am and other trawls are now used as quantitative
instruments in the estimation of the fish population, especially of
the _Pleuronectidae_.

_Use of Small Trawls for Dredging._--Although these trawls do not here
concern us, certain adaptations of small beam trawls for biological
exploration are of such identical use with the dredge, and differ from
it so little in structure and size, that they may be here described.

A small beam trawl was first used from the "Challenger" (fig. 17). It
was sent down in 600 fathoms off Cape St Vincent, the reason for its
use being the frequency with which the dredge sank into the sea-bottom
and there remained until hauling. The experiment was entirely
successful. The sinking of the net was avoided, the net had a much
greater spread than the dredge, and in addition to invertebrates it
captured several fish. After this the trawl was frequently used
instead of the dredge. Indeed tangle bar, dredge and trawl form a
series which are fitted for use on the roughest, moderately rough and
fairly firm, and the softest ground respectively, although the dredge
can be used almost anywhere.

[Illustration: From Sir Charles Wyville Thomson's _Voyage of the
"Challenger."_ By permission of Macmillan & Co., Ltd

FIG. 17.--Trawl of the "Challenger."]

The frame of the "Challenger" trawl consisted of a 15 ft. wooden beam
which in use was drawn over the sea-bed on two runners resembling
those of a sledge, by means of two ropes or bridles attached to eyes
in the front of the runners or "trawl heads." A net 30 ft. long was
suspended by one side to the beam by half-a-dozen stops. The remainder
of the net's mouth was of much greater length than the beam, and was
weighted with close-set rolls of sheet lead; it thus dragged along the
bottom in a curve approximately to a semicircle, behind the beam. The
net tapers towards the hinder end, and contains a second net with open
bottom, which, reaching about three-quarters of the way down the main
net, acts as a valve or pocket. Both heels (or hinder ends) of the
trawl heads and the tail of the net were weighted to assist the net in
digging sufficiently and to maintain its balance--an important point,
since if the trawl lands on its beam the net's mouth remains closed,
and nothing is caught.

The main differences of this trawl from the dredge are the replacement
of scraping lip by ground rope,