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ICES Journal of Marine Science, 62: 295e305 (2005)
doi:10.1016/j.icesjms.2004.11.013
Using fish-processing time to carry out acoustic
surveys from commercial vessels
Richard L. O’Driscoll and Gavin J. Macaulay
O’Driscoll, R. L., and Macaulay, G. J. 2005. Using fish-processing time to carry out
acoustic surveys from commercial vessels. e ICES Journal of Marine Science, 62:
295e305.
In some fisheries large factory freezer trawlers have periods of down time as the catch is
processed. By utilizing this time, scientific acoustic surveys can be carried out between
commercial-fishing operations without compromising fishing success. Examples are
presented from three acoustic surveys for hoki (Macruronus novaezelandiae) in New
Zealand waters during 2002 and 2003 conducted from a commercial vessel fitted with
a scientifically calibrated SIMRAD ES-60 echosounder. These surveys confirmed the
presence of a new spawning area for hoki and provided biomass estimates from known
fishing grounds. The approach described works well for small-scale acoustic surveys
adjacent to areas of high catch rates and is cost-effective because the vessel ‘‘pays for itself’’
by fishing commercially. The major limitation is that the boundaries of the survey area are
determined by the time available during processing, which is related to the size of the catch
and the time required to search for a suitable location for the next commercial trawl. In the
New Zealand hoki surveys, processing time was typically 3e8 h, which was sufficient to
carry out about 10e70 km of acoustic transects. Acoustic research was also limited to
periods of relatively good conditions by the use of a hull-mounted transducer.
Ó 2004 Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.
Keywords: acoustic survey, commercial fishing, hoki, New Zealand.
Received 21 June 2004; accepted 9 November 2004.
R. L. O’Driscoll and G. J. Macaulay: National Institute of Water and Atmospheric
Research, Private Bag 14-901, Kilbirnie, Wellington, New Zealand. Correspondence to
R. L. O’Driscoll: tel: C64 4 386 0300; fax: C64 4 386 0574; e-mail: r.odriscoll@
niwa.co.nz.
Introduction
Acoustic surveys are used worldwide to provide estimates
of fish and zooplankton abundance. Most acoustic surveys
are carried out from research vessels using specialized
scientific echosounders. However, echosounders capable of
collecting scientific-quality data have recently become
more widely available and are being installed on commercial fishing vessels. This provides the opportunity to collect
acoustic data from these vessels in support of a range of
research objectives. Acoustic data collected from commercial vessels have been used in studies of herring (Clupea
harengus) in eastern Canada (Claytor and Clay, 2001;
Melvin et al., 2002), widow rockfish (Sebastes entomelas)
in British Columbia (Stanley et al., 2000), jack mackerel
(Trachurus symmetricus murphyi) in Chile (Hector
Peña, Institute of Marine Research, Norway, pers.
comm.), walleye pollock (Theragra chalcogramma) in
Alaska (Martin Dorn and Steven Barbeaux, Alaska
Fisheries Science Center, pers. comm.), hoki (Macruronus
1054-3139/$30.00
novaezelandiae) and orange roughy (Hoplostethus atlanticus) in New Zealand (Kloser et al., 2000; Hampton and
Soule, 2003; O’Driscoll, 2003; Soule and Hampton, 2003;
O’Driscoll et al., 2004a) and Australia (Kloser et al., 2001;
Ryan and Kloser, 2002), southern blue whiting (Micromesistius australis) in New Zealand (O’Driscoll and
Hanchet, 2004), and toothfish (Dissostichus mawsoni) in
Antarctica (O’Driscoll and Macaulay, 2003).
Commercial vessels have several advantages over research vessels as platforms to collect acoustic information.
First, there are a large number of commercial vessels
available, so their spatial and temporal capacity for data
collection may exceed that of national research vessel
fleets. Second, commercial vessels are more cost-effective
than research vessels if research activities can be combined
with commercial fishing which covers part or all of the
running cost of the vessel. Finally, use of commercial
vessels requires communication and collaboration between
fishers and scientists, which is usually beneficial to both
parties (Rose, 1997).
Ó 2004 Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.
296
R. L. O’Driscoll and G. J. Macaulay
There have been three basic strategies for the collection
of acoustic data from commercial vessels: i) undirected
monitoring of the vessel’s acoustic instruments
(echosounder or sonar) during fishing operations (e.g.,
O’Driscoll and Macaulay, 2003); ii) using commercial
vessels as ‘‘scouts’’ to locate aggregations, which are then
surveyed by a research vessel (e.g., Stanley et al., 2000);
and iii) directed acoustic surveys using the commercial
vessel as the primary research platform (e.g., Hampton and
Soule, 2003).
Undirected data collection is useful for studying the
behaviour of fishers and fish, and can provide information
on mark types from new areas, but it is difficult to generate
abundance estimates from this type of data. A notable
exception is in the Canadian herring fishery, where acoustic
data collected from gillnet and purse-seine vessels have
been used to develop relative abundance indices suitable for
stock assessment (Claytor and Allard, 2001; Claytor and
Clay, 2001). Directed research surveys which follow a predetermined survey design are usually more suitable for
biomass estimation, but the value of using a commercial
vessel instead of a research vessel may be reduced if the
vessel needs to stop its commercial-fishing operation to
carry out the acoustic survey work.
In New Zealand, large (40e80 m) factory freezer trawlers
participate in spawning fisheries for hoki, orange roughy,
and southern blue whiting. These vessels target catches of
10e30 t, which are filleted and frozen at sea. Typically
vessels can process 2e5 t green weight of fish per hour
depending on the species and vessel. During the spawning
period, catch rates are usually high, so the vessel does not
have to deploy the trawl again immediately following
a successful haul. This creates a window of ‘‘down time’’
during processing, when the vessel is not actively fishing.
During this processing period, the vessel usually attempts to
maintain contact with a known aggregation of fish or
searches for new aggregations prior to the next trawl.
By utilizing the down time during processing, we have
been able to carry out directed acoustic surveys, without
compromising commercial-fishing success. This enables
research to be carried out relatively cheaply as part of
commercial fishing operations, allowing acoustic surveys of
new areas, or at more frequent intervals than those carried
out on research vessels. In this paper, we illustrate the
utility of this approach using examples from three acoustic
surveys for hoki during 2002 and 2003.
Methods
Background to hoki acoustic surveys
Hoki form New Zealand’s largest fishery, with annual
catches of 160 000e270 000 t since 1987 (O’Driscoll et al.,
2004b). Two stocks of hoki are recognized based on
morphometric and growth rate differences (Livingston and
Schofield, 1996), and these have been assessed separately
35 S
40
West coast
South Island
Cook Strait
Hokitika
Canyon
Chatham Rise
45
Pegasus Canyon
50
Campbell Plateau
200 km
55
165 E
170
175
180
175
170 W
Figure 1. A map of New Zealand showing the location of the main
spawning and feeding areas of hoki and the acoustic-survey areas
(boxes). Depth contours are 500 m (dotted line) and 1000 m (solid
line).
since 1990. The western stock resides primarily on the
Campbell Plateau, south of New Zealand, and spawns on
the west coast of the South Island (Figure 1). The eastern
stock’s ‘‘home ground’’ is the Chatham Rise, with
spawning occurring mainly in Cook Strait (Figure 1).
Juvenile hoki of both stocks mix together on the shallower
areas of the Chatham Rise, and are believed to recruit to
their respective stocks at maturity-at-age 3e8 (Livingston
et al., 1997).
Acoustic surveys from research vessels have provided
abundance indices for spawning hoki on the west coast
South Island and in Cook Strait since 1988 (Coombs and
Cordue, 1995; O’Driscoll, 2002), and these are an important
input to the stock assessment model used to set the total
allowable commercial catch (Francis, 2004). Hoki also
spawn in other areas (e.g., Livingston, 1990), but there was
little information about the magnitude of spawning aggregations away from west coast South Island and Cook Strait.
In 2002, a New Zealand fishing company, Independent
Fisheries Limited, approached us with anecdotal evidence
that marks of spawning hoki in Pegasus Canyon on the east
coast of the South Island (Figure 1) were increasing. As
part of a collaborative initiative, funded by the consortium
of hoki-quota holders (The Hoki Fishery Management
Company Ltd), we carried out the first acoustic survey of
this area from 2 to 11 September 2002 using the
commercial fishing vessel ‘‘Independent 1’’. This survey
was repeated in JulyeSeptember 2003.
The major hoki fishery is on the west coast South Island,
accounting for almost half the catch in recent years
(O’Driscoll et al., 2004b). The last acoustic survey of the
Fish-processing time to carry out acoustic surveys
west coast South Island spawning grounds was in 2000
(Cordue, 2002), and there are no plans for future research
vessel surveys in this area. We carried out a pilot acoustic
survey from 1 to 16 August 2003 from ‘‘Independent 1’’ to
determine the feasibility of using a commercial vessel to
survey spawning hoki on the west coast South Island. We also
aimed to obtain estimates of the size of the main hoki
spawning aggregation in Hokitika Canyon (Figure 1) in 2003.
Survey vessel and acoustic equipment
‘‘Independent 1’’ is a 45.6 m factory freezer, stern trawler,
fitted with a SIMRAD ES-60 echosounder with a hullmounted 38-kHz split-beam transducer. The echosounder
was calibrated on 6 August 2002 and on 26 July 2003, using
standard scientific methods (Foote et al., 1987). Details of
the acoustic system and its calibration are provided in
Table 1. Calibration coefficients were similar (within
0.3 dB) between the two calibrations.
Survey design
The survey design in both areas followed the methods of
Jolly and Hampton (1990) as adapted by Coombs and
Cordue (1995) to obtain a biomass index for transient fish
Table 1. Set-up and calibration data of the acoustic system used in
hoki acoustic surveys from ‘‘Independent 1’’ in 2002 and 2003.
Echosounder
Transducer
Operating frequency
Bandwidth
Transmit power
Pulse length
Ping interval
Sample interval
Two-way beam angle
Gain
2002
2003
Sa correction
2002
2003
Absorption (a)
Pegasus Canyon
Hokitika Canyon
Sound velocity
3 dB beam width
Alongship
Athwartship
Angle sensitivity
Alongship
Athwartship
Angle offset
Alongship
Athwartship
Time-varied gain
SIMRAD ES-60
ES38B
38 000 Hz
2 425 Hz
2 000 W
1.024 ms
2.0 s
0.192 m
20.6 dB re 1 steradian
25.17 dB
25.43 dB
0.68 dB
0.78 dB
9.27 dB km1
8.86 dB km1
1 500 m s1
7.0(
7.0(
21.9
21.9
0.04(
0.21(
20 log R C 2aR
297
populations. It was similar to that used successfully in
previous research-vessel acoustic surveys on the west coast
South Island and in Cook Strait (Coombs and Cordue,
1995; O’Driscoll, 2004). Hoki have a long spawning season
from July to early September and it is thought that during
this period there is a turnover of fish on the spawning
grounds (Coombs and Cordue, 1995). We therefore aimed
to conduct a number of sub-surveys or ‘‘snapshots’’ of the
areas during each survey to obtain estimates of the
spawning biomass at different times.
The timing of spawning in Pegasus Canyon is uncertain.
In 2002, the survey was carried out over a short time period
relatively late in the spawning season (2e11 September). In
2003, the survey design was revised to enable acoustic
snapshots to be spread over a wider period of time.
‘‘Independent 1’’ passed through Pegasus Canyon about
every 25 days on the way to fishing on the west coast South
Island and in Cook Strait. Snapshots were carried out at the
beginning or end of trips in July to September. This timing
overlapped with a research vessel acoustic survey of
spawning hoki in Cook Strait from 17 July to 28 August
2003 (O’Driscoll and McMillan, 2004). Estimates of the
timing of spawning on the west coast South Island were
available from previous acoustic surveys and from commercial catches (Harley, 2002). These data suggested peak
spawning in early August. Accordingly, the survey of
Hokitika Canyon was carried out between 1 and 16 August.
Randomly allocated parallel transects were used to
estimate the mean fish density in each snapshot. Survey
boundaries were based on depth contours with transects
oriented to run across canyon features in water depths
greater than 200 m in Pegasus Canyon, and 300 m in
Hokitika Canyon (Figure 2). Hokitika Canyon was divided
into two strata because fishing in the inner canyon, within
25 nautical miles of the coast, is restricted to vessels under
46 m total length. A total of 9e11 transects were allocated
to each snapshot of Pegasus Canyon, eight transects to each
snapshot of inner Hokitika Canyon (Stratum 5A), and three
transects in the outer canyon (Stratum 5B) (Figure 2).
Acoustic survey work was conducted in the processing
time between commercial trawls as described earlier.
Transects were run at 7e10 knots. It was not usually possible
to complete an acoustic snapshot within a single processing
window, so adjacent transects within an area were sometimes
separated by several hours viz. the time required to complete
a trawl and return to the next transect. To determine whether
this strategy might bias abundance estimates, we conducted
an experiment to investigate small-scale (hourly) temporal
variation in the hoki distribution in Hokitika Canyon. During
this experiment, a single 9.3 km transect (see Figure 2) was
run 28 times over a period of 30 h.
Trawling
Commercial trawls were used for mark identification and
the collection of biological data. The positions of tows were
298
R. L. O’Driscoll and G. J. Macaulay
A Seabird SM-37 Microcat conductivity-temperaturedepth (CTD) datalogger was mounted on the headline of the
net during five trawls in Pegasus Canyon and eight trawls in
Hokitika Canyon to collect temperature and salinity data,
which were then used to estimate the acoustic absorption
coefficient during the survey using the equation of Doonan
et al. (2003).
Pegasus Canyon
Acoustic data analysis and biomass estimation
10 km
Hokitika Canyon
25 n. mile
boundary
Stratum 5B
Stratum 5A
10 km
Figure 2. An enlargement of the acoustic survey areas in Pegasus
Canyon and Hokitika Canyon, respectively, with typical transect
allocation. Stars indicate the start positions of the commercial
trawls carried out by ‘‘Independent 1’’ during the surveys. The
thick dashed line in Stratum 5A of Hokitika Canyon shows the
transect run repeatedly to investigate the temporal variability in
hoki density. Depth contours are 500 m (dotted line) and 1000 m
(thin solid line).
determined by the fishing officers (captain and first mate)
and were usually targeted on relatively dense marks thought
to contain hoki. Trawl catch weights and species composition were estimated from the vessel trawl-catch-effortprocessing-returns, which give a tow-by-tow breakdown of
the catch and are completed by the vessel crew as a legal
requirement. A random sample of 100e200 hoki from
every tow was measured, and the sex and macroscopic
gonad stage determined.
Acoustic data were analysed using standard echo-integration methods, as implemented in the New Zealand National
Institute for Water and Atmospheric Research Echo
Sounder Package (ESP2) software (McNeill, 2001). A
systematic, ping-induced variation (triangular wave of 1 dB
amplitude with a period of 2721 pings) was identified in
ES-60 data in 2003 (Ryan and Kloser, 2004), and a modified
version of the ESP2 software which removed this error was
used for the analysis.
Echograms were visually examined, and the bottom
determined by a combination of an in-built bottom tracking
algorithm and manual editing. Regions corresponding to
various acoustic mark types were then identified. Marks
were classified subjectively, based on their appearance on
the echogram (shape, structure, depth, strength, etc.), and
using information from commercial trawls. Backscatter
from marks (regions) identified as hoki was then integrated
to produce estimates of the mean area-backscattering
coefficients (Sa) for each transect. No species decomposition of acoustic backscatter was carried out, i.e., backscatter
from all hoki regions was assumed to be 100% hoki. This
was a reasonable assumption because commercial and
research fishing on spawning hoki aggregations typically
results in very clean catches of hoki, with little or no
bycatch. Transect Sa estimates were converted to hoki
biomass using the ratio of mean weight to meanbackscattering cross-section, the linear equivalent of target
strength, for hoki. This ratio was calculated from the scaled
length frequency distribution of hoki from commercial
trawls during the survey. Acoustic target strength (TS in
dB) was derived using the TSelength relationship:
TS Z 18 log10 (L) 74 (Macaulay, 2001); and mean
hoki weight (w in kilogrammes) was determined from
the lengtheweight relationship: w Z 0.00000479 L2.89
(Francis, 2003), where L is fish total length in centimetres.
Biomass estimates and variances were obtained for each
stratum and snapshot using the formulae of Jolly and
Hampton (1990), as described by Coombs and Cordue
(1995). The snapshots were averaged to obtain the biomass
index (O’Driscoll, 2002). The sampling precision of the
biomass index was calculated by assuming the snapshot
biomass estimates are independent and identically distributed random variables. The sample variance of the snapshot
means divided by the number of snapshots is therefore an
unbiased estimator of the variance of the index, the mean of
the snapshots. Note that the sampling precision will greatly
Fish-processing time to carry out acoustic surveys
299
underestimate the overall survey variability, which also
includes uncertainty in survey timing, target strength,
calibration, and mark identification (O’Driscoll, 2004).
Table 2. Values for estimating noise from the SIMRAD ES-60
echosounder and hull-mounted transducer on ‘‘Independent 1’’.
Note that Si and Z are estimated from values for other ES38B
transducers.
Noise trials
Parameter
One noise trial was carried out from ‘‘Independent 1’’
during the 2002 survey and two noise trials were carried out
in 2003. The 2002 trial was carried out at vessel speeds of 8
and 10 knots in calm sea conditions in Pegasus Canyon
over bottom depths greater than 500 m. The first trial in
2003 was also at 10 knots in calm conditions in Pegasus
Canyon over a sloping bottom from 300e845 m. The
second trial in 2003 was at 7 and 10 knots in Hokitika
Canyon over bottom depths of 500e600 m, when the vessel
was pitching into a 2 m swell. In all noise trials, the ES-60
echosounder was operated in passive mode with a ping
interval and duration of 2.0 s and 1.024 ms, respectively
(see Table 1). Data were recorded to about 1500 m range.
The noise levels were calculated based on the ‘‘SA
method’’, taken from the SIMRAD EK500 manual
(SIMRAD, 1993), using the following algorithm:
NLZSi C10log10
PTX sA tp
2TLC10log10 w 75;
ZL
Si (dB re 1 mPa per A)
PTX (W)
tp (ms)
Z (U)
L (m)
2TL (dB)
10 log10 w (dB)
Value
209
2 000
1.024
16
20
78.5 (Pegasus Canyon)
77.7 (Hokitika Canyon)
20.6
the survey days. Echogram quality deteriorated markedly
when there was more than about 25 knots of wind and 2 m
of swell, and there was about four days when conditions
were deemed too rough for acoustic data collection.
Time available for acoustic-survey work
ð1Þ
where NL is the noise level (dB re 1 mPa) and sA is the
nautical-area scattering coefficient estimated by integrating
a 20 m layer from 990e1010 m. Si is the transducer
transmitting response (dB re 1 mPa per A); PTX, the
transmitter power (W); Z, the impedance for all four
transducer quadrants in parallel (ohms); 2TL, the two-way
transmission loss at a range of 1000 m; L, the layer thickness
(m); 10 log10 w, the equivalent two-way beam factor (dB);
and tp, the pulse length (ms). The two-way transmission loss
at range, R (m), is given by 2TLZ20log10 RC2aR, where
a is the absorption loss (dB m1).
The transducer-specific parameters (Si and Z) for the
transducer on the ‘‘Independent I’’ were unavailable and so
data for other SIMRAD ES38B transducers were obtained
and used to estimate these values. Absorption-loss values
(a) were calculated from CTD data from the two areas (see
Table 1). PTX, 10 log10 w, and tp values were recorded in
the ES-60 data files. A summary of the parameters is given
in Table 2.
Results
Acoustic data were recorded during 38 days at sea on
‘‘Independent 1’’ in 2002 and 2003. Seven acoustic
snapshots of Pegasus Canyon were completed during the
2002 survey (Table 3). In 2003, four snapshots of Pegasus
Canyon (Table 3), eight snapshots of inner Hokitika
Canyon (Stratum 5A), and three snapshots of outer
Hokitika Canyon (Stratum 5B) (Table 4) were carried
out. Sea and weather conditions were good (swell height
less than 1.5 m, windspeed less than 20 knots) for most of
‘‘Independent 1’’ targeted about 15 t of hoki per trawl to
maximize product quality and processing efficiency. A
SIMRAD ITI trawl-management system allowed accurate
catch estimation, and catches were usually close to the target
level (n Z 110 trawls, mean Z 14 t, range Z 3e25 t). Most
tows were of 15e30 min duration. The catch in both
Pegasus and Hokitika Canyons was predominantly spawning
hoki, with low levels of bycatch (Pegasus Canyon: n Z
42 trawls, mean Z 97% hoki by weight, range Z 75e100%;
Hokitika Canyon: n Z 68 trawls, mean Z 97% hoki,
range Z 83e100%). Processing time was related to the size
of the catch (Figure 3a). The time between successive trawls
was typically 3e8 h for catches of 10e20 t (Figure 3a). This
usually allowed 10e70 km of acoustic transects to be
completed (Figure 3b) and time for the vessel to return to
the area of the next trawl.
Not all of the time between trawls was available for
acoustic survey work. It typically took 1e2 h to locate and
‘‘line up’’ a suitable mark and then shoot the trawl. This is
why no acoustic transects were carried out when the time
between successive trawls was less than 2 h (Figure 3b).
The variability in the relationship between processing time
and the length of acoustic transects completed (Figure 3b)
was due to the varying distance between the location of
trawls and the start of the next acoustic transect.
Commercial trawls were usually concentrated in the part
of the survey area that gave best catch rates. For example,
all 68 trawls carried out from ‘‘Independent 1’’ in Hokitika
Canyon were in Stratum 5A, and most tows were on the
southern side of the canyon (see Figure 2). The inner
transects in Stratum 5A were adjacent to the area of commercial fishing, so almost all of the available time could be
spent running these acoustic transects. However, transects in
300
R. L. O’Driscoll and G. J. Macaulay
Table 3. A summary of the acoustic surveys carried out in Pegasus Canyon in 2002 and 2003. ‘‘No. of trawls’’ is the number of commercial
tows carried out during each snapshot.
Year
Snapshot
Start time
End time
No. of transects
No. of trawls
Hoki biomass (’000 t)
CV (%)
2002
1
2
3
4
5
6
7
3 September 05:25
3 September 21:50
5 September 13:01
7 September 08:35
8 September 06:53
9 September 07:39
10 September 15:12
3 September 20:02
5 September 05:10
6 September 16:40
8 September 01:03
9 September 02:55
9 September 14:24
11 September 06:36
9
8
9
10
9
9
11
2
5
6
5
3
2
4
84
96
79
140
90
165
44
31
29
21
50
40
55
15
100
17
56
108
109
45
18
31
33
12
79
21
Mean
2003
1
2
3
4
30
18
22
11
July 08:21
August 15:11
August 04:40
September 10:00
31
18
23
12
July 04:51
August 23:07
August 00:16
September 01:40
10
10
10
10
5
0
6
4
Mean
Stratum 5B were up to 40 km away, and the time required to
steam to and from the transect was much greater than the
time required to run the transect itself. In practice, the
captain of the vessel modified his fishing practice to allow
outer transects to be run (Allan Dillon, Independent
Fisheries Limited, pers. comm.), by targeting a larger catch
(20e25 t) as close as possible to the transect position.
Acoustic biomass estimates
Hoki biomass estimates by snapshot are given in Tables 3
and 4. The average biomass index from Pegasus Canyon
was 100 000 t in 2002 and 79 000 t in 2003 (Table 3). It is
difficult to compare biomass estimates between years
because of the different timing of the two surveys.
Snapshots on 11 September in both years gave very similar
estimates of abundance (Table 3). Sampling precision (CV)
of individual snapshots in Pegasus Canyon ranged between
12% and 55% (Table 3). The variance of the biomass
estimates was 17% from the seven snapshots in 2002 and
21% from the four snapshots in 2003.
The estimates from Pegasus Canyon were 35% and 43%
of the acoustic-biomass indices from research surveys in
Cook Strait in 2002 and 2003, respectively (O’Driscoll and
McMillan, 2004), confirming the importance of Pegasus
Canyon as a spawning area for eastern stock hoki. The
overlapping timing of the two surveys means it is unlikely
Table 4. A summary of the acoustic survey carried out in Hokitika Canyon in 2003. Stratum boundaries are shown in Figure 2.
Stratum
5A
Snapshot
Start time
End time
No. of transects
No. of trawls*
Hoki biomass (’000 t)
CV (%)
1
2
3
4
5
6
7
8
1 August 18:33
3 August 23:25
5 August 13:19
6 August 13:32
8 August 09:13
9 August 19:43
12 August 03:00
14 August 09:36
2 August 10:01
5 August 12:01
6 August 12:57
8 August 06:31
9 August 16:11
11 August 06:36
14 August 09:22
14 August 23:47
8
8
8
8
8
8
8
8
7
8
6
9
7
10
10
3
96
113
109
120
97
68
52
58
65
56
50
30
39
36
13
37
89
10
10
17
27
66
2
31
18
27
Mean
5B
1
2
3
Mean
2 August 12:41
5 August 20:22
11 August 07:24
3 August 21:32
6 August 00:33
12 August 02:21
3
3
3
0
0
0
*
Eight further trawls were carried out following the completion of snapshot 8 from 15 August 02:45 to 16 August 21:01 when the weather
was too rough for acoustic data collection.
Time between trawls (h)
Fish-processing time to carry out acoustic surveys
16
Table 5. The time-series of acoustic estimates of spawning hoki
biomass in Hokitika Canyon from 1988 to 2000. Biomass indices
from the 2003 survey were calculated using a sound absorption of
8.0 dB km1 to make them comparable with earlier surveys.
(a)
12
8
Biomass (’000 t)
4
0
0
10
20
30
Hoki catch (t)
Acoustic transect length (km)
301
80
(b)
60
Year
Stratum 5A
Stratum 5B
1988
1989
1990
1991
1992
1993
1997
2000
2003
39
94
100
124
17
201
144
69
77
47
31
29
32
15
32
57
51
15
40
2000 and the equal lowest estimate in the time-series
(Table 5).
20
0
0
5
10
15
Time between trawls (h)
Figure 3. The relationships between: (a) hoki catch and the time
between successive commercial trawls, and (b) time between
trawls and the total length of the acoustic-survey transects carried
out in that time. Note that the length of the transects does not
include steaming to and from transects or the distance between
transects.
that the fish in Pegasus Canyon were the same fish observed
spawning in Cook Strait, and fish in spawning condition
were observed simultaneously at both sites.
Hoki biomass in the inner Hokitika Canyon (Stratum 5A)
was 96 000e120 000 t for the first five snapshots in 2003,
declining to 52 000e68 000 t in snapshots six, seven, and
eight (Table 4). All eight snapshots were averaged to obtain
the abundance index for Stratum 5A of 89 000 t. Sampling
precision of individual snapshots ranged between 13% and
65% (Table 4). The variance of the biomass estimates from
the eight snapshots was 10%. Hoki biomass in outer
Hokitika Canyon (Stratum 5B) was estimated as
10 000e27 000 t in three snapshots with snapshot CVs of
2e66% (Table 4). The mean estimate for Stratum 5B was
18 000 t with a variance of 27%.
To compare biomass estimates for Hokitika Canyon with
those from the previous west coast South Island acoustic
surveys, we re-integrated the 2003 data with an absorption
coefficient of 8.0 dB km1, which was the value, based on
the formula of Fisher and Simmons (1977), used in
1988e2000 (Cordue, 2002). Estimates with the old
absorption coefficient (Table 5) were about 87% of the
estimates in Table 4. The biomass index from Stratum 5A
in 2003 was similar to the equivalent estimate from this
stratum in the 2000 survey (Table 5). However, the
estimate for Stratum 5B was only 30% of the estimate in
Temporal variation in hoki density
Visual examination of echograms from the 28 repetitions
of the same experimental transect showed strong and
consistent diurnal behaviour. Hoki occurred close to the
bottom on the southern side of the canyon during the day,
and were higher off the bottom on the north side at night. At
dawn and dusk there was a transition period, where hoki
were more dispersed and occurred along the whole transect.
These changes in hoki distribution are presented
quantitatively in Figure 4, which shows mean hoki height
(distance above bottom) and mean hoki latitude over time.
During the day, hoki were an average of 30 m above the
bottom and close to the southern end of the transect at
42(36.6#S, the transect running from 42(33.6# to
42(37.2#S. At night, hoki were about 70 m off the bottom
and further to the north, at about 42(35.6#S.
There was a marked change in estimated hoki density
associated with these diurnal changes in vertical and
latitudinal distribution (Figure 4a). Mean hoki density
along the experimental transect during the day (defined as
07:30 to 18:00 NZST; number of transects, n Z 12; mean
area-backscattering coefficient, Sa Z 0.000036) was almost
three times lower than mean density at night (n Z 16;
Sa Z 0.000099). We were concerned that the difference in
density estimates may be because hoki were somehow less
detectable acoustically during the day (e.g., very close to
the bottom in the acoustic deadzone, or lower acoustic
target strength due to changes in fish orientation). Results
from our survey transects did not indicate that this was the
case. There was no systematic dayenight difference in
transect density estimates during the eight biomass snapshots of Stratum 5A. Of the eight highest transect densities,
four were observed during the day and four at night. Rather,
diurnal differences in mean hoki density along the
302
R. L. O’Driscoll and G. J. Macaulay
0.00016
(a)
Mean Sa
0.00012
0.00008
0.00004
0.00000
0:00
8:00
16:00
0:00
8:00
Mean height above bottom (m)
Time (NZST)
100
(b)
80
60
40
Discussion
20
0
0:00
8:00
16:00
0:00
8:00
Time (NZST)
Mean latitude (42° + y minutes S)
noisier than the data collected in Pegasus Canyon. This was
because of the difference in sea conditions at the time of the
experiments. Data from Pegasus Canyon in 2002 and 2003
were collected in calm conditions, but the recording on the
west coast South Island was made when the vessel was
pitching into a 2 m swell.
Trials at different speeds indicated that ‘‘Independent 1’’
was quieter at 10 knots than at slower speeds. The reduction
in the noise level with increasing vessel speed was more
apparent in the rougher conditions during the trial in
Hokitika Canyon. This was probably due to the physical
location of the transducer and the trim of the vessel. The
transducer was located well forward and the vessel tended
to be trimmed bow-up (up to 5() to facilitate the draining of
the factory deck. At higher speeds the bow ‘‘dug in’’,
levelling the trim, reducing pitching, and increasing the
depth of the transducer. The bow-up trim of the vessel also
increased the incidence of side-lobe echoes, especially
when depth was increasing.
33.6
(c)
34.6
35.6
36.6
0:00
8:00
16:00
0:00
8:00
Time (NZST)
Figure 4. The variation in: (a) the mean area-backscattering
coefficient, Sa, (b) the vertical distribution, and (c) the spatial
distribution, of hoki during 28 repetitions of the same acoustic
transect in Hokitika Canyon on 13e14 August 2003. Bars on x axis
show night-time.
experimental transect were probably a result of movement
of fish into and out of the canyon, longitudinal eastewest
movement, occurring at the same time as the changes in
vertical and latitudinal (northesouth) distribution. It was
not possible to detect this longitudinal movement using
a single transect design.
Noise trials
Estimated noise levels are given in Table 6. The recording
from Hokitika Canyon was an order of magnitude (10 dB)
By utilizing the processing time between commercial
trawls, we were able to carry out three acoustic surveys
for New Zealand hoki from a commercial vessel during
routine fishing operations. The acoustic research requirements did not appear to seriously compromise fishing
success: ‘‘Independent 1’’ recorded a ‘‘record trip’’ during
the survey in Hokitika Canyon, filling the holds to capacity
in 17 days.
Surveys in 2002 and 2003 provided the first estimates of
hoki spawning biomass in Pegasus Canyon. This spawning
aggregation was much more extensive than previously
thought, with biomass estimates about 35e43% of those
from the main eastern spawning ground in Cook Strait.
Following the presentation of results of our 2002 survey
(O’Driscoll, 2003), interest in this area increased, and the
commercial catch from Pegasus Canyon rose from 3000 t in
2002 to 7000 t in 2003 (O’Driscoll et al., 2004b).
The survey of Hokitika Canyon in 2003 provided another
estimate of hoki spawning biomass for this area that could
be compared with results from research-vessel surveys in
1988e2000. However, results from Hokitika Canyon
probably do not provide a reliable estimate of hoki
abundance for the entire west coast South Island spawning
fishery. Biomass estimates for inner Hokitika Canyon
(Stratum 5A) are much flatter than the overall west coast
South Island acoustic index, which is derived from a much
larger survey area of over 10 000 km2 (O’Driscoll et al.,
2004a). This is consistent with the hypothesis that the inner
Hokitika Canyon is a ‘‘preferred habitat’’. The density of
fish in a preferred habitat may remain relatively constant,
even if there is wide variation in abundance (MacCall,
1990). In years when abundance is high, fish ‘‘spill-over’’
into other areas. When abundance is low a much higher
Fish-processing time to carry out acoustic surveys
303
Table 6. The results of noise trials for the SIMRAD ES-60 echosounder and hull-mounted transducer on ‘‘Independent 1’’. sA is the
nautical-area scattering coefficient estimated by integrating a 20 m layer from 990e1010 m and NL is the derived noise level from
Equation (1).
Year
Area
Sea conditions
Vessel speed (knots)
Propeller pitch (%)
sA (m2 nautical mile2)
NL (dB re 1 mPa)
2002
Pegasus Canyon
Calm
10
8
72
50
16
21
55.1
56.1
2003
Pegasus Canyon
Calm
10
72
30
57.7
2003
Hokitika Canyon
2 m swell
10
7
72
45
399
815
69.8
72.9
proportion of the population occur in the preferred area.
The perception of many fishers was that overall abundance
on the west coast South Island spawning grounds was low
in 2003, yet our acoustic estimate for Stratum 5A was
similar to 2000, and vessels fishing in this stratum
maintained high catch rates.
The survey approach described in this paper works well
for small-scale acoustic surveys adjacent to areas of high
catch rates. The major limitation is that the boundaries of
the survey area are determined by the time available during
the processing window. The length of the processing
window depends on the size of the preceding catch, which,
in turn, is related to market-driven quality requirements and
also the time required to locate a suitable mark for the next
commercial trawl. In these hoki surveys, the processing
window was typically 3e8 h. This was sufficient to cover
an area the size of Pegasus Canyon (333 km2) or inner
Hokitika Canyon (254 km2), where high densities of fish
were present. It was more difficult to survey areas such
as the outer Hokitika Canyon (529 km2) because hoki
densities were usually too low to allow the vessel to remain
in this stratum and fish commercially. Stratum 5B was
surveyed on three occasions, which required careful
planning to maximize the processing window, i.e. large
catch as close as possible to the survey area.
Other characteristics of the Pegasus Canyon and inner
Hokitika Canyon hoki fisheries make them well-suited for
this type of survey approach. The distribution of hoki in
both areas is related to bathymetry and so the location of
aggregations is relatively predictable. This reduces the time
required to locate suitable marks for commercial fishing.
Aggregations are also dense so tow duration is short. The
combination of these factors means that fish can be caught
‘‘on demand’’. In other fisheries with more dispersed
aggregations or lower catch rates or both of these features,
the down time between trawls is reduced because the vessel
must search for aggregations, or because the trawl is shot
earlier to allow for the longer tow duration. Hoki also form
single-species schools which simplifies mark identification
and reduces the need for targeted trawling to sample
confusing mark types, which is typically part of acoustic
surveys from research vessels.
This survey approach has also been applied to other New
Zealand species with similar behaviour and fishing patterns.
Hampton and Soule (2003) and Soule and Hampton (2003)
successfully carried out two acoustic surveys of an orange
roughy spawning plume on the Chatham Rise from the
commercial factory trawler ‘‘San Waitaki’’ in 2002 and
2003. Like hoki, orange roughy in this area form a spatially
predictable, dense, single-species aggregation occupying
a relatively small area (about 100 km2). Catches were
consistently large (25 t) and this created sufficient processing time to carry out repeated grid surveys (snapshots) of
the spawning plume. O’Driscoll and Hanchet (2004) carried
out a pilot acoustic survey of spawning southern blue
whiting on the Campbell Plateau in 2003 from the factory
freezer trawler ‘‘Aoraki’’. This survey was more difficult
because, although southern blue whiting form dense
aggregations, the location of these aggregations varies
between years and the schools are also mobile prior to
spawning. A two-phase strategy was devised, where the
approximate boundaries of aggregations were first located
by searching and mapping the location of other commercial
vessels, and then biomass was estimated using a grid of
random parallel transects. Two main aggregations were
surveyed over an area of about 2250 km2. Southern blue
whiting were slower to process than hoki and orange
roughy, so there was usually longer between trawls (6e12 h
for catches of 10e20 t).
The results from the repeated transect experiment have
implications for survey design. It appears that the distribution of hoki in the inner Hokitika Canyon was relatively
stable during the day and again at night, with rapid changes
over a short period of about 2 h at dawn and dusk (see Figure
4). These differences were not related to tidal flow. Ideally
all transects within a snapshot would be conducted during
either day or night to avoid the transition periods at dawn
and dusk. This strategy was not feasible with the current
design, where transects were run in the processing time
between commercial tows. There was never enough time to
run all transects in a snapshot during a single processing
period, so snapshots took 14e48 h, depending on how many
commercial trawls were carried out. Because the timing, as
well as the location, of transects was essentially random, the
304
R. L. O’Driscoll and G. J. Macaulay
diurnal patterns we observed are unlikely to have biased the
biomass estimates: there would have been some risk of
‘‘double counting’’, but a similar risk of ‘‘double missing’’.
Temporal variability in spatial distribution, could have
contributed to the relatively high sampling uncertainty
(snapshot CVs) associated with snapshot biomass estimates
(see Tables 3 and 4). This uncertainty is reduced by carrying
out a large number of snapshots.
Acoustic data from the SIMRAD ES-60 echosounder
were of generally high quality and could be analysed using
the same software and methods as data from scientific
echosounders. However, acoustic research from commercial
vessels will be limited to periods of relatively good weather
by their use of hull-mounted transducers. We were fortunate
with the weather during our hoki surveys, and there were
only four days when conditions were deemed too rough for
acoustic-data collection. Noise trials indicated data quality
was much worse when the vessel was pitching, so transect
direction was sometimes manipulated to avoid running into
the swell. Echogram quality was improved by running with
the sea, but there would still have been some attenuation of
the acoustic signal by wind-induced bubbles (Dalen
and Løvik, 1981) which was not apparent from the
echograms.
The noise levels for ‘‘Independent 1’’ in calm conditions
were below the threshold of 95 dB re 1 mPa (1 Hz band)
recommended as the acceptable vessel noise level at 38 kHz
(Mitson, 1995), but about 10 dB higher at 1000 m than
values reported by Hampton and Soule (2003) for the hull
system on ‘‘San Waitaki’’. Ideally, noise levels should be
subtracted from the measured backscatter following integration. There was no noise correction applied in this
paper. However, a preliminary analysis indicated that the
bias in our biomass estimates due to ambient noise was only
about 1% in calm conditions. The echosounder transducer on
‘‘Independent 1’’ was also tilted up to 5( forward for much
of the survey. The hoki target strengthelength relationship
(Macaulay, 2001) is based on average fish-tilt angles relative
to a level transducer. If the transducer is not level, then the
relative tilt-angle distribution of the fish changes and this
may introduce a bias to the biomass estimates. In future
surveys it would be useful to have pitch-and-roll sensors
mounted on the commercial vessel. This would allow
transducer orientation to be measured so that a new target
strengthelength relationship relative to mean transducer
attitude could be calculated, and also allow correction for
signal loss due to pitch and roll (e.g., Stanton, 1982).
Acknowledgements
Thanks to Allan Dillon and Trevor Smith (captains), the
crew of ‘‘Independent 1’’, and Independent Fisheries
Limited for their cooperation. Neil Bagley and Matt Dunn
assisted with data collection at sea. Adam Dunford helped
with the analysis of results from noise trials. This paper was
improved following critical reviews by Roger Coombs,
Adam Dunford, and an anonymous referee. Funding was
provided by the New Zealand Hoki Fishery Management
Company Limited.
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