ASSESSMENT OF SMALL PELAGIC FISH BY ACOUSTIC METHODS IN
NORTH WESTERN ADRIATIC SEA
Leonori Iole, Azzali Massimo and De Felice Andrea
CNR-ISMAR Istituto di Scienze Marine, Sezione Pesca Marittima - Ancona - Italy
Abstract
An echosurvey was performed in the summer season 2005 covering the western part of GSA 17
(from Trieste to Vieste) 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 informations 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 the net ones.
Pelagic biomass as a whole and per species (Anchovies, Sardines and Sprats) estimates are
presented; total pelagic biomass spatial distribution is illustrated and also the most probable spatial
distributions for the target species of this research are reported. Trends for total pelagic biomass and
for the biomass of the target species referred to an area of about 2/3 of western GSA 17 (from
Trieste to Giulianova) are discussed; this area corresponds to that of the historical surveys made
since 1976.
INTRODUCTION
In August-September 2005 an echosurvey was conducted in Northern and Central Adriatic Sea by
the acoustic team of ISMAR-CNR of Ancona. The covered area was between Vieste and Trieste,
from Italian coast to Mid-line. 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 1976, ISMAR-CNR has been conducting echosurveys in the north-western Adriatic Sea
(GFCM sub area 17 from Trieste to Giulianova), from Italian coast to Mid-line, for the stock
assessment of small pelagic fish, in particular anchovy (Engraulis encrasicolus), sardine (Sardina
pilchardus) and sprat (Sprattus sprattus). The area covered by the surveys was on average 7623 nm2.
Since 1987, acoustic surveys were also conducted in Central Adriatic from Giulianova to Vieste.
(Azzali et al. 2002a).
Recently, due to the need to have a unique evaluation for all western side of GSA 17, the North
Adriatic area was monitored jointly with the Central Adriatic, covering an area of 12233 nm2.
These surveys were carried out annually, in the warm season, using the same mid-water trawl. 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.
MATERIALS AND METHODS
Study area
The total area covered by the acoustic survey in the western part of GSA 17 is between Trieste and
Vieste, from Italian coast to Mid-line (12233 nm2). Following historical acoustic surveys the area is
divided into two sub areas: the Northern Adriatic and the Central Adriatic. The Northern Adriatic
area, between Giulianova and Trieste, (A  7600 nm2) is almost entirely within the 100 m
bathymetry (Azzali, 2002).
Data collection
Collection of acoustic data was performed along systematic zig-zag acoustic transects (Fig. 1),
during the navigation and also during the biological samplings.
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 and 200
kHz frequencies are used for discriminating small pelagic fish with the 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 (Simrad, 1996). In June 2005 an intercalibration between Italian and Croatian acoustic
systems including 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
2
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.
Environmental measurements included CTD profiles made by SEABIRD 911 PLUS probe. CTD
(temperature, salinity) stations were regularly distributed along 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 ferry-boats.
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 package 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 Nautical
Mile. The area of the North and Central Adriatic Sea is contained in an electronic window 100x270
ESDU. There are three basic functional modules of GFRDBS: Data Base Module (DBM), Mapping
and Presentation Module (MPM), Computing and Processing Module (CPM).
The species maps were implemented in ArcView. The Voronoi diagram was applied on the
positions of the hauls carried out in the studied area (Azzali et al., 2002b); the total SA
(backscattering area per sea surface) collected along survey routes was divided according to the
extensions of the polygons and for each species the specific SA values were reconstructed on the
base of local percentages in weight of the species derived from the net samplings polygon by
polygon. With each set of specific Sa values the Inverse Distance Weighted interpolator was used.
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.
3
RESULTS
The acoustic survey in GSA 17
The survey results are reported in Table 1. In the entire area investigated (12233 nm2) the average
density of the whole pelagic biomass was estimated in 27.3 t/nm2. The biomass of anchovies is
around 74.7% of the total biomass (av.density 20.4 t/nm2, biomass 249688 t). The biomass of
sardines was 11.4% of the total biomass (average density 3.1 t/nm2, biomass 38105 t). Sprats were
6.2% of the total biomass (average density 1.7 t/nm2, biomass 20724 t). The other pelagic species
(mainly T. trachurus) formed 7.7% of the total biomass (average density 2.1 t/nm2, biomass 25738
t). The presumed error in the biomass estimation was 17.92% (5 t/nm2). The acoustic sampling
intensity was very good (CV = 15.1 >> 6).
The distribution pattern of the total pelagic biomass is shown in Figure 2. The high density areas
(av.density 89.3 t/nm2) represent 11.7% of the covered area and contain 38.1% of the total biomass.
The medium density areas (av.density 35.5 t/nm2) represent 33.2% of the investigated area and
contain 43.1% of the total pelagic biomass. Finally the low density areas (av.density 9.3 t/nm2)
represent 55.1% of the investigated area and contain only 18.8% of the total biomass. The low
density region is of no interest to fishermen.
In Table 2 are reported the results of the biological samplings. 27 hauls were carried out along the
acoustic transects. The average length of the small pelagic fish was 11.42 cm (S.D. = 2.73;
Individuals number = 10447). In the bathymetric range 0 - 40 m the catches were more abundant.
The mean length of the anchovy samples was 10.62 cm (S.D. = 2, Table 2) and the mean length of
sardine samples was 14.48 cm (S.D. = 1.72, Table 2). Sprats mean length was estimated at 10.92
cm (S.D. = 1.18, Table 2).
In Figure 3 length frequency distributions of anchovies and sardines derived from all the hauls
together are reported.
30 CTD stations to measure temperature, salinity, turbidity and fluorescence were conducted.
The maps of spatial distribution of the target species were represented in Figures 4, 5, 6. The SA
intervals used for these maps were chosen with the same principle of the ones of total pelagic
biomass.
The trend of total pelagic biomass, derived from echosurveys in the time interval 1976-2005, is
shown in Figure 7. These trends refer to the north-western part of the Adriatic Sea according to the
traditional division considered by the bioacoustics group of ISMAR-CNR that takes into account
4
100 m bathymetric as the southern boundary and is about 2/3 of GSA 17 western side. The graph
indicates that the total pelagic biomass fluctuates in a way which is almost periodical. The highest
peaks (130 and 190 t/nm2, observed in 1978 and 1983, respectively) and the largest periods of
fluctuation (around 4-5 years) occurred before 1987. Since 1987 the peaks have become smaller
(85, 110 and 100 t/nm2) and accordingly the fluctuation periods seem to shorten (4-3 years). In
recent years the biomass is decreasing below the mean. Mean biomass density from 1976 to 2005 in
the Northern Adriatic was estimated to be 72.8 t/nm2.
The fluctuations of the target species of the echosurvey in Northern Adriatic are illustrated in Figure
8 (anchovy), 9 (sardine), 10 (sprat).
The anchovy stock reached the maximum peak (85 t/nm2) in the years 1978-79, then, after a minor
peak (1984, 35 t/nm2) collapsed for the period 1986-90. The partial recovery began in 1994. The
2005 value is slightly below the mean value. Mean density of anchovy biomass from 1976 to 2005
was estimated at 27.9 t/nm2.
The sardine stock increased from a very low level (around 10 t/nm2 in the period 1976-80) to the
maximum peak (around 85 t/nm2) in the years 1981-83. After a period of normality (1986-95), the
sardine population has decreased and now seems to be stable at a very low level. Mean density was
calculated as 27.1 t/nm2 (1976-2005).
The pelagic community of the North Adriatic Sea is dominated by sardines and anchovies that are
about 80% of the total biomass.
The biomass of sprats (Fig. 10, mean density 9.7 t/nm2) show very irregular changes. Figure 10
shows that from 1994 to 2002 sprats have been rapidly decreasing, almost to extinction level. In the
recent years (2003-2005) there was a minor fluctuation with a peak around the mean value of all the
considered period.
It is interesting to note that a decrease in abundance of sardines, sprats occurred in the same years
(1985-86) as for anchovy. However these populations, in contrast to anchovy, seemed to recover
quickly and return to normal conditions (1987-96). It could be concluded that the pelagic biomass
as a whole in the North Adriatic fluctuates almost “normally” around average value, whereas its
species components are affected by drastic and unpredictable changes.
A second basic question about pelagic resources is their spatial structure and distribution, which for
fisheries may be more important than simple measurement of total biomass.
Generally the basic structure of pelagic populations is patchiness (high density strata), irrespective
of the abundance level of biomass.
However the surveys seem to indicate that high-density strata movements are related to the biomass
abundance. For example, when the anchovy stock collapsed the high-density strata approached the
5
coast, whereas when the stock recovered (1995) they moved offshore. It seems that the extension of
the high-density strata remains constant and the density within these strata changes in a way which
is apparently not related to the changes of the total biomass. Therefore the catchability of the
pelagic populations may remain high even when the level of abundance of a stock is low.
These results illustrate the importance of the variability of the pelagic resources and the importance
of acoustic techniques to determine these changes at the level of total population, species and spatial
patterns. Fishery management needs this information to reduce the uncertainty in the exploitation of
pelagic resources and in order to make long-term plans.
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 relations between abundance estimates and oceanographic parameters. In
the present period where small pelagic populations show quite low levels of abundance, a
reasonable suggestion is to maintain stable the fishing effort on them.
Acknowledgements
This project could be realized thanks to Italian Ministry for Agriculture and Forestry Policies; a
special thank goes to the crew of R/V Dallaporta and also to other collaborators from ISMAR-CNR
that worked on board during echosurvey.
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, 2002a: 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.
Azzali M., A. De Felice, T. Kariš, M. Luna, V. Tičina, M. Franicevic, 2002b: Pilot Joint Echo6
Survey in a Northern part of the Adriatic Sea on small pelagic fish and its implications on the
historical surveys. FAO-ADRIAMED report presented at GFCM-SAC Working Group on Small
Pelagic Fish.
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.
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Fig. 1. Transects plan in GSA 17
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45°
44°
43°
42°
12°
13°
14°
15°
16°
45°
SA
44°
43°
42°
17°
Fig. 2. Map of the spatial distribution of total pelagic biomass in GSA 17
0-155
155-388
> 388
Tab. 1. Acoustic survey results in GSA 17
GSA 17 Western Side 2005
Av. Length
Density
Biomass
(cm)
(t/ nm²)
(t)
respect to Total
Biomass
%
ANCHOVY
10.6
20.4
249,688
74.7
SARDINE
14.5
3.1
38,105
11.4
SPRAT
10.9
1.7
20,724
6.2
O.P.S.
12.8
2.1
25,738
7.7
Average Density: 27.3 t/ nm²; Total Biomass: 334254.6 t;
Surveyed Area: 12233 nm²; Sampled Area: 1666 nm²
Interpolation Error 5 t/ nm² (17.92%); CV: 15.1
Stratum
Density
Biomass
Area
Low density
(t/ nm²)
9.3
%
18.8
(t)
62,772
%
55.1
( nm²)
6,745
Med. density
35.5
43.1
144,115
33.2
4,062
High Density
89.3
38.1
127,367
11.7
1,426
10
Tab. 2. Demography per species and bathymetric in GSA 17
Biological samplings elaboration - GSA 17 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
10.62
9.65
11.71
12.19
2
1.84
0.77
1.3
6656
3994
728
1934
100
68
7
25
74.7
79.2
38.4
82.3
Sardine
0/200
0/40
40/60
60/200
14.48
13.76
16.02
15.86
1.72
1.64
0.46
0.57
1339
902
241
196
100
49
27
24
11.4
8.6
24.4
11.9
Sprat
0/200
0/40
40/60
60/200
10.92
11.11
10.38
11.09
1.18
1.25
0.71
0.83
1126
823
292
11
100
24
76
0
6.2
2.3
36.9
0.1
O.P.S.
0/200
0/40
40/60
60/200
12.8
12.2
14.11
15.87
4.45
4.48
2.37
2.77
1326
1106
9
211
100
83
0
17
7.7
9.9
0.3
5.7
TOT
Pelagic
0/200
0/40
40/60
60/200
11.42
10.78
12.24
12.82
2.73
2.84
2.06
2.01
10447
6825
1270
2352
100
64
13
23
Sardines
Anchovies
20
20
%
%
0
0
2
4.5
7
9.5
12
14.5
17
19.5
22
24.5
27
29.5
2
4
6
8
10
12
LT (cm)
Fig. 3. Size distribution for anchovies (left) and sardines (right) in GSA 17
14
16
18
LT (cm)
20
22
24
26
28
30
12°
13°
14°
15°
16°
17°
46°
46°
45°
45°
SA
0-104
44°
44°
43°
43°
42°
42°
12°
13°
14°
15°
16°
104-312
> 312
17°
Fig. 4. Map of the spatial distribution of anchovy biomass in GSA 17
12
12°
13°
14°
15°
16°
17°
46°
46°
45°
45°
SA
0-20
44°
44°
43°
43°
42°
42°
12°
13°
14°
15°
16°
20-84
> 84
17°
Fig. 5. Map of the spatial distribution of sardine biomass in GSA 17
13
12°
13°
14°
15°
16°
17°
46°
46°
SA
45°
45°
0-14
14-164
> 164
44°
44°
43°
43°
42°
42°
12°
13°
14°
15°
16°
17°
Fig. 6. Map of the spatial distribution of sprat biomass in GSA 17
14
TOT. PELAGIC DENSITY (t/Nm2)
TREND OF TOTAL PELAGIC BIOMASS IN NORTH ADRIATIC
200
180
160
140
120
100
80
60
40
20
0
1976
Av. Area (Nm²) 1976-2005 :
Av. Biomass per year (t) :
Max. Biomass per year (t) :
Min. Biomass per year (t)
1979
1982
7,623
562,167
1,214,725
245,079
1985
(Trieste/Giulianova within Mid-Line)
Av .Density (t/Nm²) :
72.8
Max. Density (t/Nm²) :
159.4
Min.Density (t/Nm²) :
32.2
1988
1991
(- - - - - - - - -)
+119%
-56 %
1994
1997
2000
2003
2006
Fig. 7. Trend of total pelagic biomass in North Adriatic from 1976 to 2005
ANCHOVIES DENSITY (t/Nm2)
TREND OF ANCHOVIES BIOMASS IN NORTH ADRIATIC
100
90
80
70
60
50
40
30
20
10
0
1976
Av. Area (Nm²) 1976-2005 :
Av. Biomass per year (t) :
Max. Biomass per year (t) :
Min. Biomass per year (t)
1979
1982
7,623
214,837
651,995
10,825
1985
(Trieste/Giulianova within Mid-Line)
Av .Density (t/Nm²) :
27.9
Max. Density (t/Nm²) :
85.5
Min.Density (t/Nm²) :
1.4
1988
1991
1994
(- - - - - - - - -)
+207%
-95 %
1997
2000
2003
2006
Fig. 8. Trend of anchovy biomass in North Adriatic from 1976 to 2005
15
TREND OF SARDINES BIOMASS IN NORTH ADRIATIC
SARDINES DENSITY (t/Nm2)
120
Av. Area (Nm²) 1976-2001 :
Av. Biomass per year (t) :
Max. Biomass per year (t) :
Min. Biomass per year (t)
7,623
208,870
693,159
25,156
(Trieste/Giulianova within Mid-Line)
Av .Density (t/Nm²) :
27.1
Max. Density (t/Nm²) :
90.9
Min.Density (t/Nm²) :
3.3
(- - - - - - - - -)
+236%
-88 %
100
80
60
40
20
0
1976
1979
1982
1985
1988
1991
1994
1997
2000
2003
2006
Fig. 9. Trend of sardine biomass in North Adriatic from 1976 to 2005
TREND OF SPRATS BIOMASS IN NORTH ADRIATIC
SPRATS DENSITY (t/Nm2)
40
Av. Area (Nm²) 1976-2005 :
Av. Biomass per year (t) :
Max. Biomass per year (t) :
Min. Biomass per year (t)
7,623
74,745
237,228
2,134
(Trieste/Giulianova within Mid-Line)
Av .Density (t/Nm²) :
9.7
Max. Density (t/Nm²) :
31.1
Min.Density (t/Nm²) :
0.3
(- - - - - - - - -)
+222%
-97 %
30
20
10
0
1976
1979
1982
1985
1988
1991
1994
1997
2000
2003
2006
Fig. 10. Trend of sprat biomass in North Adriatic from 1976 to 2005
16