tkptli to be sanrl)led, and to riw the sampler which under any given
conditioiis most nearly achitwes this.
For purely nrcclranical reasons of handling there is a limit to the
volume of sediment which can be sieved or sorted, so that there is in
practice an upper limit, of the order of 1 m2 area, to the area which can
be sampled by grab a t each station. On the other hand, many important
spec.ies may o ( ~ u r
at densities of much less than one per square metre.
If tlredgc., trawl, u n d e r w a t ~ ~
( * m r ( w , or television are also employed a
much lurgcr area of the sarfaw may he samplcd or scanned, although
such iiivextigatioti is neccwarily lirnitcd to the epifauna and to such
Imrrowiiig animals as call h(. takrn t)y t h e dredge or trawl or are
rcqqiizable by their bui-rows in canicra and television pictures.
‘rherr is no way of estimating t he densities of sparsely occurring
1)urrowing animals which esc;tl)t capture by the dredge or trawl.
Stwcn (1930) has pointed out that fish are much better collectors
than we arc, and he cites instances of invertebrates found in fish
stomachs which wcre not otherwise known on the feeding grounds.
A. Dredging
The c*arly history of dredging has already been outlined, and we
may now coitsidcr what improvements in gear and technique have
occurred since 0. F. Muller’s four-sided dredge was first used. The
wide mouth of Mriller’s dredge evidently caused washing-out of the
sample when hauling (Thomson, 1873, p. 237) and we find the most
generally adopted shape in later years was the rectangular dredge, an
early example of which was Ball’s dredge (Fig. 2) (Thomson, 1873;
Fowler and Allen, 1928, p. 329), and which, with little modification,
is that used today.
1)rcdging is one of the simplest of operations : the dredge frame, to
the back of which is attached a net bag, is lowered to the bottom,
rope or wire equal to three times the depth of water being paid out.
The ship drifts or steams slowly for 5 or 10 niin, watch being kept
on the tugs exerted by the dredge as transmitted up the tow-rope, and
then the dredge is hauled up. When a vegetable or synthetic rope is
used, the tow-rope may be almost straight between ship and dredge,
but with a steel rope the warp is curved under its own weight so that
a t the bottom the pull on the dredge is practically horizontal. A weight
or heavy chain may be fitted in front of the dredge if required to
provide a more nearly horizontal pull. I n deep-sea work, where the
time occupied in lowering and hauling up may be several hours, it is
important that time should not be wasted by paying out excessive
lengths of warp, and calculations have been made by Kullenberg
conditioiis most nearly achitwes this.
For purely nrcclranical reasons of handling there is a limit to the
volume of sediment which can be sieved or sorted, so that there is in
practice an upper limit, of the order of 1 m2 area, to the area which can
be sampled by grab a t each station. On the other hand, many important
spec.ies may o ( ~ u r
at densities of much less than one per square metre.
If tlredgc., trawl, u n d e r w a t ~ ~
( * m r ( w , or television are also employed a
much lurgcr area of the sarfaw may he samplcd or scanned, although
such iiivextigatioti is neccwarily lirnitcd to the epifauna and to such
Imrrowiiig animals as call h(. takrn t)y t h e dredge or trawl or are
rcqqiizable by their bui-rows in canicra and television pictures.
‘rherr is no way of estimating t he densities of sparsely occurring
1)urrowing animals which esc;tl)t capture by the dredge or trawl.
Stwcn (1930) has pointed out that fish are much better collectors
than we arc, and he cites instances of invertebrates found in fish
stomachs which wcre not otherwise known on the feeding grounds.
A. Dredging
The c*arly history of dredging has already been outlined, and we
may now coitsidcr what improvements in gear and technique have
occurred since 0. F. Muller’s four-sided dredge was first used. The
wide mouth of Mriller’s dredge evidently caused washing-out of the
sample when hauling (Thomson, 1873, p. 237) and we find the most
generally adopted shape in later years was the rectangular dredge, an
early example of which was Ball’s dredge (Fig. 2) (Thomson, 1873;
Fowler and Allen, 1928, p. 329), and which, with little modification,
is that used today.
1)rcdging is one of the simplest of operations : the dredge frame, to
the back of which is attached a net bag, is lowered to the bottom,
rope or wire equal to three times the depth of water being paid out.
The ship drifts or steams slowly for 5 or 10 niin, watch being kept
on the tugs exerted by the dredge as transmitted up the tow-rope, and
then the dredge is hauled up. When a vegetable or synthetic rope is
used, the tow-rope may be almost straight between ship and dredge,
but with a steel rope the warp is curved under its own weight so that
a t the bottom the pull on the dredge is practically horizontal. A weight
or heavy chain may be fitted in front of the dredge if required to
provide a more nearly horizontal pull. I n deep-sea work, where the
time occupied in lowering and hauling up may be several hours, it is
important that time should not be wasted by paying out excessive
lengths of warp, and calculations have been made by Kullenberg