180
N. A . HOLME
may have to I)c carried out, in deposits ranging from soft mud to coarse
gravel, and t.hc anim:ils nixy bc large members of the epifauna, like
starfish, occurring in denkties of one per 20 ma, or small crustacea
with densities of hundreds or even thousands to the square metre.
I n some places, such as in anaerobic muds, the fauna may be restricted
to the top few centimetres, while in others larger individuals may occur
at depths of 30 or more centinietres. Such a range of conditions may
be often found during a day’s collecting, and off Plymouth it has
often been noticed that a slight change in position, as will occur
between successive hauls when working the grab at a station, may
result in marked changes in sediment and its contained fauna (Ford,
1923; Holme, 1953).
When sampling in mid-water, whether with water bottles, plankton
nets, or mid-water fishing gear, the conditions under which the equipment is operating can be more-or-less calculated or are known from
past experience. The ambient medium has properties which are well
known, and such variations in working of the gear as occur will depend
on the degree of disturbance of the sea surface, the effects of currents
a t various depths, and the speed and handling behaviour of the ship
under different conditions. With experience the effects of such variables
may be a t least partially counterbalanced. Not so when sampling
the sea bed. While i t is true that we can determine in advance the
depth and surface contour of the bottom by means of the echo-sounder
or asdic, the detailed nature of the bottom, the presence of ripple
marks, small undulations and low outcrops of rock cannot readily
be ascertained in advance. The gr8.b or corer is lowered virtually at
random on to the bottom, and has to contend with any local irregularities which may exist. It follows that hazards to the gear are
considerable as compared with those operated in mid-water, even
more so if the apparatus is to be towed along the bottom.
Apart from fouling a submerged obstacle there is an additional
hazard, due to the “ suction ” produced when pulling an instrument
vertically out of the sediment. This may result in distortion of shackles
or even parting of the wire.
Such considerations have necessitated the construction of extremely
robust, sampling gear and equipment for handling it, this being all
the more necessary where sudden strains may result from rolling or
pitching of the ship in the open sea.
At present there secms to be no need to attempt to standardize
the types of gear in use, since this would hinder modification and
improvements. We are a long way from the ideal sampler, but what
could perhaps be attempted is to state a target in terms of area and
N. A . HOLME
may have to I)c carried out, in deposits ranging from soft mud to coarse
gravel, and t.hc anim:ils nixy bc large members of the epifauna, like
starfish, occurring in denkties of one per 20 ma, or small crustacea
with densities of hundreds or even thousands to the square metre.
I n some places, such as in anaerobic muds, the fauna may be restricted
to the top few centimetres, while in others larger individuals may occur
at depths of 30 or more centinietres. Such a range of conditions may
be often found during a day’s collecting, and off Plymouth it has
often been noticed that a slight change in position, as will occur
between successive hauls when working the grab at a station, may
result in marked changes in sediment and its contained fauna (Ford,
1923; Holme, 1953).
When sampling in mid-water, whether with water bottles, plankton
nets, or mid-water fishing gear, the conditions under which the equipment is operating can be more-or-less calculated or are known from
past experience. The ambient medium has properties which are well
known, and such variations in working of the gear as occur will depend
on the degree of disturbance of the sea surface, the effects of currents
a t various depths, and the speed and handling behaviour of the ship
under different conditions. With experience the effects of such variables
may be a t least partially counterbalanced. Not so when sampling
the sea bed. While i t is true that we can determine in advance the
depth and surface contour of the bottom by means of the echo-sounder
or asdic, the detailed nature of the bottom, the presence of ripple
marks, small undulations and low outcrops of rock cannot readily
be ascertained in advance. The gr8.b or corer is lowered virtually at
random on to the bottom, and has to contend with any local irregularities which may exist. It follows that hazards to the gear are
considerable as compared with those operated in mid-water, even
more so if the apparatus is to be towed along the bottom.
Apart from fouling a submerged obstacle there is an additional
hazard, due to the “ suction ” produced when pulling an instrument
vertically out of the sediment. This may result in distortion of shackles
or even parting of the wire.
Such considerations have necessitated the construction of extremely
robust, sampling gear and equipment for handling it, this being all
the more necessary where sudden strains may result from rolling or
pitching of the ship in the open sea.
At present there secms to be no need to attempt to standardize
the types of gear in use, since this would hinder modification and
improvements. We are a long way from the ideal sampler, but what
could perhaps be attempted is to state a target in terms of area and