M E T H O D S O F SAMPLING T H E BENTHOS
235
A. Sieving
While some workers have used a nest of sieves of decreasing mesh
size, and others have used but a single sieve, it is the mesh of the finest
sieve only, which regulates the minimum size of animal retained, that
need concern us here.
In freshwater deposits or in marine muds it is usually possible to
employ a much finer sieve than when working on the coarse sediments
of the continental shelf. Thus JBnasson (1955) reports that the mesh
normdly used in fresh water is about 0.6 mm, while in the sea a mesh
of around 1.5 mm is more gcneral (Reish, 1959a). These two papers
contain interesting comparisons of the numbers of animals retained by
sieves of different meshes.
Losses through sieves are surpri3ingly high and vary with the
nature of the organism: with an object of constant dimensions the
limiting measurement is the second greatest (i.e. breadth or height,
whichever is larger), it being assumed that in limiting cases the object
will eventually approach the mesh end-on. However, with worms
whose dimensions are indefinite and which may actively wriggle
through the meshes, the results are somewhat unpredictable.
In plankton sampling the mesh is selected according to the size of
the organism to be sampled, but in sieving bottom-deposits it is more
likely to be controlled by the grade of sediment. For example, off
Plymouth, sediments with some 30-50y0 of the total over 2 mm occur
(Holme, 1953), making sieving even with a 2.2 mm sieve practically
impossible. When using this sieve it was found, at one station on sand,
that only about 30% of total numbers and 60% of the total weight of
animals, as compared with a 1.2 mm. sieve, were retained on the
coarse sieve. At other stations probably about 90% of the total
biomass was retained on the coarse sieve.
Reish (1 Y69a) studied the effccts of sieve mesh on a mud in which
polychaete worms predominated. He gives asymptotic curves (Fig. 36)
showing the percentage of the " total '' biomass, number of species, and
number of specimens retained on different meshes. Over 90% of the
biomass was retained on a sieve mesh of 1-4 mm ; over 90% of the number of species on mesh of 0-86 mm and over goo/, of the number
of specimens on a mesh of 0.27 mm. Clearly a larger mesh can be
accepted when only biomass is to be determined. It is almost impossible to make any general statement on mesh size, as this will vary
with the size of animals to be investigated. When starting an investigation it would be advisable t o make some comparisons, using different
meshes to discover the magnitude of losses. The greater losses
235
A. Sieving
While some workers have used a nest of sieves of decreasing mesh
size, and others have used but a single sieve, it is the mesh of the finest
sieve only, which regulates the minimum size of animal retained, that
need concern us here.
In freshwater deposits or in marine muds it is usually possible to
employ a much finer sieve than when working on the coarse sediments
of the continental shelf. Thus JBnasson (1955) reports that the mesh
normdly used in fresh water is about 0.6 mm, while in the sea a mesh
of around 1.5 mm is more gcneral (Reish, 1959a). These two papers
contain interesting comparisons of the numbers of animals retained by
sieves of different meshes.
Losses through sieves are surpri3ingly high and vary with the
nature of the organism: with an object of constant dimensions the
limiting measurement is the second greatest (i.e. breadth or height,
whichever is larger), it being assumed that in limiting cases the object
will eventually approach the mesh end-on. However, with worms
whose dimensions are indefinite and which may actively wriggle
through the meshes, the results are somewhat unpredictable.
In plankton sampling the mesh is selected according to the size of
the organism to be sampled, but in sieving bottom-deposits it is more
likely to be controlled by the grade of sediment. For example, off
Plymouth, sediments with some 30-50y0 of the total over 2 mm occur
(Holme, 1953), making sieving even with a 2.2 mm sieve practically
impossible. When using this sieve it was found, at one station on sand,
that only about 30% of total numbers and 60% of the total weight of
animals, as compared with a 1.2 mm. sieve, were retained on the
coarse sieve. At other stations probably about 90% of the total
biomass was retained on the coarse sieve.
Reish (1 Y69a) studied the effccts of sieve mesh on a mud in which
polychaete worms predominated. He gives asymptotic curves (Fig. 36)
showing the percentage of the " total '' biomass, number of species, and
number of specimens retained on different meshes. Over 90% of the
biomass was retained on a sieve mesh of 1-4 mm ; over 90% of the number of species on mesh of 0-86 mm and over goo/, of the number
of specimens on a mesh of 0.27 mm. Clearly a larger mesh can be
accepted when only biomass is to be determined. It is almost impossible to make any general statement on mesh size, as this will vary
with the size of animals to be investigated. When starting an investigation it would be advisable t o make some comparisons, using different
meshes to discover the magnitude of losses. The greater losses