
(MacGinitie, 1048, Fig. 2 ; Zoutrtrdyk, 1964). It is particularly necessary
to have a weak link to free thv dredge should it become caught up,
and this is usually achieved by shackling one arm to the towing cable,
the other being attached by several turns of twine (Fig. 2) which will
part and release the dredge if nwessary. This precaution is advisable
a t all times, but particularly so on rocks. Other methods of avoiding
excessive strains when dredging are mentioned on pp. 223-4.
Nalwalk et al. (1962) have illustrated a dredge especially designed
for collectiiig stones in deep water where the bottom may be of boulders
or massive rocky outcrops. The bag is of chain mail, and the frame
free to pivot to some extent to clear rocks. A weak link and safety
chain are incorporated in the design, and by the use of acoustic pulses
from a pinger attached to the warp (see p. 230) it is possible to dredge
in deep water with wire cqual to the depth plus only 10% paid out.
The development, of a dredge designcd to dig deeply into the
sediment) dates back to Robertson’s (1868) bucket dredge (Fig. 3).
This is, however, rather a small instrument and not very effective,
although successfully used for a recent survey of shallow-water faunas
in the Clyde by Clark and Milne (1955). Any departure from a straight
cutting edge as on the rectangular dredge no doubt increases the power
of digging, and a t Plymouth we use a dredge in which the sides are well
bowed out to give the frame an almost oval shape (Fig. 5). A much
larger and heavier ring dredge, successful in capturing deeper burrowing Crustacea, is described by Custafson (1 934).
At one time the conical dredge (Fig. 6) was a popular instrument
in Britain for obtaining samples of the sediment. It consists of a
cone-shaped inetad frame with a circular cutting cdge, inside which is
mounted a conical canvas bag. The dredge is towed by two or, more
usually, three eliaiiis, and a full description is given by Borley (1923).
The coiiicrtl dredge was extensivcly used by Borley for his survey of
North Sea sediments, and it is clear that other things being equal it
was preferred t o the grab even by Davis (1925) for obtaining a satisfactory sample of deposit. Smith (1032) surveyed the Eddystone
gravels with the conical dredge, Z’oweII (1937) used this instrument
* \ I H - - 9
c