Seafloor Videos From Bering Sea Research Project:
Modifying Trawl Sweeps to Reduce Effects on Sessile Benthic Animals
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Figure 1. Map showing locations of sled and trawl tows during May – June 2006 research on reducing seafloor effects of
trawling. Light green areas closed to bottom trawling. Figure 2. Launching the seafloor sled with DIDSON sonar and camera with strobed lighting. (Photo by Carwyn Hammond.) |
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Scientists with the AFSC's Midwater Assessment and Conservation Engineering (MACE) program compared the effects of conventional and modified sweeps (herding cables ahead of the trawl net) on sessile invertebrates. For most the flatfish trawlers in the Bering Sea, sweeps cover approximately 90% of the seafloor swept by during trawling, herding the fish from that area into the path of the trawl net. Conventional sweeps, which are 2 – 3 inches in diameter along their entire length, were modified by installing clusters of larger diameter disks every thirty feet. This raised the rest of the sweep cables 2 – 4 inches above the seafloor, reducing direct contact by more than 90% and providing space for small and flexible seafloor animals to pass beneath.
These videos were taken at four study sites on the eastern Bering Sea shelf (Fig. 1). We selected sites with high abundances of such animals as well as a variety of the most common types. A site about 60 nautical miles (nmi) west of St. Paul Island (A) was dominated by sea whips and basketstars. Sites 45 nmi east of St. Paul (B) and 100 nmi west of Cape Newenham (C) had mostly ascidians (Halocynthia, Boltenia and Styela). Finally, sponge dominated the sessile seafloor fauna at a site 60 nmi NNE of Port Moller (D).
A seafloor sled (Fig. 2) with both sonar and video sensors was then towed across the parallel trawl tracks at several points to compare the condition of seafloor animals in areas affected by these different gears. An acoustic camera (DIDSON) provided an image of seafloor terrain on which trawl marks could be consistently identified, making it possible to discern which part of which trawl track the sled was in or whether it was between tracks. A video camera with strobed lights was then used to assess the condition and abundance of seafloor invertebrates associated with each area. The imagery from these sensors will be analyzed to estimate the relative effects of the alternative sweep designs on each kind of structure-forming invertebrate.
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