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Midwater Assessment & Conservation Engineering (MACE) Program

Development and Evaluation of Trawl Groundgear Modifications to Reduce Damage to Living Structure in Soft Bottom Areas

Scientists from the the Conservation Engineering team of the MACE program have been working with the fishing industry to modify groundfish trawls to reduce their effects on the seafloor environment. We are initially focusing on areas with soft-bottom (sand and mud) substrates where most groundfish fishing occurs. In those areas, the seafloor features considered most likely to be both significant habitat elements and be vulnerable to fishing are sessile invertebrates such as anemones, ascidians, sponge, and basketstars. Because they have relatively low profiles and flexible bodies, trawl modifications that create more space between the trawl and the seafloor are being considered to reduce damage to these animals.

From 23 May to 7 June, MACE scientists compared the effects of conventional and modified sweeps (herding cables ahead of the trawl net) the on sessile invertebrates 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 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).

Map showing locations of sled and trawl tows
Figure 1.  Locations of sled and trawl tows during May June 2006 research on reducing seafloor effects of trawling.  Principal sites (A-D), Fishing boundary transition sites are indicated inside circles, skate nursery area in square.

At each site, experimental trawling created parallel tracks of four types of modified sweeps and two types of conventional sweeps. Modified sweeps had clusters of larger diameter disks at 30-foot (9.1-m) intervals, lifting the sweep cables above the seafloor. Conventional sweeps had the same diameter throughout, causing more continuous seafloor contact. Both disk diameter and sweep material were varied.

Photo of seafloor sled being launched
Figure 2.  Launching the seafloor sled with DIDSON sonar and camera with strobed lighting.  Photo by Carwyn Hammond.

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.

Having the seafloor sled also provided many opportunities for additional observations while running between stations. These included:

  1. Whenever possible (21 sites), the sled was deployed when crossing over stations (or points halfway between stations) that are sampled by the Bering Sea trawl survey. This will allow comparisons of trawl catches with direct observations of the seafloor as well as putting the study sites in the context of a wider range of comparable observations.
  2. At three locations, the sled was towed several times across boundaries set by fishery managers that created contrasts between heavily fished areas and adjacent unfished areas. Comparisons of seafloor conditions across the boundaries should indicate the resultant effects of fishing.
  3. The sled was towed at a series of eight stations at 5 nmi intervals north from Unimak Island, crossing an area of highest fishing intensities as well as adjacent nearshore habitats. A similar transect of three tows was made west from Unimak Island.
  4. A skate nursery area was crossed with the sled east-west and north-south. On the southern end of the later transect, very high densities of skate egg cases were observed. This supplemented previous trawl observations by confirming the nursery location and providing distribution information on a finer scale.

The next phase of this project will use twin trawls equipped with conventional and modified sweeps to determine whether they affect catch rates of commercial species. A pilot cruise in fall 2005 found this technique effective and detected no significant loss of target species while trawling on the Bering Sea slope.

By Craig Rose

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