ASSESSMENT OF SMALL PELAGIC FISH BY ACOUSTIC METHODS IN
SOUTH WESTERN ADRIATIC SEA
Leonori Iole, Azzali Massimo and De Felice Andrea
Abstract
An echosurvey was performed in the summer season 2005 covering the north-western part of GSA
18 (from Vieste to Brindisi) according to FAO division of the Mediterranean Sea. Acoustic data on
small pelagic fish were logged by an EK500 scientific echosounder working at three frequencies:
38, 120 and 200 kHz. Biological information on fish acoustically monitored was derived from net
samplings made with a mid-water trawl. In addition temperature, salinity and other environmental
parameters were measured by means of CTD samplings made in correspondence of net samplings.
Pelagic biomass as a whole and per species (Anchovies and Sardines) estimates are presented; total
pelagic biomass spatial distribution is illustrated. Trends for total pelagic biomass and for the
biomass of the target species of north-western GSA 18 are discussed; this area was studied since
1987.
INTRODUCTION
In August 2005 an echosurvey was conducted in the Southern part of the Adriatic Sea by the
acoustic team of ISMAR-CNR of Ancona. The covered area was between Vieste and Brindisi, from
Italian coast to 200 m bathymetric. The acoustic survey is part of the research project funded by the
Italian Ministry of Agriculture and Forestry Policies (MIPAF) for the monitoring of the small
pelagic fish in the Italian side of Adriatic Sea (GFCM sub areas 17 and 18).
National Research programme: VALUTAZIONE DEGLI STOCK DI ALICI E SARDINE NEI
MARI ITALIANI AL FINE DI UNA GESTIONE RAZIONALE DELLA PESCA- U. O. 4
Since 1987, ISMAR-CNR has been conducting echosurveys in the south-western Adriatic Sea
(GFCM sub area 18 from Vieste to Brindisi), from Italian coast to the 200 m bathymetric, for the
stock assessment of small pelagic fish, in particular anchovy (Engraulis encrasicolus) and sardine
(Sardina pilchardus).
The area covered by the surveys was on average 2823 nm2. These surveys were carried out
annually, in the warm season, using the same mid-water trawler. An environmental monitoring was
performed during surveys. The main results are the temporal changes of the pelagic biomass as a
whole and per species and the variability of the spatial distribution (Azzali et al., 2002).
MATERIALS AND METHODS
Study area
The area covered by the historical acoustic surveys in the Southern Adriatic goes from Vieste to
Brindisi, from Italian coast to 200 m bathymetry (Azzali, 2000). This area is characterized by a
narrow shallow water strip near the coastline except for Manfredonia Gulf and a rapidly increasing
depth profile going offshore.
Collection of acoustic data was performed along systematic zig-zag acoustic transects (Fig. 1),
during the navigation and also during biological samplings. The surveyed area was the same of the
previous surveys.
Data collection
The acoustic data were collected using an EK 500 system (SIMRAD), operating at 38, 120 and 200
kHz simultaneously with hull-mounted split-beam transducers on the R/V G. Dallaporta.
The main frequency for the assessment of small pelagic biomass is 38 kHz. The 120 kHz e a 200
kHz frequencies are used for discriminating small pelagic fish with swimbladder from fish without
swimbladder, plankton, noise, etc.: moreover, within the species with swimbladder, comparing the
frequencies just the sizes between 5 cm and 50 cm are left.
Before the cruise, in April 2005, the hydroacoustic system was calibrated following the SIMRAD
procedure. In June 2005 an intercalibration between Italian and Croatian acoustic system included
the sampling nets was performed. The results were presented to the GFCM in September 2005
(Azzali et al., 2005).
The speed of the ship during the survey was between 8 and 10 knots and the integration readings
were recorded every 1 nautical mile. The speed and the position of the vessel were obtained by
means of GPS. The geographic position of the ship and its routes were displayed during the survey
on an electronic chart equipment (Shark 12 NT).
The pelagic trawl samplings were collected to determine size and species composition. The net
samplings were 4 per day (about a haul every 6 hours). The mid-water sampling trawl, designed by
the acoustic team of ISMAR, has been used in historical surveys in the Adriatic since 1982. It has
800 mm nominal mesh size in the front and 18 mm in the codend. The mouth area was about 90 m2
(about 7 m in height and 12 m in width). The spatial position of the net in the water was monitored
by the ITI (Simrad’s Integrated Trawl Instrumentation). The receiving sensor was installed on a
towed body, properly built by the ISMAR acoustic group. The ITI measures the trawl depth, the
vertical opening of the trawl mouth and the temperature at the trawl depth.
2
Environmental measurements included CTD profiles made by SEABIRD 911 PLUS probe. CTD
(temperature, salinity) stations were regularly distributed along systematic zig-zag transects.
Satellite images concerning surface temperature (since 1982) and a-chlorophyll (since 1998) of the
Adriatic Sea were acquired thanks to the collaboration with ISAC-CNR of Bologna; visual census
on cetaceans was performed on board during echosurvey cruises and the data integrated with
sightings made by selected personnel of Tirrenia company along the routine travels of their ferryboats.
Data processing
Acoustic data were recorded on Hard Disk and CD using the Simrad BI500 system and Sonar Data
EchoView software and printed for each frequency. Data set originated by acoustic survey (acoustic
data, catch data, position of the ship) were processed using the Sw Geographical Fishery Resources
Data Base System for multifrequency analysis.
From 1985 until now the SW GFRDBS originally designed by ISMAR acoustic team (Burczynski
et al., 1997), has been implemented, tested and adjusted. This package system processes the
acoustic, catch and satellite data in a geographical context, converting Lat&Lon into X&Y
coordinates. The Elementary Sampling Distance Unit (ESDU) of the X,Y map is 1 nm. The area of
the Southern Adriatic Sea is contained in an electronic window 100x180 ESDU. There are three
basic functional modules of GFRDBS: Data Base Module (DBM), Mapping and Presentation
Module (MPM), Computing and Processing Module (CPM).
The biomass assessment and its spatial distribution was performed using the standard method of
echo integration (Simmonds & MacLennan, 2005), implemented by the Split-beam and Multifrequency technology.
RESULTS
Echosurvey in GSA 18
The area covered by acoustic survey was GSA 18 (from Vieste to Brindisi).
The echosurvey began on the 6th of August at 22:34 in front of Vieste and ended on the 12th of
August at 00:43 in the same point of the start. The acoustically sampled nautical miles were 639 in
an area of about 3500 nm2. Sampling was made along predetermined transects; due to bad weather
3
conditions the echosurvey was interrupted on the 7th of August at 19:18 (the ship entered Barletta
harbour where it remained two days).
The coverage coefficient was 18.1%, very higher respect to the requested minimum of 5% for an
affordable estimate.
10 net samplings on small pelagic fish with a non-selective mid-water pelagic trawl were carried
out; also 10 CTD samplings to determine temperature, salinity, turbidity and fluorescence profiles
were made.
The survey results are reported in Table 1. In the entire area investigated (3531 nm2) the average
density of the whole pelagic biomass was estimated in 12.1 t/nm2. The biomass of anchovies is
around 68.3% of the total biomass (av. density 8.2 t/nm2, biomass 29072 t). The biomass of sardines
was 15.7% of the total biomass (average density 1.9 t/nm2, biomass 6683 t). Sprats were 0.1% of
the total biomass (average density 0.1 t/nm2, biomass 43 t). The other pelagic species (OPS) formed
15.9% of the total biomass (average density 1.9 t/nm2, biomass 6768 t). The error in the biomass
estimation was 18.25% (2.8 t/nm2). The acoustic sampling intensity was very good (CV = 10.8 >>
6).
The distribution pattern of the total pelagic biomass is shown in Figure 2. The high density areas
(av. density 36.5 t/nm2) represent 10.2% of the area covered and contain 30.9% of the total biomass.
The medium density areas (av. density 14.6 t/nm2) represent 45.6% of the area investigated and
contain 55.3% of the total pelagic biomass. Finally the low density areas (av. density 3.8 t/nm2)
represent 44.2% of the area investigated and contain only 13.8% of the total biomass.
In Table 2 are reported the results of the biological samplings. The average length of the small
pelagic fish was 8.3 cm (S.D. = 3.47; Individuals number = 3783). In the bathymetric range 60 200 m the catches are more abundant. The mean length of the anchovy samples was 7.23 cm (S.D.
= 2.26, Table 2) and the mean length of sardine samples was 12.4 cm (S.D. = 2.22, Table 2).
In Figure 3 size distributions of anchovies and sardines derived from all the hauls together are
reported.
The trends of total pelagic biomass derived from echosurveys in the time interval, 1987-2005 is
shown in Figure 4. These trends refer to the south-western part of the Adriatic Sea that corresponds
to the Italian portion of coastal area (within 200 m bathymetric) of GSA 18. Total pelagic biomass
presents fluctuations that start from a low level then rise to higher values to decline again in recent
years. The highest peaks occurred in 1994 and 1996; minimum values of pelagic biomass were
registered in 1988, 1993 and 2005. In recent years the biomass is decreasing below the mean as was
observed in the first years of the studied period; unfortunately in these two periods there was
discontinuity in yearly sampling due to financial problems and consequently the trend is flatter.
4
Mean total pelagic biomass density from 1987 to 2005 in the Southern Adriatic was estimated to be
48.45 t/nm2. Anchovy, as can be seen in the graphic (Fig. 5), became the most important species in
late 90’s exceeding sardine. Recently there has been a decline also for this species that presents
levels similar to 1993 and 1996. Sardine presents several higher fluctuations in the first years of the
studied period until the peak in 1996 (Fig. 6), after which there was a sudden decline with much
minor oscillations below the mean value of the entire period.
Conclusions
The results of the studies of small pelagic populations derived from the historical surveys in the
western Adriatic Sea show that these populations are affected by large variability in time and space.
This variability is mostly due to environmental factors and this is why an effort is currently in
progress to find possible relationships between abundance estimates and oceanographic parameters.
In the present period where small pelagic populations show quite low levels of abundance, we
suggest not to increase fishing effort on them.
References
Azzali M., V. Ticina, A. De Felice, I. Leonori, E. Paschini, M. Marini, B. Grbec, O. Vidjak, L.
Grubisic, A. Pallaoro, F. Matic, 2005: Inter - ship calibration to compare acoustic estimations of
small pelagic fish in the Adriatic Sea. SCSA 2005 proceedings.
Azzali M., 2002: Valutazione acustica della biomassa, distribuzione e struttura delle popolazioni
pelagiche in Adriatico, in relazione con i dati ambientali ricavati da satellite. MIPAF final report.
Azzali M., A. De Felice, G. Cosimi, M. Luna, F. Parmiggiani, 2002: The state of the Adriatic Sea
centered on the small pelagic fish populations. P.S.Z.N.: Marine Ecology, 23, Supplement 1 (2002),
pp. 78-91.
Burczynski, J., M. Azzali, J. Rucabado, 1997: Geographical Fishery Resources Data Base System
for management and geographical presentation of
Data from Acoustic Surveys and Catches:
AdriaBase as a case study. In: T-ECHO AIR1 CT92 0314 - Final report and scientific & technical
annexes - Section Specific Software Tools.
Simmonds J., MacLennan D., 2005: Fisheries Acoustics; 2nd edition, Eds. Blackwell, 437 pp.
Simrad, 1996: EK500 Scientific Echo Sounder – Operator manual. Simrad AS. Norten, Norway.
5
16°00 '
16°30 '
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17°30 '
18°00 '
42°00 '
42°00 '
41°30 '
41°30 '
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40°30 '
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Fig. 1. Transects plan in GSA 18
17°00 '
17°30 '
40°30 '
16°
42°
41°
17°
18°
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40°
19°
SA
42°
0-108
108-300
> 300
41°
40°
16°
17°
18°
19°
Fig. 2. Map of the spatial distribution of total pelagic biomass in GSA 18
Tab. 1. Acoustic survey results in GSA 18
Av. Length
Density
Biomass
(cm)
(t/ nm²)
(t)
respect to Total
Biomass
%
ANCHOVY
7.2
8.2
29,072
68.3
SARDINE
12.4
1.9
6,683
15.7
SPRAT
10.0
0.1
43
0.1
O.P.S.
9.6
1.9
6,768
15.9
Average Density: 12.1 t/ nm²;
Total Biomass: 42565.8 t;
Surveyed Area: 3531 nm²
Sampled Area: 639 nm²
Interpolation Error 2.8 t/ nm² (18.25%); CV: 10.8
Stratum
Density
(t/ nm²)
Biomass
%
Low density
3.8
Med. density
High Density
Area
13.8
%
10.2
( nm²)
1,560
14.6
55.3
45.6
1,611
36.5
39.9
10.2
360
8
Tab. 2. Demography per species and bathymetric in GSA 18
Biological samplings elaboration - GSA 18 North-Western Side 2005
Species Depth (m) Mean Size (cm)
St. Dev.
N°
Weight (%)
referred to TOT catch (%)
Anchovy
0/200
0/40
40/60
60/200
7.23
5.97
9.38
8.07
2.26
1.2
1.08
2.56
2494
1161
256
1077
100
30
10
60
68.3
73.2
26.9
88.2
Sardine
0/200
0/40
40/60
60/200
12.4
9.11
12.15
14.04
2.22
1.42
0.53
1.34
354
75
111
168
100
2
66
31
15.7
1.3
41.1
10.6
Sprat
0/200
0/40
40/60
60/200
10
0
0
10
0
0
0
0
5
0
0
5
100
0
0
100
0.1
0
0
0.1
O.P.S.
0/200
0/40
40/60
60/200
9.59
8.49
15.83
12.32
4.7
4.15
1.55
5.88
930
779
126
25
100
45
51
3
15.9
25.4
32
1.1
TOT
Pelagic
0/200
0/40
40/60
60/200
8.3
7.06
11.65
8.95
3.47
3.03
2.92
3.27
3783
2015
493
1275
100
28
25
46
Sardines
Anchovies
20
20
%
%
0
0
2
4
6
8
10
12
14
16
18
20
2
4
6
8
LT (cm)
10
12
14
16
18
20
LT (cm)
Fig. 3. Size distribution for anchovies (left) and sardines (right) in GSA 18
9
TREND OF TOTAL PELAGIC BIOMASS IN SOUTHERN ADRIATIC SEA
YEAR
AREA
S
I
E
II
A
S
III
O
IV
N
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2662
1811
2597
2597
2597
2852
1919
3364
3064
1634
2383
3136
3144
3533
3596
3569
16.1
15.4
34
55
58.5
29.6
58.3
66.3
81.7
65.4
52.7
71.5
53.1
44.2
14
18.7
39.8
58.3
54.4
24.2
81
48.8
90.9
53
61.3
68.4
48.8
21.8
13
23.1
45.4
60.2
47.6
25.4
92.6
44.7
88.5
45.7
68.6
63.6
18.8
13.4
28.3
50.6
60.4
38.5
36.6
85
60.8
78.6
46.3
71.9
58.2
Av. Area (Nm²) 1987-2005
Av. Biomass per year (t)
Max. Biomass per year (t)
Min. Biomass per year (t)
2,823
142,821
239,757
39,861
(Vieste/Brindisi)
Av .Density (t/Nm²) : 48.45
Max. Density (t/Nm²) : 84.93
Min.Density (t/Nm²) : 14.12
2003
2004
2005
52.1
57.7
46.9
25
45.3
54.7
56.5
42.2
18.6
45.8
47.1
56.7
54.2
36.9
12.1
44.3
49.5
57.8
51
31.1
3531
(- - - - - - - - -)
+75%
-71%
TOTAL PELAGIC DENSITY (t/nm 2)
120
100
80
60
40
20
0
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Fig. 4. Trend of total pelagic biomass in South Adriatic from 1987 to 2005
10
TREND OF ANCHOVIES IN SOUTHERN ADRIATIC SEA
YEAR
AREA
S
I
E
II
A
S
III
O
IV
N
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2662
1811
2597
2597
2597
2852
1919
3364
3064
1634
2383
3136
3144
3533
3596
3569
0.6
3.4
9.7
15.3
15.2
5.9
12.6
24.1
8
18.7
42.7
58.7
50.2
40.2
0.9
4.8
11.4
16
13.7
4
19.3
20.2
7.6
24.7
47.9
59.8
44.2
0.2
1.5
6.4
12.9
16.2
11.5
4.1
24.4
15.5
9.7
31
52.4
58.9
0.4
2.3
8
14.2
16
8.6
6.8
25.8
11
13.5
37.1
56.1
55.7
Av. Area (Nm²) 1987-2005 : 2,823
Av. Biomass per year (t) : 71,960
Max. Biomass per year (t) : 164,524
Min. Biomass per year (t)
3,755
(Vieste/Brindisi)
Av .Density (t/Nm²) : 24.24
Max. Density (t/Nm²) 58.28
Min.Density (t/Nm²) : 1.33
2003
2004
2005
49.8
41.8
29.3
15.4
43.3
49
38.9
26
11.8
39.8
46.6
47.1
35.8
22.5
8.2
38.7
49.1
44.6
32.6
19
3531
(- - - - - - - - -)
+140%
-95 %
ANCHOVIES DENSITY (t/nm 2)
120
100
80
60
40
20
0
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Fig. 5. Trend of anchovy biomass in South Adriatic from 1987 to 2005
11
TREND OF SARDINES IN SOUTHERN ADRIATIC SEA
YEAR
AREA
S
I
E
II
A
S
III
O
IV
N
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2662
1811
2597
2597
2597
2852
1919
3364
3064
1634
2383
3136
3144
3533
3596
3569
12.8
11.5
27.3
44.6
45.4
15.3
32.2
33
72.9
41.7
2.5
10.5
1
2.2
10.8
14.3
32.2
47.1
40.8
9
49.6
20.4
83.3
21.2
7.4
6.9
2.4
18.2
9.7
18.1
36.9
48.2
33.8
8.1
57.5
22.9
78
6.4
11.7
3.3
15.3
9.9
22.5
41.1
47.7
24.6
15.7
49.3
46.4
62.4
0.9
12.5
1
Av. Area (Nm²) 1987-2001 : 2,823
Av. Biomass per year (t) : 62,739
Max. Biomass per year (t) : 209,325
2,258
Min. Biomass per year (t)
(Vieste/Brindisi)
Av .Density (t/Nm²) : 21.349
Max. Density (t/Nm²) : 74.15
Min.Density (t/Nm²) :
0.8
2003
2004
2005
1.9
13.8
14.8
7.2
0.8
4.9
15.2
13.5
4.6
3.5
0.1
8.4
15.7
11.7
1.9
3.3
0.1
11.5
15.6
9.6
3531
(- - - - - - - - -)
+247%
-96 %
120
SARDINES DENSITY (t/nm 2)
100
80
60
40
20
0
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Fig. 6. Trend of sardine biomass in South Adriatic from 1987 to 2005
12