Although the global literature clearly indicates that communities on hard substrates are more vulnerable to bottom trawling (see reviews in Dayton et ah, 1995; Kaiser et ah, 1998; Watling and Norse, 1998; NRC, 2002; Barnes and Thomas, 2005), the lack of impacts to unconsolidated sandy sediments at a depth of 170 m as observed in this study indicates that this type of sediment is much less vulnerable, at least at the level of fishing effort undertaken for this study with a bottom trawl that met the federal requirement of a small footrope
(<20 cm in diameter).
The trawl, with an 8.5 m headrope and a 10.4 m footrope
, was towed for 10 min at 3.7 km/h at each sampling location.
Each tow covered 0.37 km at 0.5 m/s, with a standard Southern California Coastal Water Research Project bottom trawl that had a footrope
of 5 m and net width of 3.5 m during fishing.
While their mobility may permit avoidance of the doors, it only slightly delayed contact with the sweeps or footrope
. Whether a crab passed over or under a trawl component was mostly determined by the relative size of the crab and the component encountered.
The Poly-Nor'eastern bottom-trawl footrope
comprised 10-cm disks interspersed with bobbins 36 cm in diameter.
This was done by: 1) replacing the long-link chain running through the rubber disks with 19 mm cable, 2) removing the chain fishing line, and 3) attaching the ground gear directly to the footrope
Crabs were more likely to die after encountering the footrope
than the sweeps of the trawl, and higher death rates were noted for the side sections of the footrope
than for the center footrope
The sampling gear is a standard AFSC eastern otter trawl with a 25.3 m headrope and 34.1 m footrope
. Otter doors are 1.8 x 2.7 m and weigh about 800 kg each.
The net modifications included replacement of the standard footrope
with rockhopper gear, the addition of heavier bridles (1.9 cm), and double meshes in the belly of the net.
The only variables contributing significantly to the models were month, area, depth, year, ln(length), ln(footrope
), and In(days fished/trip).
The major modifications to the net were heavier netting material in the belly of the net, a footrope
with tire gear through the center, and continuous roller gear through the sweeps.
Most observed vessels utilized a transducer, or net sounder, placed on either the headrope or footrope
to determine the depth and opening of the net.