ICES Journal of Marine Science, 60: 658–661. 2003
doi:10.1016/S1054–3139(03)00052-3
Acoustic observation and assessment of fish in
high-relief habitats
K. Cooke, R. Kieser, and R. D. Stanley
Cooke, K., Kieser, R., and Stanley, R. D. 2003. Acoustic observation and assessment of fish
in high-relief habitats. – ICES Journal of Marine Science, 60: 658–661.
Acoustics present an alternative sampling strategy in areas characterized by steep slopes and
rugged terrain where fishing is impractical. However, when the interference between echoes
from fish targets and boundaries is severe, acoustic observations require careful interpretation
of the echo returns. This article outlines a method of generating a representative 3D model of
the bottom topography that can assist in near-boundary fish discrimination. Images provide
greater insight to echo source and highlight some of the difficulties associated with
classifying acoustic sign. The results emphasize the importance of good survey design aimed
at minimizing side-lobe interference and reducing acoustic-shadow zones.
Crown Copyright Ó 2003 Published by Elsevier Science Ltd on behalf of International Council for the
Exploration of the Sea. All rights reserved.
Keywords: acoustics, acoustic-shadow zones, assessment methods, mapping, 3D imaging.
K. Cooke, R. Kieser, and R. D. Stanley: Pacific Biological Station, Fisheries and Oceans,
Canada, 3190 Hammond Bay Road, Nanaimo, British Columbia, Canada V9T 6N7.
Correspondence to K. Cooke: tel.: þ1 250 756 7125; fax: þ1 250 756 7053; e-mail:
[email protected].
Introduction
Acoustic observation and assessment of fishes is highly
influenced by fish behaviour, target size, species mix, and
soundbeam characteristics as well as bottom proximity,
roughness, and scattering properties. In regions where the
acoustic-scattering properties of the substrate are unknown,
near-boundary detection problems suggest that the application of acoustics for biomass estimation of some demersal
species is problematic (Stanley et al., 2000) and, in some
cases, impractical (Richards et al., 1991; Stanley et al.,
1999).
This work emphasizes that better near-boundary detection requires knowledge of the 3D boundary shape as
well as acoustic parameters such as target size, beam
pattern, and beam attitude. We describe a sampling strategy
using conventional sounders for surveying an area where
acoustic bottom returns modified, obscured, or appeared
like returns from the target species (Stanley et al., 1999,
2000). Our methods help to visualize the acoustic data in a
way that leads to a better understanding of bottom structure
and of fish behaviour in relation to their habitat.
Background
The study site was located at the continental-shelf edge off
the west coast of Vancouver Island, British Columbia near
1054–3139/03/000658þ04 $30.00
Pisces Canyon, and had been termed ‘‘Pisces Pinnacles’’ by
local fishers (Figure 1). Their acoustic observations showed
‘‘dense rockfish schools’’, but these schools were extremely
difficult to fish. Their trawling efforts typically resulted in
lost or damaged gear and they were quick to caution that
‘‘what you see is not what you get’’. The area had long been
recognized by fishers as one that, despite its relatively innocent look, was considered virtually untrawlable (B. Mose,
pers. comm.). We agreed to examine the area acoustically
to assess what it was that appeared to be very dense aggregations of rockfish but was in fact ‘‘something more’’ than
just fish.
Methods
Acoustic system and data acquisition
Survey operations were carried out from the Canadian
Coast Guard Ship ‘‘W.E. Ricker’’, a 58-m stern trawler
using a calibrated SIMRAD EK500 sounder, ram-mounted,
38 Hz split-beam transducer (Foote et al., 1987; Simrad,
1993a; Cooke et al., 1996), and BI500 data logger (Simrad,
1993b). We sounded continuously during a 32-h period, 5–
6 February 1998, repeating a sector pattern constructed
of six lines bounded by an outer circle of about 2-nmi
diameter. Each line crossed a central area of intersection at
increments of about 30 . We introduced minor adjustments
Crown Copyright Ó 2003 Published by Elsevier Science Ltd on behalf of
International Council for the Exploration of the Sea. All rights reserved.
Acoustic observation and assessment of fish in high-relief habitats
659
Figure 1. The ‘‘Pisces Pinnacles’’ study site near Pisces Canyon off the west coast of Vancouver Island, British Columbia, with an
example of the sector survey pattern used.
to line spacing and course heading after each completed
circumference of the grid. Geo-referenced, ping-by-ping
volume-backscattering strengths (Sv) at 0.5-m depth resolution were continuously logged along the ship’s track to
a maximum range of 250 m.
Data processing and visualization
Sounder-detected, bottom-depth data were used to create an
interpolated surface and a series of 3D elevation maps to
visualize the bottom topography from various directions
and viewpoints (Golden Software, 1996). Echo-editing software (SonarData, 1999) was used to view echograms and to
identify ping number, range, Sv, and bottom-detected
values. Frame grabs of 2D and 3D bottom images and portions of zoomed echograms were stored in jpeg format
(Jasc Software, 2000) and overlaid using Powerpoint97
(Que, 1997) to show ping location and echo returns in relation to bottom structures. Since automatic bottom tracking was often intermittent when the trackline crossed the
steepest terrain, we pieced together manually a new ‘‘true
bottom’’ outline of the pinnacles based on maximum Sv
colour value observed along all transects. Our new ‘‘true
bottom’’ outline was superimposed on each of the passes
to provide operators with a better understanding of signal source, regardless of proximity of each pass to the
formations.
Results
Survey pattern and mapping
We completed five rotations of the grid and added two
separate lines for a total of 32 crossings over the central
area of intersection (Figure 1). All bottom-depth data were
used to generate an interpolated surface and create
multiple 2D and 3D views of the same ground (Figure 2).
Elevation could only be measured with limited accuracy
owing to the motion of the ship caused by wave heights in
excess of 3 m. No attempt was made to correct for roll and
pitch of the soundbeam. We chose to exaggerate the depth
scale to illustrate bottom features more clearly. The images
showed a series of closely spaced pinnacles with a maximum height of 12–15 m that rise sharply on all sides,
each from a base of about 20 m in diameter. The overall longitudinal extent of the feature is about 150 m with
a northwest–southeast orientation. Pinnacle separation varied; some were poorly defined given the relatively few
data points used to identify individual structures. Our
confidence in the reality of the maps is derived from the
overall density of measured depths, the repeatability of
their locations as shown from the various passes and
knowledge of the relative size of the soundbeam footprint.
The interpolated maps provide a visual appreciation of
the topography that is essential for improving echogram
interpretation.
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K. Cooke et al.
Figure 2. An example of a 3D interpolated elevation map with portions of echograms from two passes overlaid on a ‘‘true bottom’’ image
showing ping-by-ping trackline, bottom-detection locations, and Sv values relative to pinnacles. Circled areas indicate the region zoomed
for a ‘‘best fit’’ alignment of ping-by-ping positions. Pass 3303 ran parallel but adjacent to the longitudinal axis of the pinnacles; pass 3503
crossed directly over the middle section of the pinnacles. The yellow lines and markers represent sounder-detected bottom tracking for
each pass. The darker blue outline depicts the ‘‘true bottom’’ overlay we created to define more clearly acoustic sign near boundaries and
identify possible side-lobe signals.
Echogram editing
Our sounder was unable to track bottom continuously along
any transect that crossed directly over all or most of the
pinnacles owing to the steepness of the terrain (Figure 2,
Pass 3503 bottom track). Little of the high-density acoustic
sign observed along these tracks could be classified with
certainty, since there was little separation from the bottom
signal. For passes that ran near, but not over, the formations,
the sounder was able to track bottom (Figure 2, Pass 3303
bottom track). At first, we viewed much of the off-bottom
backscatter as typical of high-density fish aggregations.
Acoustic observation and assessment of fish in high-relief habitats
However, by overlaying the ‘‘true bottom’’ outline, we can
more clearly identify possible bottom-tracking limitations
and shadow zones associated with side-lobe echoes of the
formations. This new insight was the key for classification
of echo sign that appeared removed from bottom. Using
different perspectives of the same image, and by rotating
the images around the formations, we were better able to
identify transect bearings that were less affected by bottom
structure and soundbeam characteristics, and which were
more appropriately oriented for the detection of fish targets.
Zones of uncertainty could be better defined or removed
entirely from the integration process.
Discussion
Our images help explain possible reasons for the gear loss
experienced by the fishers and serve as a reminder that even
the most experienced of operators can easily misinterpret
acoustic sign. We were not able to ground truth the acoustic
signal but, nonetheless, our work illustrates that having
even a limited knowledge of bottom structures can help in
survey design and may provide a strong warning against
conducting fishing operations in regions of uncertainty. Our
results emphasize the need to map and preview study sites
and to incorporate all data in the scrutinizing process,
especially when near-boundary detection is attempted. In
the absence of high-resolution bathymetric data, a single
pass with a downward-looking multibeam sonar, or several
closely spaced transects with an echosounder could map
a narrow survey corridor and assess the acoustic suitability
of the area. Once identified, the corridors could be revisited
on subsequent surveys. This approach will be valuable for
the planning of assessment surveys for near-bottom fish in
areas with difficult bottom structure. Although ship time is
a major consideration, a single survey may be sufficient to
characterize an area and produce suitable maps that could
be used over the course of subsequent surveys.
Imaging of acoustic data in 3D is an effective tool for
examining target distributions (Greene et al., 1998) and
shoal behaviour (Stanley et al., 2002). This work describes
a new technique for mapping habitat and visualizing echo
returns in relation to bottom signal, side-lobe echoes,
and beam characteristics. Our results offer a greater under-
661
standing of echo source and improved interpretation of the
acoustic returns.
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