METHODS OF SAMPLING THE BENTHOS
191
Agassiz trawl, a small double-side beam trawl. When fitted with a
fine mesh net it is particularly valuable, although its design does not
allow it to catch nisny fish.
Although it is possible, to some extent, to estimate the distance
over which a trawl has been towed (Steven, 1930), such instruments
fail to collect but a fraction of the fauna on the ground over which they
pass (see p. 215). An attempt to measure the distance towed is incorporated in the “epibenthic dredge’’ of Beiri and Bradshaw (see
Gunter, 1957, pp. 76 and 77) which haa a rotating ‘‘ bottom-walker ”
the rotations of which measure the ground covered. Another more
elaborate device is described by Riedl (1955).
Mention may be made of certain plankton nets fitted with runners
or similar devices to sample the plankton just over the bottom (e.g.
Mortensen, 1926 ; Russell, 1928 ; Bossanyi, 1961 ; Wickstead, 1963;
Beyer, 1958 (not yet described in detail-see Fig. 10)). In some
instances these may also sample truly benthic species, and an interesting
sledge sampler (Fig. 9) developed by Ockelmann (1964) at the Marine
Laboratory, Elsinore, Denmark, has a tickler chain to stir up newlysettled members of tho benthic fauna, which are then taken by the
plankton net which glides over the bottom on broad runners. Marr
( 1963) reports conspicuous success in obtaining undamaged specimens
of comatulids on the Ross Sea shelf by using Russell’s (1928) bottom
net fitted with an ordinary dredge bag instead of the usual stramin net.
Other collecting methods
Valuable information on the local bottom fauna may often be
acquired by those without the means for making their own collections
at sea. These include examination of the contents of lobster or crab
pots, trawl rubbish ” and the assortment of specimens washed up
on the beach after storms, In some instances animals not obtainable
by other methods will be collected in this way.
C . Urabs and other quantitative samplers
The first instrument used for quantitative investigation of the
bottom fauna was the grab used by Petersen (1918) in the Danish
fiords. This is a small version of the grab used for loading coal, etc., and
consists of two hinged buckets, each forming a sector of a circle, which
are lowered to the bottom in the open position (Figs. 12, 13). When
the lowering rope slackens, a release is actuated so that on hauling
U P the two scoops first close together, so taking a ‘‘ bite ” of semicircular cross section of the sediment. The area covered by the Petersen
grab was originally’ 0.1 ms but later a larger model was built covering
0.
191
Agassiz trawl, a small double-side beam trawl. When fitted with a
fine mesh net it is particularly valuable, although its design does not
allow it to catch nisny fish.
Although it is possible, to some extent, to estimate the distance
over which a trawl has been towed (Steven, 1930), such instruments
fail to collect but a fraction of the fauna on the ground over which they
pass (see p. 215). An attempt to measure the distance towed is incorporated in the “epibenthic dredge’’ of Beiri and Bradshaw (see
Gunter, 1957, pp. 76 and 77) which haa a rotating ‘‘ bottom-walker ”
the rotations of which measure the ground covered. Another more
elaborate device is described by Riedl (1955).
Mention may be made of certain plankton nets fitted with runners
or similar devices to sample the plankton just over the bottom (e.g.
Mortensen, 1926 ; Russell, 1928 ; Bossanyi, 1961 ; Wickstead, 1963;
Beyer, 1958 (not yet described in detail-see Fig. 10)). In some
instances these may also sample truly benthic species, and an interesting
sledge sampler (Fig. 9) developed by Ockelmann (1964) at the Marine
Laboratory, Elsinore, Denmark, has a tickler chain to stir up newlysettled members of tho benthic fauna, which are then taken by the
plankton net which glides over the bottom on broad runners. Marr
( 1963) reports conspicuous success in obtaining undamaged specimens
of comatulids on the Ross Sea shelf by using Russell’s (1928) bottom
net fitted with an ordinary dredge bag instead of the usual stramin net.
Other collecting methods
Valuable information on the local bottom fauna may often be
acquired by those without the means for making their own collections
at sea. These include examination of the contents of lobster or crab
pots, trawl rubbish ” and the assortment of specimens washed up
on the beach after storms, In some instances animals not obtainable
by other methods will be collected in this way.
C . Urabs and other quantitative samplers
The first instrument used for quantitative investigation of the
bottom fauna was the grab used by Petersen (1918) in the Danish
fiords. This is a small version of the grab used for loading coal, etc., and
consists of two hinged buckets, each forming a sector of a circle, which
are lowered to the bottom in the open position (Figs. 12, 13). When
the lowering rope slackens, a release is actuated so that on hauling
U P the two scoops first close together, so taking a ‘‘ bite ” of semicircular cross section of the sediment. The area covered by the Petersen
grab was originally’ 0.1 ms but later a larger model was built covering
0.