sometimes with distance-metering wheels alongside, are a modern version of this
ancient concept.
Fig. 13.2 An Agassiz trawl, an early, two-sided version of a beam trawl.
(^) (After Gage & Tyler 1991.)
For many purposes, it is more useful to collect inhabitants of sediment together with
the sediment, doing the sieving on deck. Thus, the sediment can be characterized and
correlations defined between sediment properties (grain size, organic content,
porosity) and fauna. A modification of the dredge, a Sanders “anchor” dredge, is
sometimes used. Landing either side up on the bottom, it is dragged along for a short
distance until the large, non-filtering bag or box (Carey & Hancock 1965) behind is
full of sediment. When less disturbance of the sediment is desired, grabs and corers
are deployed. Every sort of construction scoop has been used in benthic collecting:
draglines, clamshell buckets, and orange-peel grabs. Mostly those depend upon their
weight to hold them down as they dig into the sediment. Many grabs are designed
expressly for benthic sampling. The simplest designs, for example the van Veen grab,
are paired scoops suspended from long lever arms. Deployed by cable open, the arms
are released on hitting the bottom so that pulling upward on the cable closes the
scoop, collecting a mass of sediment. Mechanically more complex grab designs are
numerous, but can be represented by the Smith–McIntyre (Smith–Mac) grab (Fig.
13.3a). Two spring-driven clamshell scoops are hinged into a heavily weighted frame
and are cocked before deployment with a lever. Trip pads extending below the frame
release the springs on impact with the bottom, and the scoops are driven into the
sediment, closing together below a half cylinder of 0.1 m^2 surface area. The opening
above the enclosed sediment is covered first with a screen, then with a rubber flap.
Water passes under the flap during closure, but on retrieval it seals against the screen
to prevent washout.
Fig. 13.3 (a) Design of a Smith–McIntyre grab
(after Smith & McIntyre 1954).
(b) A box corer, shown with core-box dolly and stages of deployment and recovery
(I–IV).
(After Gage & Tyler 1991.)