Resource Assessment & Conservation Engineering (RACE) Division
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).
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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.
Figure 2. Launching the seafloor sled with DIDSON sonar and camera with strobed lighting. Photo by Carwyn Hammond. |
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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:
- 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.
- 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.
- 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.
- 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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