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ICES Journal of Marine Science, 61: 53e63. 2004
doi:10.1016/j.icesjms.2003.10.001
RoxAnn bottom classification system, sidescan sonar and
video-sledge: spatial resolution and their use in assessing
trawling impacts
Odd-Børre Humborstad, Leif Nøttestad, Svein Løkkeborg,
and Hans Tore Rapp
Humborstad, O.-B., Nøttestad, L., Løkkeborg, S., and Rapp, H. T. 2004. RoxAnn bottom
classification system, sidescan sonar and video-sledge: spatial resolution and their use in
assessing trawling impacts. e ICES Journal of Marine Science, 61: 53e63.
Three complementary seabed characterization tools with different spatial resolution were
used to locate a research site and to assess physical effects of experimental otter trawling in
the Barents Sea: an acoustic seabed classification system (RoxAnn), sidescan sonar and a
video-sledge. The marine protected area (MPA) around Bear Island was chosen as it offered
unfished reference sites. The area was topographically complex which resulted in certain
challenges for choice of the experimental site due to the requirements of representativity and
homogeneity and suitable sampling substrate. Systematic waylines with RoxAnn gave broadscale patterns of bottom conditions, the more informative sidescan revealed topographic
reliefs, whilst detailed information on sediment composition and small-scale seabed features
was provided by the video-sledge. Accurate positioning of towed gears (trawl, sidescan and
video-sledge) ensured unbiased data acquisition. Trawl doors and rockhopper gear created
furrows that were visible by sidescan sonar and video. Intensive trawling also caused changes
in the acoustic properties by increasing roughness and decreasing hardness. Results are
consistent with a possible resuspension of the sediment and a homogenizing effect from the
trawl doors and ground gear ploughing the area. The suitability and advantages of using
spatially overlapping tools in trawl impact studies are discussed.
Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: impact studies, ITI positioning system, otter trawling, RoxAnn, sidescan sonar,
video-sledge.
Received 13 December 2002; accepted 3 October 2003.
O.-B. Humborstad, L. Nøttestad, and S. Løkkeborg: Institute of Marine Research, PO Box
1870 Nordnes, N-5817 Bergen, Norway. H. T. Rapp: Department of Fisheries and Marine
Biology, Bergen High-Technology Centre, N-5020 Bergen, Norway. Correspondence to
O.-B. Humborstad; tel: +47 55236939; fax: +47 55236830; e-mail: odd-boerre.
[email protected].
Introduction
The environmental effects of fishing have aroused a growing degree of interest during the past few decades (Hall,
1999). The most obvious effect of fishing has been the
decline in many major fish stocks (Myers et al., 1996), and
fishing down the marine food web (Pauly et al., 1998), but
secondary effects on non-target species and habitats have
also caused growing concern (Kaiser and De Groot, 1999).
Habitat alteration is believed to be having serious
consequences for many species of invertebrates and fishes
(see e.g. Jennings and Kaiser, 1998; Langton and Auster,
1999).
In order to meet the requirements of the fishing industry
and its need to exploit new and deeper areas with rougher
bottom conditions to meet the increasing competition for
limited fish resources, heavier gears have gradually evolved
1054-3139/$30
(Van Beek et al., 1990; Jones, 1992), which in turn leaves
fewer areas and habitat types undisturbed (Mortensen et al.,
2000). The Barents Sea covers approximately 1.2 million
km2, where bottom conditions vary widely from sand and
mud to coarse ground consisting of cobble and rock
(Figure 1), and which is the home of an important demersal
trawl fishery. The majority of impact studies to date have
been performed on relatively flat sandy and muddy
sediments (Collie et al., 2000), although some have
reported effects on rougher bottom types such as pebble,
cobble and boulders (Collie et al., 1997; Freese et al.,
1999). Established methods, tools and equipment for
studies of environmental effects of fishing on relatively flat
homogeneous bottoms may not therefore be applicable to
these more topographically complex habitats, and an
evaluation of such methods is therefore needed to ensure
unbiased data acquisition.
Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved..
54
O.-B. Humborstad et al.
Figure 1. The research site inside the 20 nm MPA around Bear Island and bottom types in the adjacent areas (reproduced with permission
from Elverhøi and Solheim, 1983).
An 18 month project with the aim of studying the impact
and recovery of demersal bottom trawling started in May
2000 in the Barents Sea. In order to perform a quantitative
BACI (before/after, control/impact) design for biological
sampling, the experimental area had to fulfil the requirement
of no influence by any recent previous fishing activity.
Furthermore the area should be representative for commercial trawling grounds in terms of species composition,
bottom habitat, topography, depth distribution and also be
suitable for biological sampling. The marine protected area
(MPA) around Bear Island in the Barents Sea (established
1977) was chosen as the only site, which could provide true
undisturbed areas for control. Here we describe and discuss
the resolution and spatial variability seen from the sidescan,
video-sledge and RoxAnn data, and how to use the
combined information for experimental site location, unbiased sampling and physical impact assessment. To the
best of our knowledge this is the first investigation examining the environmental effects of fishing on the seabed in the
Barents Sea.
Materials and methods
Experimental area
The experiment was conducted from 10 to 24 May 2000 at
74(30#N 18(12#E, 9 nautical miles west of Bear Island in
the Barents Sea (Figure 1). The exact location of the
experimental area was based on acoustic mapping from the
RoxAnn bottom classification system, sidescan sonar
recordings and video-sledge observations (see descriptions
below).
We selected one area (Ti) for intensive fishing with 10
continuous overlapping trawl tracks within a 200 m wide,
2.2 km long corridor, and one moderate fishing area (Tm)
with 10 continuous overlapping trawl tracks within a 600 m
wide, 2.2 km long corridor (Figure 2). All trawling was
done in the eastewest direction, evenly distributing effort
with parallel tracks inside the corridors. Three reference
areas of 100 m width (C1e3) with no fishing activity were
also located between the trawled corridors. The distance
between corridors was 200 m. Trawling was carried out by
a commercial trawler equipped with a standard bottom
trawl (Cotesi maxi 404/A) with Rockhopper gear (21$
diameter, 19 m long), bobbins (11$ diameter, 37.5 m on
each side towards the sweeps), sweeps (140 m), otter
boards (2300 kg each) and a door spread of approximately
140 m measured by Scanmar sensors.
Instrumentation
The ultrasonic processor, RoxAnn, offers real-time classification of seabed features via processing of signals from
the ship’s echo sounder. RoxAnn was connected to a Simrad
RoxAnn bottom classification system
55
Figure 2. Relative positions of the different corridors in the research area: Ti Z trawled intensively, Tm Z trawled moderately and C1e3 Z
control. Grids indicate sledge-sampling blocks. Outlined areas indicate approximate positioning of sidescan tows shown in Figures 4 and 7.
EK 500 scientific echo sounder connected to a 38 kHz splitbeam transducer positioned on the vessel’s centreboard, and
gathered data at 5-s intervals. RoxAnn discriminates between types of seabed material, and output data in a digital
format ready for computer analysis, E1 (roughness) and E2
(hardness), are obtained by integrating different parts of the
first and second echoes (Caddel, 1998). The system was
calibrated during a test survey in a fjord outside Bergen in
southwestern Norway where bottom conditions were
known in detail, giving the track of the ship a colour on a
mapping plotter that corresponded to the substrate below it
(Figure 3). Additional ground-truthing was subsequently
performed by acquiring sledge samples, video and sidescan
sonar recordings at the study site. At approximately 95 m
depth, the beam has a width of about 12 m and a footprint
of approximately 106 m2 (at 7( 3 dB point).
A Simrad MS992 sidescan sonar was used for detailed
monitoring of the seabed. A towfish operating at 120 kHz
was used for acoustic observations and the data were
recorded as hardcopy prints and DAT tapes. The towfish,
which carried sensors for depth, bearing, temperature, pitch
and roll, was towed about 10e30 m above the bottom and
collected high-resolution data on bottom structure and
topography from a sector covering about 200 m of the
seabed on each side. A Simrad Integrated Trawl Instrumentation (ITI) sensor was connected to the wire for
exact positioning.
The ITI system is a cordless trawl positioning and
monitoring system. The system, which was used to position
the trawl, video-sledge and sidescan sonar into the preselected corridors, is based on hydro-acoustic communication between a transducer mounted on the vessel and
a sensor mounted on the device to be positioned. The sensor
determined the position of the different devices relative to
the vessel by measuring the depth, the distance to the vessel
and the angular deviation of its position relative to the
heading of the vessel. The system is operated using a menu
of commands displayed on a colour video screen. The ITI
accuracy is G5 m in straight-line length calculation
between ship and sensor and G1( error in bearing (ITI
Technical specification).
Sampling
A van Veen grab was tested for possible use as the main
sampling tool, but it was soon recognized that it could not
assure quantitative samples as it collected small unequal
amounts of the substrate, and also suffered from the
inability of visual supervising. In order to catch more
mobile fauna an Agassiz trawl (a type of beam trawl) was
tested. However, this gear was not appropriately constructed and was smashed and destroyed during a tow on
coarse ground. A sledge (Sneli, 1998) was finally chosen to
provide quantitative samples of benthic assemblages. The
sampling sledge is designed to sample epifauna and parts of
the infauna in the upper few centimetres of soft bottom. The
sledge measures 200!80!20 cm and weighs about 80 kg.
In order to prevent the sledge from flipping over, 10 buoys
(12$) were connected on top of the sledge and four weights
(10 kg each) to the lower part. A camera and light was
placed in front of the sledge in order to monitor the seabed
habitat and sledge performance. Batteries and a recording
unit were placed on top of the sledge. Sledge samples data
will be used to analyse the effect of trawling on epifauna in
a future publication, whereas sledge data are used here for
ground-truthing purposes.
56
O.-B. Humborstad et al.
Figure 3. RoxAnn track lines showing different bottom types inside the 20 nm Bear Island MPA: dark grey Z hard-packed sand and mud;
white Z sand and gravel/stones. Dashed line indicates the selected research area.
Results
Seabed description
An area of approximately 6:8!4:4 km on the northwest
side of Bear Island was mapped using the RoxAnn system
with tracks 300e400 m apart zigzagging the area
(approximately 8% coverage). Track lines with different
colours indicated variations in substrate types within this
area (Figure 3). Hardpacked sand/mud and sand/gravel/
stones were the only bottom types in the area as revealed by
RoxAnn. The two bottom types were spatially separated
and a smaller research area of 1:9!2:3 km with a more
uniform bottom substrate was selected for further characterization. Additional tracks were conducted in the easte
west direction giving Ti an estimated coverage of 54% and
Tm 15%. No additional tracks were made in reference
corridors due to time constraints. The mean depth of the
research area was 94G7 m, and RoxAnn indicated
a substrate that consisted of sand and mud according to
the preset calibration. This area was divided into five
corridors as described above (Figure 2).
Observations from 16 sidescan tows in the eastewest
direction, covering the whole research site, showed that the
research area was not as uniform as the RoxAnn data
indicated, but consisted of 5e6 parallel ridges (30e90 m
wide) of rougher material oriented in a southwest/northeast
direction in Ti (Figure 4) and dispersed smaller patches in
Tm and reference corridors. By combining the sidescan
prints the bottom could be seen like a snapshot throughout
the research site.
Video and sledge samples revealed that the rougher areas
consisted of small stones and rocks, occasionally up to 1 m
diameter, in contrast to the areas in between which
consisted of mixed silt/sand, gravel and shell fragments
from Mya truncata and Balanus spp., often with a patchy
distribution along the direction of the haul. Few signs of
sessile three-dimensional building fauna were observed in
the areas in between the ridges, yet some structurally
forming fauna (erect hydroids, encrusting cirripeds and
bryozoans) was seen at the ridges which provided hard
substrate for their attachment. Visually recognized fauna
from video was mostly echinoids, ophiuroids, and a range
of mobile free-swimming crustaceans, cephalopods and
small pelagic and demersal fish escaping in front of the
sledge.
Sampling and positioning
The video recordings enabled us to perform a critical
evaluation of every sledge haul in addition to visually
confirming that the samples after trawling originated in an
impacted area since trawl marks were readily seen at
several locations. Patches of rough substrate (rocks and
stones), sledge speed (both too low and too high) and wave
action (during periods of fresh breeze with resulting wave
height up to 2.5 m) had an adverse effect on sledge
performance. In periods of harsher weather than fresh
breeze no sampling could be adequately carried out. Of
a total of 102 hauls, 65% were regarded as successful and
suitable for quantitative analyses.
Positioning the trawl in the corridors Ti and Tm was done
by inspection of real-time ITI positional data and gave
a 100% success rate. This exercise gave Ti trawl coverage
of 700% and Tm a coverage of 230%, calculated from the
distance between otter boards (140 m). The coverage
calculated from the width of the rockhopper gear and
bobbins (approximately 40 m) gave 200% and 67% for Ti
and Tm, respectively.
RoxAnn bottom classification system
57
Figure 4. Sidescan sonar recording showing parallel ridges of coarser ground with stones extending across the towing direction. Maximum
width of left and right ridges: 50 and 90 m, respectively. Approximate position of record shown in Figure 2.
The average speed of the sledge hauls was approximately
0.5 m s1 and the mean haul length was 65 m. No hauls
were made outside the corridors, nor were two hauls made
in the same position, so that all hauls were successful with
respect to location. The differences in pre-selected and
actual positions at bottom contact for the sledge hauls, as
determined by ITI, are given in Figure 5. In terms of
latitude the deviation was small, with 60% being less than
20 m away from the desired location. Along the towing
direction and the corridors’ longitude, 79% of hauls was
more than 60 m away from the pre-selected longitude.
Post-processing and immediate physical effects
Post-processing of pre-trawling RoxAnn data showed only
minor differences in mean values in roughness (E1) and
hardness (E2) values between the corridors (Table 1). These
parameters indicated a hard bottom devoid of any large
structures, although later ground-truthing observations altered this interpretation. Some of the variations, especially
the peaks in roughness in the intensive area (Figure 6),
could be explained by comparing it with the position of
ridges from the sidescan recordings (Figure 4). However,
hardness failed to show increasing values at the ridges. The
values of hardness were generally high before trawling, in
that 49% and 42% for the intensive (Ti) and moderate (Tm)
fishing areas, respectively, had values that reached the
upper voltage capacity (4.095 V) of the RoxAnn system.
Figures changed to 42% and 50% after trawling.
The spatial variation in E1 and E2 values corresponded
well before and after trawling (Figure 6). In Ti, trawling
caused an increase in surface relief (E1: sign-test,
z ¼ 4:5643, p!0:001) and a decrease in sediment hardness
(E2: z ¼ 3:8340, p!0:001). Mean difference overall in Ti
before and after impact was 0.01 and 0.11 V for E1 and E2,
respectively. In Tm, no differences between pre- and posttrawling values were found (E1: z ¼ 1:6431, p ¼ 0:10; E2:
z ¼ 0:5477, p ¼ 0:58). The number of observations from
control transects was too low to run any tests.
Sidescan sonar recordings showed no evidence of
physical disturbance prior to trawling. After trawling, the
tracks from otter boards were highly visible except at the
ridges. Parallel tracks could often be seen about 140 m
apart corresponding to the door spread. In several places,
smaller depressions made by the rockhopper gear were also
visible. In Figure 7b (lower part), at least 8 of a total of 10
hauls can be identified by the marks made by the otter
boards.
From the video, trawl door tracks were seen as U formed
depressions approximately 10 cm deep and 20 cm width
and an adjacent rounded berm of sediment set off at one of
the sides approximately 10 cm high. The rockhopper marks
evident from sidescan sonar could, however, not be
detected from the video with certainty.
Discussion
Area description
We first employed the RoxAnn bottom classification
system for systematic selection of a suitable homogeneous
(in order to reduce variability) research area on a large
spatial scale (kilometres). Although its performance has
been found to be dependent on vessel speed (Hamilton
et al., 1999), RoxAnn can be operated at high speed (10
knots in this study), and with wide spacing between track
lines it covers a large area in a short period of time
(Magorrian et al., 1995). A weakness was that with the
58
O.-B. Humborstad et al.
Figure 5. Deviation from pre-selected sledge positions measured by ITI. (a) Along towing direction (eastewest) and (b) perpendicular to
towing direction (northesouth). Note difference in scales for (a) and (b).
preset calibration of RoxAnn, the recordings indicated a flat
seabed consisting of hardpacked sand not showing the
patchiness of rougher material present in the experimental
site. However, post-processing of the RoxAnn data showed
some variations along the research site that could be
attributed to the ridges observed by sidescan and video
(Figure 6). This deficiency may be due to the size of these
patches being small relative to the size of the footprint
(106 m2) of the echo sounder beam, or to RoxAnn bottom
classes being difficult to define, as suggested by Hamilton
et al. (1999). The reason why the ridges did not show up as
harder bottom may be due to the system capacity, but also
growth of biota (erect hydroids, encrusting cirripeds and
bryozoans), which may decrease hardness. The use of
rectangular boxes for bottom classification is a very crude
way of grouping data and may result in misidentification
(Greenstreet et al., 1997). The RoxAnn system, however, is
cost-effective and has the advantage of offering a rapid
classification of the seabed structure. Furthermore, RoxAnn
data can be collected while the vessel is performing other
activities.
Sidescan sonar was primarily intended to assure that the
trawl disturbances had made the predetermined impact, and
to reveal how the sediment surface structure was affected by
the different components of the trawl. At least 8 tracks out
of 10 were observed in the intensively trawled corridor, and
it is likely that the latter two were disguised by consecutive
trawl hauls (Friedlander et al., 1999). By combining the
sidescan prints, the bottom could be seen like a snapshot,
enabling us to get a good impression of the actual bottom
contours and in turn aiding us when selecting areas for
sampling with the sledge by avoiding the coarse stony
areas. To give sonograms of sufficiently high resolution, the
sonar had to be towed close to the bottom at a speed of 3e5
knots. This operation demanded the data provided by the
RoxAnn on bottom topography and depth in order to plan
Table 1. Basic statistics from the RoxAnn survey before trawling in
the research area.
Corridor
Ti
C1
C2
C3
Tm
Valid n
Mean E2
St.d. E2
Mean E1
St.d. E1
2254
170
82
84
1906
3.908
3.448
3.663
3.755
3.836
0.281
0.414
0.386
0.316
0.320
0.225
0.205
0.276
0.300
0.298
0.067
0.053
0.070
0.101
0.087
RoxAnn bottom classification system
59
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7L
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7L
(+DUGQHVV
(5RXJKQHVV
7P
7P
(+DUGQHVV
P
P
HDVW
P
P
ZHVW
Figure 6. Transects of E1 and E2 values along the intensively (Ti) and moderately (Tm) trawled area before and after trawling. Note
difference in scale for E1 and E2.
waylines that would not be obstructed by rapidly changing
depth or structures on the bottom. Sidescan sonars cover
large areas in a short time (Brown et al., 2002) as RoxAnn
does, but its operation demands the full attention since it is
a vulnerable and expensive towed gear. Details on topography were better than revealed by RoxAnn, however,
post-processing of sidescan data is a complicated task that
involves image-processing software, and trawl tracks still
have to be counted manually (Friedlander et al., 1999).
Neither the sidescan nor the RoxAnn data provided
information about the actual composition of the seabed,
which could only be provided by ground-truthing video
observations and benthic samples. The video-sledge gave
the final and detailed characterization of the area consisting
of silt/sand and shell debris interspersed with longitudinal
patches of gravels and stones. This heterogeneous pattern
complicated sampling since only samples from the soft
bottom were thought to be quantitative, and the assemblages were likely to differ between different types of
bottom. Here, video was crucial in order to select only the
hauls made outside the stony areas. The samples from the
video-sledge also showed that the benthic assemblage was
suitable for quantitative analyses. In a preliminary study,
two sites further south were rejected, one because the fauna
was too sparse and the other because the animals found
could not be sampled quantitatively with the Sneli sledge.
Seabed description and impact assessment normally
involve one or more of the sampling techniques we applied
in this study (Kaiser and Spencer, 1994; Schwinghamer
et al., 1998; Tuck et al., 1998). Conventional sampling
may, however, provide an incorrect impression of the
distribution of epibenthic communities (Magorrian et al.,
1995). Our results clearly demonstrate that single tools
cannot provide a decisive basis alone without comparison
60
O.-B. Humborstad et al.
Figure 7. Sidescan sonar recordings in intensively trawled area: (a) before trawling and (b) after trawling. Circles indicate the same
structure. The white mid-panel reflects the distance from the towed fish to the bottom, and changes in this distance affect the brightness of
the recording and the area covered. Approximate position of records shown in Figure 2.
across observational platforms. The RoxAnn was capable
of a broad-scale, rapid mapping of potential areas for
conducting our experiment, and showed where the least
rough areas were situated within the MPA. The sidescan
revealed smaller features, although the height and size
could not be provided from this tool. Sediment samples had
to be collected in order to get detailed information. After
ground-truthing we could zoom out and generalize over
larger areas based on RoxAnn and sidescan data. With
RoxAnn, we could cover a larger area within the MPA
borders than with the other tools. Had we not used the
sidescan, we would not have been able to evaluate the
spatial distribution of the ridges that were not sufficiently
mapped by RoxAnn. Finally there would have been a great
risk of misinterpreting the spatial patterns of the area, if we
only had used the video-sledge, which was essential for
ground-truthing.
Positioning and sampling
Working in small research areas on tight time schedules
(Figure 2) in order to avoid spatial and temporal differences
between treatment and control sites (e.g. Morrisey et al.,
1992a, b; Underwood, 1992; Hewitt et al., 2001) make
accurate real-time observations of position of towed gears
a necessity. The positioning of the trawl relative to the
vessel has been shown to vary greatly between and within
hauls, depending on the vessel’s heading to wind and
current and on warp length (Engås et al., 2000). Therefore,
the ship’s position could not be used to determine the
position of the trawl and the sledge. Deviations of the
position of the sledge in the longitudinal direction were not
considered crucial, since no hauls overlapped and a good
coverage of the transects was achieved. However, precise
positioning in the latitudinal direction was crucial, as the
smallest corridors were only 100 m wide and therefore
substantial effort was put into this task. This was achieved
by towing the sledge along the corridors in the eastewest
direction.
Generally, grab samples have been chosen in impact
studies since they are known to be more quantitative than
sledges and other towed gears. However, grabs are not
suitable for sampling patchy distributed and low abundance
fauna (Bergman and Van Santbrink, 1994). The main
difficulty in obtaining quantitative sampling with towed
gears seems to accurately determine the time the gear is in
RoxAnn bottom classification system
contact with the seabed and the length of a tow (Rice et al.,
1982). Towing distances have previously been estimated by
odometer wheels (Collie et al., 1997; Prena et al., 1999),
although their accuracy is questionable due to their variable
performance on soft sediments (Carney and Carey, 1980;
Prena et al., 1996; M. J. N. Bergman pers. comm.). In our
study, distance measurements of sledge hauls were made by
post-processing of ITI data in conjunction with video
observations of the time with bottom contact, which
allowed us to get better and more reliable area estimates
compared to estimates based on the duration of the hauls.
61
1999), and 5 months after the trawl disturbance, the door
tracks could not be seen from either sidescan or video
recordings (authors’ observations). The shallow depth in
the experimental area suggests a possible weather induced
sediment transport (Pfirman, 1985; Amos and Judge, 1991
(cited in Solheim and Elverhøi, 1996)) in addition to
winnowing by strong currents in the area (Huthnance, 1981;
Solheim and Elverhøi, 1996), both factors contributing to
a possible high level of natural variation in the sandy
habitat. Sand is generally a mobile sediment type and long
lasting topographical features are not expected (DeAlteris
et al., 1999).
Post-processing and physical effects
Trawling was shown to cause physical disturbance in the
intensively (700%) and moderately (230%) trawled area.
Furrows and berms created by the trawl doors were clearly
visible both on video recordings (single trawl door tracks)
and sidescan sonograms (view of many tracks). Sidescan
sonar was also used to demonstrate door tracks in a similar
trawling experiment which was carried out on the Grand
Banks off Newfoundland (Schwinghamer et al., 1998) and
in other areas (Service and Magorrian, 1997; Tuck et al.,
1998; Friedlander et al., 1999). Reports of scouring depths
up to 0.3 m exist (Krost et al., 1990; Jones, 1992).
However, the depth and longevity of furrows are dependent
on the sediment type (DeAlteris et al., 1999), but also on
weight, cable length, angle of attack, door type and depth.
Reported scouring depths of trawl doors on sandy seabeds
are in the 0e5 cm range (Brylinsky et al., 1994; Gilkinson
et al., 1998). In our experiment the scouring depth was up
to 10 cm, and is consistent with a sediment type between
sand and mud, and the heavier trawl doors used in our
experiment.
Physical effects of trawling were also reflected in the
RoxAnn data in that overall hardness decreased after
trawling, indicating that the hardpacked shellsand was
resuspended and made less compact by the trawl (Churchill,
1989; Pilskaln et al., 1998). Most studies have concentrated
on sediment biota and studies that examine the contribution
of fishing to sediment resuspension are urgently required
(Kaiser et al., 2002), hence the RoxAnn system may be an
important tool for documentation of this phenomenon.
RoxAnn also indicated an increase in surface roughness,
and this observation was supported by the sidescan
sonograms that showed small depressions made by the
rockhopper gear in addition to the door marks. The ability
to detect any changes with RoxAnn was dependent on the
level of effort deployed, as consistent changes were not
demonstrated for the moderately trawled area. Changes in
sediment surface characteristics following intensive trawling were also demonstrated by RoxAnn data in similar
experiments conducted on the Grand Banks (Schwinghamer
et al., 1998) and in a Scottish sea loch (Tuck et al., 1998).
Effects of trawling are related to the level of natural
variations (Auster and Langton, 1999; DeAlteris et al.,
Conclusions
The need for spatial overlapping and complementary tools
and instrumentation, to be able to evaluate any possible
trawling impact on bottom fauna has been demonstrated.
We used three different tools all of which provided valuable
information within their respective resolutions, zooming
and covering across in an area that fulfilled the preset
requirements for the research site. Conclusions based on
observations with a single tool in isolation would have lead
to misinterpretation. In areas where detailed information of
bottom conditions is not known, or where the topography is
expected to vary, our approach should be appropriate. In
areas of known homogenous substrate (e.g. Prena et al.,
1999) this experimental setup may not be necessary for
appropriate sampling, but may well be used for quantification of physical effects (this study; Schwinghamer et al.,
1998). Accurate positioning of the towed equipments and
visual observations during sampling is crucial for unbiased
data acquisition.
Acknowledgements
We thank skippers and crew of survey vessels. Jon-Arne
Sneli for lending us the sledge. Tore Høisæter for planning
and experimental design. Bjørn Totland, Svein Floen and
Ingvald Svellingen for being helpful with instrumentation
and post-processing. Anne Britt Skaar Tysseland for preparation of the map. Students and volunteers for help during
cruises. The Norwegian Research Council for funding this
study.
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