METHODS OF SAMPLING THE BENTHOS
216
the Petersen, the number of specimens taken was no higher. In a second
trial (Ursin, 1956) the 0-1 m2 Knudsen sampler was compared with a
0.2 ma van Veen grab. The Knudsen sampler brought up a very
much larger volume of sediment per unit area, but the number of
specimens was not very much greater. Ursin notes that in the lower
half of the Knudscn samples (the samples were about 22 ern deep)
the sand and shclls were blackish and foul-smelling. Such soils are
avoided by many animals and this may account for the comparatively small increase in numbers of animals in the deeper samples
taken by the Knudsen.
Birltett (1958) has shown that, thc 0-2 in2 Petersen grab misses the
larger, deeper-burrowing individuals of the lamellibranch Tellina
fabula which are, however, taken by the van Veen grab. The very
largest specimens are not taken by either grab, and are oiily known
as dead shells.
Barriard and Hartman (1959) record the echiuroid Listriolobus in
orange-peel grab hauls in which the sampler had penetrated up to
60 cm (in mud). Often the animals had been snagged by the bottom
claws of thc grab, indicating that they were deep burrowers and that
possibly others lived still deeper, out of reach of the grab.
From these various accounts we may conclude that the majority
of benthic animals may be found in the top 10 cm of the sediment.
Those below this depth, although often large, are infrequent, and i t
may be necessary to sqtmple to depths of 30 cm or more to collect all
individuals of certain species. On a firmly-packed, sandy, ground
both the 0.1 and 0.2 ,m2 Petersen grabs fail to take an adequate sample,
while samplers digging rather deeper, such as the van Veeii grab and
Holme scoop-sampler do tdke a reasonably satisfactory sample. Not
very many more specimens are taken by the Knudsen sampler as
compared wit,h the deeper-digging grabs.
Sampling the ep@tiinu
The efficiency of various methods of sampling the epifauna has
been tested by McIntyre (1956), who compared the sampling abilities
of a 0-1 m2 van Veen grab and a 6-ft (182-cm) Agassiz trawl against
an underwater camera.
When sampling brittle-stars, which occurred in large numbers on
the surface of the ground, the Agassiz trawl was the least efficient of
the three, while the, camera and grab gave fairly comparable results,
dependent on the densities of brittle stars. Estima,tes of brittle-stars
with the camera were from 16 to 117 times greater and, with the grab,
18 to 152 times greater, than with the trawl. For the less common
A.Y.B.-2
U
216
the Petersen, the number of specimens taken was no higher. In a second
trial (Ursin, 1956) the 0-1 m2 Knudsen sampler was compared with a
0.2 ma van Veen grab. The Knudsen sampler brought up a very
much larger volume of sediment per unit area, but the number of
specimens was not very much greater. Ursin notes that in the lower
half of the Knudscn samples (the samples were about 22 ern deep)
the sand and shclls were blackish and foul-smelling. Such soils are
avoided by many animals and this may account for the comparatively small increase in numbers of animals in the deeper samples
taken by the Knudsen.
Birltett (1958) has shown that, thc 0-2 in2 Petersen grab misses the
larger, deeper-burrowing individuals of the lamellibranch Tellina
fabula which are, however, taken by the van Veen grab. The very
largest specimens are not taken by either grab, and are oiily known
as dead shells.
Barriard and Hartman (1959) record the echiuroid Listriolobus in
orange-peel grab hauls in which the sampler had penetrated up to
60 cm (in mud). Often the animals had been snagged by the bottom
claws of thc grab, indicating that they were deep burrowers and that
possibly others lived still deeper, out of reach of the grab.
From these various accounts we may conclude that the majority
of benthic animals may be found in the top 10 cm of the sediment.
Those below this depth, although often large, are infrequent, and i t
may be necessary to sqtmple to depths of 30 cm or more to collect all
individuals of certain species. On a firmly-packed, sandy, ground
both the 0.1 and 0.2 ,m2 Petersen grabs fail to take an adequate sample,
while samplers digging rather deeper, such as the van Veeii grab and
Holme scoop-sampler do tdke a reasonably satisfactory sample. Not
very many more specimens are taken by the Knudsen sampler as
compared wit,h the deeper-digging grabs.
Sampling the ep@tiinu
The efficiency of various methods of sampling the epifauna has
been tested by McIntyre (1956), who compared the sampling abilities
of a 0-1 m2 van Veen grab and a 6-ft (182-cm) Agassiz trawl against
an underwater camera.
When sampling brittle-stars, which occurred in large numbers on
the surface of the ground, the Agassiz trawl was the least efficient of
the three, while the, camera and grab gave fairly comparable results,
dependent on the densities of brittle stars. Estima,tes of brittle-stars
with the camera were from 16 to 117 times greater and, with the grab,
18 to 152 times greater, than with the trawl. For the less common
A.Y.B.-2
U