Towards coordinated acoustic surveys in Northwest Africa: results of parallel surveys and intercalibrations

Abstract

The coastal pelagic fish off Northwest Africa are stocks of high commercial and social importance that are shared between the countries of the subregion. Since 1995, these resources are subject to regular evaluation by the Norwegian research vessel R/V Dr. Fridtjof Nansen through regional acoustic surveys covering the area from southern Senegal to Cape Cantin in Morocco. This activity is undertaken within the framework of a programme of cooperation between the countries, FAO and the NANSEN Programme. With the acquisition of national research vessels with acoustic equipment, scientific activities were initiated to prepare for the national vessels to take over the responsibility for the acoustic surveys carried out by the R/V Dr. Fridtjof Nansen. To achieve this, several parallel surveys and intercalibration exercises were carried out to ensure comparable results

Full text

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Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques
Towards coordinated acoustic
surveys in Northwest Africa:
results of parallel surveys and
intercalibrations
Vers des campagnes acoustiques coordonnées en Afrique du nord-ouest:
résultats des campagnes parallèles et intercalibrations
(English only/En anglais seulement)
A. Sarré1, N. Charouki2, E.M. Mahmoud3, M.A.O.Taleb3, E. Mbye4,
J. Jallow4, S. El Ayoubi2, J.O. Krakstad5 and O. Alvheim5
1 Centre de recherche océanographique de Dakar-Thiaroye, BP 2241, Dakar,
Sénégal
2 Institut national de recherche halieutique, Maroc
3 Institut mauritanien de recherches océanographiques et des pêches, BP22
Nouadhibou, Mauritanie
4 Fisheries Department, Banjul, Gambie
5 Institute of Marine Research, P.O. Box 1870 Nordnes, N-5817 Bergen, Norway
Sarré, A., Charouki, N., Mahmoud, E.M., Taleb, M.A.O., Mbye, E., Jallow, J.,
El Ayoubi, S. Krakstad, J.O. and Alvheim, O. 2011. Towards coordinated acoustic
surveys in Northwest Africa: Results of parallel surveys and intercalibrations. In/
Dans S. Garcia, M. Tandstad & A.M. Caramelo, eds. Science and Management of Small
Pelagics. Symposium on Science and the Challenge of Managing Small Pelagic Fisheries
on Shared Stocks in Northwest Africa, 11–14 March 2008, Casablanca, Morocco/
Science et aménagement des petits pélagiques. Symposium sur la science et le défi de
l’aménagement des pêcheries de petits pélagiques sur les stocks partagés en Afrique nord-
occidentale, 11-14 mars 2008, Casablanca, Maroc, FAO Fisheries and Aquaculture
Proceedings/FAO Comptes rendus des pêches et de l’aquaculture, No. 18. Rome,
FAO. 606 pp.
ABSTRACT
The coastal pelagic fish off Northwest Africa are stocks of high commercial and
social importance that are shared between the countries of the subregion. Since
1995, these resources are subject to regular evaluation by the Norwegian research
vessel R/V Dr. Fridtjof Nansen through regional acoustic surveys covering the area
from southern Senegal to Cape Cantin in Morocco. This activity is undertaken
within the framework of a programme of cooperation between the countries,
FAO and the NANSEN Programme. With the acquisition of national research
vessels with acoustic equipment, scientific activities were initiated to prepare for
the national vessels to take over the responsibility for the acoustic surveys carried
out by the R/V Dr. Fridtjof Nansen. To achieve this, several parallel surveys and
intercalibration exercises were carried out to ensure comparable results between

Science and Management of Small Pelagics/Science et aménagement des petits pélagiques
260
the national vessels and the R/V Dr. Fridtjof Nansen, using the latter as a reference.
This study describes some of the results of these parallel surveys and intercalibrations,
shows the recorded improvements made, underlines technical or methodological
problems encountered and makes recommendations to improve the precision and
the reliability of the abundance estimates of the main pelagic resources in the area.
Keywords: acoustics, intercalibrations, parallel surveys, Northwest Africa, shared
marine resources, small pelagics, biomass estimates.
RÉSUMÉ
Les poissons pélagiques côtiers en Afrique du nord-ouest représentent des stocks
considérables partagés entre le Sénégal, la Gambie, la Mauritanie et le Maroc. Depuis
1995, ces ressources font l’objet d’évaluations par le navire de recherche norvégien
le N/R Dr. Fridtjof Nansen à travers des campagnes acoustiques couvrant toute la
région. Cette activité est entreprise dans le cadre d’un programme de coopération
avec les pays de la sous-région, le Programme Nansen, sous l’auspice de la FAO.
Avec l’acquisition de navires locaux équipés de matériels acoustiques, des activités
scientifiques sont en cours pour suppléer le N/R Dr. Fridtjof Nansen. Il s’agit
principalement de conduire des campagnes parallèles et d’effectuer des exercices
d’intercalibration pour aboutir à des résultats comparables. Ainsi plusieurs
exercices ont été conduits par les navires nationaux avec comme référence le N/R
Dr. Fridtjof Nansen.
Cette étude présente certains résultats de ces campagnes parallèles et intercalibrations,
évoque les améliorations enregistrées, souligne des problèmes techniques ou
méthodologiques rencontrés et formule des recommandations pour améliorer la
précision et la fiabilité des estimations d’abondance dans la région.
Mots clés: acoustique, intercalibrations, campagnes parallèles, Afrique du
nord-ouest, stocks marins partagés, petits pélagiques, estimations de
biomasses
1. INTRODUCTION
During the third Planning Group of acoustic activities in Northwest Africa (Dakar,
October 2004), one of the main recommendations was to conduct future regional
surveys with the three local research vessels in Senegal, Mauritania and Morocco.
One of the reasons was that the R/V Dr. Fridtjof Nansen was expected to leave the
region in the near future. Combined surveys with three different research vessels can
be achieved only if the local vessels have the same references, the same procedures and
produce comparable results. This was to be assessed by performing parallel surveys and
intercalibrations between the vessels in the region and the R/V Dr. Fridtjof Nansen. A
parallel survey was therefore conducted in October–December 2004 and the results
analysed during a workshop in Casablanca the following January. The recommendations
from that workshop were that the parallel survey and intercalibrations generally had
been successful, providing valuable experience and much needed results. It highlighted
several problem areas were the countries in the region had to focus their effort in
order to be able to run a regional assessment survey with three different vessels. One
of the recommendations from the workshop was that a second parallel survey and
intercalibration exercise should be conducted in 2005 in order to compare the results
from 2004, assess any improvements in performance and indicate areas where more
effort is still needed to reliably assess the biomass in the region with the three local

261
research vessels. The survey took place from 31 October to 1 December 2005 and was
generally successfully conducted. The workshop of April 2006 gave the opportunity to
compare results, correct procedures and standardize methods of processing the acoustic
surveys across the region to prepare the research institutions in the region for future
regional surveys with local vessels without the assistance of R/V Dr. Fridtjof Nansen.
2. METHODS
The methodology used during intercalibrations and parallel surveys is as follows. The
R/V Dr. Fridtjof Nansen conducted the regional survey in the usual way following the
“Guidelines for acoustic surveys in the Northwest African region”. A parallel survey
was conducted by the local research vessels following the same methodology. All vessels
were operating with newly calibrated acoustic equipments. In general, the methodology
is described in the respective cruise reports. All vessels produced:
a) survey maps with cruise tracks;
b) distribution maps of target species;
c) biomass estimates of target species;
d) catch results;
e) length frequency distributions for target species;
f) SA values from intercalibrations.
The intercalibration exercises were carried out according to standard procedure
with one vessel 0.5 NM ahead of the other, and 0.5 SA degrees offset at starboard side
(Figure 1). The course tracks included whenever possible shallow and deep waters with
areas of high fish concentrations.
For each intercalibration, total SA
values per nautical mile sailed by the
two vessels and the correlation between
the nautical mile values recorded by
the vessels are recorded, plotted and
compared. It has not been always
possible to reach a total duration of
six hours as recommended for a good
intercalibration but the exercises always
lasted between four and ten hours. All
vessels used EK60 Simrad echosounders
to collect acoustic raw data that was
processed using BI500 or BI60 software.
3. RESULTS FROM SENEGAL
3.1 The 2004 parallel survey
a) Specific arrangement of the 2004 parallel survey
During this survey, 397 nm were jointly surveyed. The target species were sardinellas,
horse mackerels and chub mackerel. In total 33 trawls hauls and 19 hydrographic
stations were performed.
b) Biomass estimates
The total biomass estimates obtained by the R/V Itaf Deme and R/V Dr. Fridtjof Nansen
(Table 1) are close, reflecting the fact that a similar acoustic energy was received by the
two vessels. The ratio between estimated total biomasses was 1.2. The differences in
estimated biomass by target species can be explained as follows:
• There may be difficulties to interpret echograms in this area because of mixed
plankton and high number of species in the Senegalese area.
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques
FIGURE 1
Diagram of the parallel survey design

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262
• Onboard the R/V Itaf Deme, when direct interpretation of echogram cannot be
performed, the trawling does not always give enough information to allow good
allocation of species.
TABLE 1
Senegal 2004 parallel survey. Biomass estimates by area calculated from R/V Itaf Deme (ID) and R/V Dr. F. Nansen
(FN) respectively. Data in 000’s tonnes.
Species R/V Itaf Deme Ratio;
ID/FN1R/V Dr. Fridtjof Nansen2
Casamance Central North Total Casamance Gambia Gambia/
Cape Verde Cape Verde/
St Louis Total
S. maderensis 185 80 6.2 271 0.61 249 107 154 39 548
S. aurita 140 92 23 255 1.02 117 21 108 25 270
Horse mackerels 52 128 7 187 3.97 0 29 33 15 76
Mackerel 3 22 4 28 - 0 0 0 0 0
Other pelagics3151 140 46 337 2.15 58 56 79 20 212
Total 531 461 86 1078 1.21 424 213 374 99 1106
1Senegal only.
2The data for Gambia are excluded from the ratio FN/ID. The Gambia/Cape Verde and Cape Verde/St Louis areas for the FN
correspond respectively to the central and north areas for the ID.
3Scomber sp. is included in “Other pelagics”.
c) 2004 intercalibrations
Two intercalibration exercises have been performed in 2004. The software used for
interpretation was BI500. A minimum Sv threshold of -79db was used by both vessels.
The first intercalibration (Figure 2) has given close results between R/V Itaf Deme and
R/V Dr. Fridtjof Nansen as shown in the table below presenting total SA values, average
and standard deviation from the ships.
TABLE 2:
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 338 323 450 101 1.33
Average 3 192 4 129 1.29
Standard dev. 3 610 4 778
FIGURE 2
Senegal 2004 first intercalibration. a) Total SA values per nautical mile sailed for R/V Itaf Deme and
R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values recorded by the two
vessels (right)

263
A linear regression was made between the two data sets. According to the regression,
the relation between the biomass estimated by the two vessels was:
Dr. Fridtjof Nansen = 1.1832 Itaf Deme +326.97 with R = 0.88
The second intercalibration (Figure 3) had confirmed those results. According to the
regression made between the two data sets, the relation between the biomass estimated
by the two vessels was:
Dr. Fridtjof Nansen = 1.0017 Itaf Deme +202.8 with R = 0.95
The software used onboard R/V Itaf Deme for interpretation was BI500. The
minimum Sv threshold of -79db was used by both vessels. The values obtained by
the R/V Dr. Fridtjof Nansen values were very similar to those of R/V Itaf Deme. The
difference was not significant and it was concluded that the two vessels had similar
acoustic performance.
3.2 The 2005 parallel survey
a) Specific arrangement of the 2005 parallel survey
In total, 1 309 nm were jointly surveyed, of which 277 nm in Gambian waters. The
target species were sardinellas, horse mackerels and chub mackerel. Twenty-eight trawl
hauls were performed, of which 14 bottom trawls, two trawls with floats and 12 pelagic
trawls. Fifty-three hydrographic stations were also performed. The target strength (TS)
of herring was used for biomass estimates.
b) Biomass estimates
The total biomass estimates from R/V Itaf Deme and R/V Dr. Fridtjof Nansen
(Table 3) are close with a ratio of 1.13, representing a noticeable amelioration from
the 2004 result (1.2). Biomass in Gambia represented 31 percent of total biomass on
R/V Itaf Deme and 37 percent of total biomass on R/V Dr. Fridtjof Nansen.
FIGURE 3
Senegal 2004 second intercalibration. a) Total SA values per nautical mile sailed for R/V Itaf Deme and R/V
Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values recorded by the two vessels
(right)
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

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TABLE 3
Senegal 2005 parallel survey. Biomass estimates per area calculated from R/V Itaf Deme (ID) and R/V Dr. Fridtjof
Nansen (FN) respectively. Data in thousand tonnes.
Species R/V Itaf Deme Ratio;
ID/FN
R/V Dr. Fridtjof Nansen
Casamance Gambia Gambia
– Cape
Verde
Cape
Verde
St Louis
Total Casamance Gambia Gambia
– Cape
Verde
Cape
Verde
St Louis
Total
S. maderensis 396 127 99 0.13 622.2 1.14 201 188 148 8 545
S. aurita 2 8 87 0.7 97.8 0.35 19 73 191 - 283
Horse mackerels 2 125 35 42 204.8 1.77 3. 4 12 97 116
Chub mackerel - - 0.2 - 0.2 -- - - - -
Other pelagics 76 107 69 17 269.8 2.35 59 19 26 11 115
Total 476.1 368.24 289.79 60.65 1194.8 1.13 279 284 377 116 1059
c) 2005 intercalibrations
Several intercalibration exercises were conducted as part of the parallel survey with the
R/V Dr. Fridtjof Nansen. Generally, at least two exercises were made with each vessel.
Several problems were encountered that complicated the initial data analysis. These
have been listed below.
• Different Sv thresholds. The vessels had been using different minimum Sv
threshold when scrutinizing and exporting data. These ranged from -70 db to
-79 db. It has been suggested that a minimum threshold of -79 db be used for all
vessels in the future.
• Bottom fish detection. Settings in the bottom detection algorithm may affect what
is detected as fish close to the bottom. A general recommendation was difficult
to give but it was underlined that the operator should alert when scrutinizing fish
close to the bottom.
• Software issues: Morocco had used the same –Sv threshold as the R/V Dr. Fridtjof
Nansen (-79 db). However, the export function in the BI60 software v2.1.0 had
a bug, and it was decided to upgrade the software to v2.1.1 for all analyses of
acoustic data in the region.
Because of these problems, each country had to re-examine its intercalibration
data with a minimum threshold of -79 db in order to have comparable results. As an
example of the difference introduced to the total estimate by using different minimum
Sv threshold, the intercalibration data for Senegal has been analysed twice, and two
different datasets and results are presented below.
d) Analyses with different Sv thresholds (R/V Dr. Fridtjof Nansen versus R/V Itaf
Deme)
Three intercalibration exercises had been performed in 2005. The software used for
interpretation was BI500. A minimum Sv threshold of -79db has been used onboard
R/V Dr. Fridtjof Nansen while a threshold of -70 db has been used onboard R/V Itaf
Deme.
First intercalibration
The distance covered was 56 nm, from 600 m depth (offshore) to 20 m depth
(inshore). The intercalibration survey was carried out according to standard
procedure (cf. Figure 1). The conditions were good for intership comparison.
Relatively high recordings, mainly of plankton, were made offshore, while sardinella
concentrations where recorded inshore at the end of the intercalibration tracks. The
average recorded values were slightly higher for R/V Dr. Fridtjof Nansen as shown
in the table below.

265
TABLE 4
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 358 027 367 339 1.03
Average 7 783 8 163 1.05
Standard dev. 10 732 11 185
Figure 4a shows the recorded integrator values (total SA values per nautical mile)
vs time recorded for the two ships. Figure 4b shows the regression between the
intercalibration data from the two vessels. The figures illustrate that the vessels made
similar acoustic recordings during the intercalibration with generally slightly higher
recordings with the R/V Dr. Fridtjof Nansen. The higher variance may also indicate
that this vessel had encountered slightly more fish schools. The relation between the
biomass estimated by the two vessels was:
Itaf Deme = 0.9362 Dr. Fridtjof Nansen +294.15 with R = 0.94
Second intercalibration
It was conducted off Cape Verde on the 5–6 November 2005 from 22:00 to 04:20
covering approximately 40 nm (Figure 5). Although northerly winds were not
very strong, it made sea conditions
difficult throughout the first part
of the night. In addition, the two
vessels did not adhere strictly to the
procedures for intercalibration and
moved further apart from each other
than 0.5 mile. Evaluation of the data
from the intercalibration suggested
that they were not suitable to evaluate
the difference in performance between
the vessels. The R/V Itaf Deme
generally recorded higher values than
the R/V Dr. Fridtjof Nansen when
going against the sea and wind,
cruising seawards, and similar values
after turning shoreward.
FIGURE 4
Senegal 2005 first intercalibration analysed with -70db threshold. a) Total SA values (m2/NM2) vs time
for R/V. Itaf Deme and R/V Dr. Fridtjof Nansen (left) and b) correlation between average SA recorded per
nautical mile values recorded by the two vessels (right)
FIGURE 5
Senegal 2005 second intercalibration analysed with
-70db threshold.Total SA values (m2/nm2) vs. time for
R/V ITAF DEME and R/V Dr. Fridtjof Nansen
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

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Third intercalibration
It was conducted from south of St. Louis to Cayar Canyon starting on 8 November
2005 around midnight and lasted until 07:50 the following morning. The vessels
covered a course track of approximately 72 nm. The conditions were generally good
for intercalibration and the vessels followed the recommended procedures. Several
zig-zag transects were conducted between bottom depths of 200 and 20 m. During this
intercalibration, the average recorded values were slightly lower for R/V Dr. Fridtjof
Nansen as shown in the table below.
TABLE 5
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 475 600 444 400 0.93
Average 6 606 6 088 0.92
Standard dev. 5 347 6 079
Figure 6a shows the recorded integrator values (total SA values per nautical mile)
vs time recorded for the two ships, and Figure 6b shows the regression between
the intercalibration data from the two vessels. The figures illustrate that the vessels
made similar acoustic recordings during the intercalibration with generally slightly
lower recordings with R/V Dr. Fridtjof Nansen. The higher variance recorded with
R/V Dr. Fridtjof Nansen may indicate that the vessel has encountered slightly more
fish schools. Although the average SA values recorded during the third intercalibration
was consistently higher for R/V Itaf Deme than R/V Dr. Fridtjof Nansen, the data are
still comparable and the differences between the vessels are probably best explained
by natural variation in the amount of encountered fish. A linear regression was made
between the two data sets. The relation between the biomass estimated by the two
vessels was:
Dr. Fridtjof Nansen = 0.9204 Itaf Deme +1270.4 with R = 0.87
FIGURE 6
Senegal 2005 third intercalibration analysed with -70db threshold. a) Total SA values vs time recorded
for R/V Itaf Deme and R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values
recorded by the two vessels (right)

267
Conclusions
Generally, the two vessels recorded the same acoustic densities and the first and the
third intercalibration experiment seem to have been successful. However the first
intercalibration showed that R/V Itaf Deme recorded lower acoustic densities (average
difference in SA values of -379.9) than R/V Dr. Fridtjof Nansen while the third showed
that R/V Itaf Deme had somewhat higher acoustic recordings than R/V Dr. Fridtjof
Nansen (average difference in SA values of 517.9). The conditions were good for
intercalibration during both exercises. Since the vessels used different Sv threshold
settings, it is believed that both intercalibrations should have shown that R/V Itaf
Deme had somewhat higher recordings than R/V Dr. Fridtjof Nansen during both
surveys. The second intercalibration illustrates several issues. The most important is
the role of the weather factor as the quality of the intercalibration is sharply reduced
with bad weather. This factor affected the R/V Itaf Deme most because of its smaller
size and the absence of roll sensor onboard and drop-down keel. The second factor
that may have affected the estimate was the fact that the vessels did not manage to keep
equal distance at all times during this intercalibration.
e) Analyses with the same threshold (-79 db) (R/V Dr. Fridtjof Nansen versus R/V
Itaf Deme)
The stored raw data collected from October 2005 intercalibrations and interpreted
previously by BI500 Simrad software, are now interpreted using BI 60. A minimum Sv
threshold of -79db is used for both vessels.
First intercalibration
The analysis gives much higher SA values for R/V Dr. Fridtjof Nansen as shown in the
table below.
TABLE 6
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 84 044 201 829 2.4
Average 2 272 5 455 2.4
Standard dev. 2 570 8 922
R20,42
R 0,65
These big differences are introduced by the SA values of R/V Dr. Fridtjof Nansen in
areas with high concentrations where R/V Itaf Deme gives relatively small SA values
(Figure 7). If we exclude these areas with high concentrations (Figure 8), the recordings
for both vessels are in the same level (with a ratio of 1.07 between averages) even
though the correlation factor is low (R=0.48).
TABLE 7
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 34 249 37 242.2 1.09
Average 1 317 1 432 1.07
Standard dev. 1 302 1 388
R20.23
R 0.48
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

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TABLE 8
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Dr. Fridtjof Nansen Ratio FN/ID
Total SA 64 120 59 196 0.92
Average 4 275 3 946 0.92
Standard dev. 1 106 1 222
R20.54
R 0.73
Second intercalibration
The analyse gives similar SA values for both R/V Dr. Fridtjof Nansen and R/V Itaf
Deme (relative difference in SA of 8.32) as shown in the table below but it concerns
only the beginning of the intercalibration where low concentrations were met (Figure
9 ). The data were unreadable for the last part of the exercise.
FIGURE 7
Senegal 2005 first intercalibration analysed with -79db threshold a) Total SA values vs time recorded
for R/V Itaf Deme and R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values
recorded by the two vessels (right)
FIGURE 8
Senegal 2005 first intercalibration analysed with -79db threshold and without areas with high
concentrations a) Total SA values vs time recorded for R/V Itaf Deme and R/V Dr. Fridtjof Nansen (left) and
b) correlation between the nautical mile values recorded by the two vessels (right)

269
Conclusion
When interpreting with same threshold of -79db on both vessels and using BI60 as
processing software, the average recorded values are much higher for R/V Dr. Fridtjof
Nansen (about 58.36 percent difference in average).
f) Intercalibrations between R/V Itaf Deme and R/V Al Awam
Two intercalibration exercises were performed between R/V Itaf Deme and R/V Al
Awam, the first in March 2005 and the second in November 2007. During both
exercises, a minimum Sv threshold of -79db had been used and the software for
interpretation was BI60 for both vessels.
First intercalibration
The intercalibration was done over a total distance of 48 nm from shallow to deep
waters, in the two directions (seaward and shoreward). The intercalibration was carried
out according to standard procedure (cf. Figure 1). The intercalibration was performed
during night time from 22:00 to 04:00 on the following morning. The depth ranged
sampled was 20–500 m. Some results of the intercalibration are given in the table below
and in Figure 10 showing the level of integrator values (total SA per nautical mile) along
the course line for the two ships.
TABLE 9
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Al Awam Ratio AW/ID
Total SA 76 332 107 783 1.42
Average 1 527 2 156 1.41
Standard dev. 1 418 2 288
Figure 10 shows a bad correlation between the two vessels in deep waters (from time
00:23 to 00:42 corresponding to depth >220 m). This is due to the fact that the R/V Itaf
Deme’s integrator was set to operate only from 0 to 220 m deep. When we exclude the
data from deep waters (depths <220 m, see Figure 11) we have the results shown in the
text table below.
FIGURE 9
Senegal 2005 third intercalibration analysed with -79db threshold. a) Total SA values vs time recorded
for R/V Itaf Deme and R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values
recorded by the two vessels (right)
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

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TABLE 10
Total SA values, average and standard deviation from the ships
R/V Itaf Deme R/V Al Awam Ratio AW/ID
Total SA 67 910 67 449 0.99
Average 1 698 1 686 0.99
Standard dev. 1 516 2 123
Even if the correlation coefficient is
still bad (R=0.05; Figure12), we can see
from this table that, on average, the two
vessels had met the same concentrations,
but not simultaneously. Accordingly, it
may be concluded that the two vessels
have equal performance. This must be
confirmed by other intercalibrations. In
addition, an intercalibration with more
homogeneous population, in depths less
than 220 m has been recommended for
the future.
FIGURE 10
Senegal 2006 intercalibration with Mauritania: Total SA
values per nautical mile sailed for R/V Itaf Deme and R/V Al
Awam
FIGURE 11
Senegal 2006 intercalibration with Mauritania and with
depths >220 m excluded. Total SA values per nautical mile
sailed for R/V Itaf Deme and R/V Al Awam
FIGURE 12
Senegal 2006 intercalibration with Mauritania and
with depths >220 m excluded. Plot of the SA values of
R/V Itaf Deme and R/V Al Awam

271
Second intercalibration
The results of the intercalibration are shown in the table below.
TABLE 11
Total SA values, average and standard deviation from the ships
Itaf Deme Al Awam Ratio AW/ID
Total SA 127 389 221 830 1.70
Average 2 498 44 376 1.63
Standard dev. 1 995 3 243
The total SA values per nautical mile sailed for R/V Itaf Deme and R/V Al Awam
during this intercalibration, are shown in Figure 13. A linear regression was made
between the two data sets (Figure 14). According to the regression, the relation
between the biomass estimated by the two vessels were
Al Awam = 1. 0822 Itaf Deme + 1 716 3 with R = 0.67
Conclusions on the R/V Itaf Deme intercalibrations
Since 2006, the SA values recorded by
the R/V Itaf Deme have been relatively
low compared with those from the
R/V Dr. Fridtjof Nansen. The ratio
between total biomass averages rises
from 1.29 in 2005 to 2.4 in 2006. The SA
values recorded by the R/V Itaf Deme
are also lower than those from the
R/V Al Awam with a ratio around
1.63. The acoustic response of the
echosounder has been updated by
calibration of the equipment onboard
the R/V Itaf Deme but this cannot
explain such a reduction in the SA values
registered by the R/V Itaf Deme since
the interpretation concerned the same
raw data stored from ER60 sounder. As
the only significant change operated in
2006 was the use of the BI60 software
for scrutinizing raw data, it may be
concluded that it is this change that has
generated the reduction in SA values
registered by the R/V Itaf Deme.
4. RESULTS FROM MAURITANIA
4.1 The 2004 parallel survey
a) Specific arrangement of the 2004
parallel survey
A distance of 1 200 nm was jointly covered. Target species were: sardinella, anchovy,
sardine, mackerel and horse mackerels. Twenty-seven trawls hauls were performed.
The Target Strength (TS) used for biomass estimates was that for herring.
FIGURE 13
Senegal 2007 intercalibration with Mauritania. Total SA
values per nautical mile sailed for R/V Itaf Deme and
R/V Al Awam
FIGURE 14
Senegal 2007 intercalibration with Mauritania. Correlation
between the nautical mile values recorded by R/V Itaf Deme
and R/V Al Awam
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b) Biomass estimates
Biomass estimates of species and groups of species (Table 12) are relatively different
for the two vessels: The ratio between the total biomass estimated by the two vessels is
0.39 and the highest difference is for flat sardinella (Sardinella maderensis) with a ratio
of 0.16. The differences in biomass estimates may be partly explained by the differences
in catch composition. It can be seen that R/V Al Awam samples reflect apparently
better the small individuals than R/V Dr. Fridtjof Nansen which seems to sample bigger
fishes. This may be due to the limited speed of R/V Al Awam, the engine of which was
overheating and could not reach 4 knots. In addition, big individuals of horse mackerel
are underestimated by R/V Al Awam because its maximum fishing depth is 100 m
TABLE 12
Mauritania 2004 parallel survey. Biomass estimates per area calculated from R/V Al Awam (AW) and
R/V Dr. Fridtjof Nansen (NAN) respectively. Data in 000’s tonnes. S. MAD: Sardinella maderensis;
S. AUR: Sardinella aurita; PILCH: Sardina pilchardus; H. MACK: Chinchards (Trachurus trecae,
Trachurus trachurus, Decapterus rhonchus); ANCH. Engraulis encrasicolus, O/PEL: autres pélagiques
Species R/V Al Awam Ratio
AW/NAN
R/V Dr. Fridtjof Nansen
St. Louis –
C. Timiris C. Timiris –
C. Blanc Total %St. Louis –
C. Timiris C. Timiris –
C. Blanc Total
S. MAD 194.3 17.3 212 0.16 1244 91 1335
S. AUR 156.4 177.1 333 1.56 189 24 213
PILCH 48.8 67.4 93 0.23 20 384 404
C. RHON 87.1 3 90.1
T. TRE 21 2.7 23.7 0.29 29.8 53 83
ANCH. 0 16.2 16.2 0 0 0
O/PEL 90 0.60 106 42 148
Total 858 0.39 1588.8 594 2183
c) 2004 intercalibrations
Two intercalibration exercises were performed with the R/V Dr. Fridtjof Nansen. Only
the first one gave acceptable results.
First intercalibration
The total SA values per nautical mile sailed by R/V Dr. Fridtjof Nansen and R/V Al
Awam during the first intercalibration are shown in Figure 15. A ratio of 1.41 is found
between the averages of the estimated biomass by the two vessels as shown in the table
below.
TABLE 13
Total SA values, average and standard deviation from the ships
R/V Al Awam R/V Dr. Fridtjof Nansen Ratio FN/AW
Total SA 97 369 137 610 1.41
Average 2 562 3 719 1.45
Standard dev. 1 941 28 489 -
A linear regression was made between the two data sets. The relation between the
biomass estimated by the two vessels is:
F. NANSEN = 0.956 AL AWAM +1 207 with R = 0.75
The SA values registered from R/V Al Awam are lower than those from
R/V Dr. Fridtjof Nansen with a difference of around 30 percent.

273
4.2 The 2005 parallel survey
a) Specific arrangement of the 2004 parallel survey
A distance of 1 200 nm was jointly surveyed. Due to a technical problem, the last part
of the survey, from Cape Timiris was conducted two days after the R/V Dr. Fridtjof
Nansen. The target species: sardinella, anchovy, sardine, mackerel and horse mackerels.
Sixteen trawl hauls were performed. No hydrographical stations were made. The target
strength (TS) used for biomass estimates was that for herring.
b) Biomass estimates
Biomass estimates of species and groups of species (Table 14) made by the two vessels
are still relatively different, as in October 2004. The ratio between the total biomass
estimated by the two vessels is 0.41 and the highest difference is for sardine (Sardina
pilchardus) with a ratio of 0.23.
TABLE 14
Mauritania 2005 parallel survey Biomass estimates per area calculated from R/V Al Awam (AW)
and R/V Dr. Fridtjof Nansen (NAN) respectively. Data in 000’s tonnes
Species R/V Al Awam Ratio
AW/NAN
R/V Dr. Fridtjof Nansen
St. Louis –
C. Timiris C. Timiris –
C. Blanc Total %St. Louis –
C. Timiris C. Timiris –
C. lanc Total
Sardinella maderensis 52 202 253 0.48 302 225 527
Sardinella aurita 16 57 74 1.00 6 68 74
Sardina pilchardus 395 97 492 0.23 1075 1103 2178
Engraulis encrasicolus 45 45 0.46 0 98 98
Trachurus trecae 76 103 179 0.90 106 92 198
Other pelagics 48 202 250 3.85 62 3 65
Total 587 706 1293 0.41 1551 1589 3140
c) 2005 intercalibrations
Two intercalibration exercises have been performed with the R/V in 2005. Only the
first one gave acceptable results. The software used for interpretation was BI 60. A
minimum Sv threshold of 79db was used by both vessels.
FIGURE 15
Mauritania 2004 first intercalibration a) Total SA values per nautical mile sailed for R/V Al Awam and
R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values recorded by the two
vessels (right)
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First intercalibration
The total SA values per nautical mile sailed by R/V Dr. Fridtjof Nansen and R/V Al
Awam during the first intercalibration, are shown in Figure 16. A ratio of 1.1 is found
between the averages of the estimated biomass by the two vessels as shown in the table
below.
TABLE 15
Total SA values, average and standard deviation from the ships
R/V Al Awam R/V Dr. Fridtjof Nansen Ratio FN/AW
Total SA 42 659 46 989 1.1
Average 2 666 2 936 1.1
Standard dev. 2 133 2 579
A linear regression was made between the two data sets. The relation between the
biomass estimated by the two vessels was:
R/V Al Awam = 0.6947 R/V Dr. Fridtjof Nansen +625.96 with R = 0.84
4.3 Comparison of intercalibrations of 2004 and 2005
The table below summarizes the results of 2004 and 2005 intercalibrations.
TABLE 16
Overview of results from 2004 and 2005 intercalibrations
2004 2005
SA total Nansen/SA total R/V Al Awam 1.7071 1.1015
Mean Nansen/Mean R/V Al Awam 1.4528 1.1012
R20.64 0.71
R 0.80 0.84
A noticeable improvement is registered between the 2004 and 2005 intercalibrations.
The ratio between total SA is 1.1 in 2005, compared to 1.7 in 2004. In addition, the
coefficient of correlation had improved from 0.63 to 0.70.
FIGURE 16
Mauritania 2004 second intercalibration a) total SA values per nautical mile sailed for R/V Al Awam and
R/V Dr. Fridtjof Nansen (left) and b) correlation between the nautical mile values recorded by the two
vessels (right)

275
4.4 Intercalibration with AL AWAM with AL AMIR
The exercise was performed in November 2007. The course tracks followed by the two
vessels (Figure 17a) cover shallow and deep waters. The total SA values per nautical mile
sailed for R/V Al Amir and R/V Al Awam during the first intercalibration are shown
in Figure 17b. A linear regression (Figure 18) was made between the two data sets. The
relation between the biomass estimated by the two vessels was:
R/V Al Amir = 0.5 R/V Al Awam + 873 with R = 0.74
4.5 Conclusion on R/V Al Awam intercalibrations
A noticeable improvement is
registered between the 2004 and 2005
intercalibrations for R/V Al Awam even
through its recordings are still lower
than those from the R/V Dr. Fridtjof
Nansen.
5. RESULTS FROM MOROCCO
5.1 The 2004 parallel survey
a) Specific arrangement of the 2004
parallel survey
A distance of 1 400 miles was jointly
covered. The target species were:
sardine, anchovy, mackerel and horse
mackerel. Twenty-four trawl hauls
were performed. The target strength
(TS) used for biomass estimates was
that of herring.
b) Biomass estimates
The total biomass estimates obtained
from R/V Al Amir and R/V Dr. Fridtjof
Nansen (Table 17) are quite close. The
ratio between the total biomasses is 0.87.
A high difference is found between the
estimated biomass of horse mackerel.
FIGURE 17A
Mauritania 2007 intercalibration with Morocco. Course
tracks followed by R/V Al Awam and R/V Al Amir
FIGURE 17B
Mauritania 2007 intercalibration with Morocco. Total
SA values per nautical mile sailed for R/V Al Awam and
R/V Al Amir
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques
FIGURE 18
Mauritania 2007 intercalibration with Morocco. Correlation
between the nautical mile values recorded by R/V Al Awam
and R/V Al Amir

Science and Management of Small Pelagics/Science et aménagement des petits pélagiques
276
TABLE 17
Morocco 2004 parallel survey biomass estimates per area calculated from R/V Al Amir (left) and
R/V Dr. Fridtjof Nansen (right) respectively. Data in 000’s tonnes
Species R/V Al Amir Ratio
AM/FN
R/V Dr. Fridtjof Nansen
C.Juby –
C.Ghir C.Ghir –
C.Cantin Total C.Juby –
C.Ghir C.Ghir –
C.Cantin Total
Pilchard 408 218 626 0.75 627 210 837
Anchovy 40 26 66 230 2 32
Scomber 174 112 286 0.85 242 96 338
Horse mackerel 64 21 85 42.5 022
Total 686 377 1063 0.87 899 310 1209
c) 2004 intercalibrations
Four intercalibration exercises had been performed in 2004 with the R/V Dr. Fridtjof
Nansen. The software used for
interpretation was BI500. A minimum
Sv threshold of -79db was used by both
vessels. The results are summarized
below.
First intercalibration
The total SA values per nautical mile
sailed for the R/V Dr. Fridtjof Nansen
and R/V Al Amir during the first
intercalibration, are shown in Figure
19. A ratio of 1.02 is found between the
average estimated biomasses from the
two vessels as shown in the table below.
TABLE 18
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM
Total SA 154 734 158 517 1.03
Average 2 668 2 733 1.02
According to the regression (Figure 20 ), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.8712 R/V Fridtjof Nansen + 286.86 with R = 0.9
Second intercalibration
The total SA values per nautical mile sailed for R/V Dr. Fridtjof Nansen and R/V Al
Awam during the first intercalibration, are shown in Figure 21. A ratio of 1.37 is found
between the averages of the estimated biomass by the two vessels as shown in the table
below.
TABLE 19
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM
Total SA 159 539 218 792 1.37
Average 3 468 4 756 1.37
FIGURE 19
Morocco 2004 first intercalibration. Total SA values
per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen

277
According to the regression (Figure 22 ), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.7808 R/V Fridtjof Nansen + 245.47 with R = 0.97
FIGURE 20
Morocco 2004 first intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 21
Morocco 2004 second intercalibration. Total SA
values per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen
Theme 3: Methods and models for assessing small pelagic fish stocks/
Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques
FIGURE 22
Morocco 2004 second intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen

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Third intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration are shown in Figure 23.
A ratio of 1.06 is found between the
averages of the estimated biomass by
the two vessels as shown in the table
below.
TABLE 20
Total SA values, average and standard deviation from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM
Total SA165 463 175 102 1.05
Average 3 848 4 072 1.06
According to the regression (Figure 24) the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.7705 R/V Fridtjof Nansen + 710.48 with R = 0.73
FIGURE 23
Morocco 2004 third intercalibration. Total SA
values per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 24
Morocco 2004 third intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen

279
Fourth intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration, are shown in Figure
25. A ratio of 1.19 is found between the
averages of the estimated biomass by
the two vessels as shown in the table
below.
TABLE 21
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM
Total SA sum 82 255 98 514 1.19
Average 3 290 3 941 1.19
According to the regression (Figure 26), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.1657 R/V Dr. Fridtjof Nansen + 2 637.4 with R = 0.2
5.2 The 2005 parallel survey
a) Specific arrangement of the 2004 parallel survey
A distance of 2 239 nm was covered. The target species were sardine, anchovy, mackerel
and horse mackerel. Thirty-three trawl hauls were made. Fifty-four biological samples
were collected. No hydrographical stations were made. The herring target strength was
used for biomass estimates.
b) Biomass estimates
The biomass estimated by each vessel from Cape Blanc to Cape Bojador is presented in
Table 22. The ratio between the total biomasses is 0.72, with a noticeable shift compared
FIGURE 26
Morocco 2004 fouth intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 25
Morocco 2004 fourth intercalibration. Total SA
values per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen
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to 0.87 in 2004. The biomass ratio for horse mackerel 0.33 is now inversed compared to
2004 where it was 42.5. The values for anchovy are still higher with R/V Al Amir than
R/V Dr. Fridtjof Nansen.
TABLE 22
Morocco 2005 parallel survey Biomass estimates per area calculated from R/V Al Amir (left) and
R/V Dr. Fridtjof Nansen (right) respectively. Data in 000’s tonnes.
Species R/V Al Amir Ratio AM/FN R/V Dr. Fridtjof Nansen
Cape Blanc to Cape Bojador %Cape Blanc to Cape Bojador
Pilchard 4 628 0.79 5 830
Anchovy 33 3.30 10
Chub mackerel 189 0.79 240
Horse mackerel 370 0.33 1 125
Sardinella 409 0.68 600
Total 5 629 0.72 7 805
c) The 2005 intercalibrations
First intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration, are shown in Figure
27. A ratio of 0.1 is found between the
averages of the estimated biomass by
the two vessels as shown in the table
below.
TABLE 23
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM
Total SA 79 441 8 214.9 0.1
Average 1 986 205 0.1
According to the regression (Figure 28), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.0595 R/V Dr. Fridtjof Nansen + 87.145 with R = 0.5
FIGURE 27
Morocco 2005 first intercalibration total SA values
per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 28
Morocco 2005 first intercalibration. Correlation between
the nautical mile values recorded R/V Al Amir and
R/V Dr. Fridtjof Nansen

281
Second intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration, are shown in Figure
29. A ratio of 2.86 is found between the
averages of the estimated biomass by the
two vessels as shown in the table below.
TABLE 24
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM %
Total SA 97 412 278 837 2.86 65.06
Average 1 680 4 807 2.86 65.06
According to the regression (Figure 30), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.1366 R/V Dr. Fridtjof Nansen + 993.01 with R = 0.5
FIGURE 29
Morocco 2005 second intercalibration. Total SA
values per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 30
Morocco 2005 second intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen
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Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

Science and Management of Small Pelagics/Science et aménagement des petits pélagiques
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Third intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration are shown in Figure
31. A ratio of 3.9 is found between the
averages of the estimated biomass by
the two vessels as shown in the table
below.
TABLE 25
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM %
Total SA41 007 158 993 3.9 74.21
Average 954 3 698 3.9 74.21
According to the regression (Figure 32 ), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.0043 R/V Dr. Fridtjof Nansen + 937.84 with R = 0.06
Fourth intercalibration
The total SA values per nautical mile
sailed for R/V Dr. Fridtjof Nansen
and R/V Al Awam during the first
intercalibration are shown in Figure
33. A ratio of 5.6 is found between the
averages of the estimated biomass by
the two vessels as shown in the table
below.
FIGURE 32
Morocco 2005 third intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen
FIGURE 33
Morocco 2005 fourth intercalibration. Total SA values per
nautical mile sailed R/V Al Amir and R/V Dr. Fridtjof Nansen
FIGURE 31
Morocco 2005 third intercalibration. Total SA
values per nautical mile sailed for R/V Al Amir and
R/V Dr. Fridtjof Nansen

283
TABLE 26
Total SA values and average from the ships
R/V Al Amir R/V Dr. Fridtjof Nansen Ratio FN/AM %
Total SA65 626 368 700 5.6 82.20
Average 2 524 14 181 5.6 82.20
According to the regression (Figure 34), the relation between the biomass estimated
by the two vessels is:
R/V Al Amir = 0.0765 R/V Dr. Fridtjof Nansen + 1 438.6 with R = 0.56
Discussion
The results from intercalibrations in 2004 and 2005 are very similar between the
research vessels R/V Al Amir and R/V Itaf Deme and give comparable acoustic
response with the R/V Dr. Fridtjof Nansen. In the same period, R/V Al Awam had
improved its performance from a 29 percent difference in SA values with the R/V Dr.
Fridtjof Nansen to a nine percent difference only. The last intercalibration between
R/V Al Awam and R/V Dr. Fridtjof Nansen give similar acoustic responses for both
vessels. From these first exercises, it appears that the SA values from the local vessels
are comparable with those from the R/V Dr. Fridtjof Nansen.
However, the last analyses in Senegal for the 2005 intercalibration and those in
Morocco for 2006 give results that need to be confirmed by further exercises. In
Senegal, the differences found with the R/V Dr. Fridtjof Nansen during the second
analysis using the BI60 software seem to be related to the use of this software with
which SA values drop by more than 6 db compared to values obtained with the BI500
software. In Morocco, the important decrease of SA values registered during the 2006
intercalibrations seems to be related with weather conditions. A strong wind was
registered during the exercise and this factor affects R/V Al Amir most because of its
smaller size and lack of stabilizing equipment.
Total biomass estimates made by R/V Itaf Deme, R/V Al Amir and R/V Dr. Fridtjof
Nansen are generally comparable. Differences in biomass estimates in Mauritania may
be partly explained by the differences in catch composition: R/V Al Awam seems able
to better sample the small individuals (or is less able to catch the larger ones) than
R/V Dr. Fridtjof Nansen. This may be due to the limited speed of R/V Al Awam that
could not reach 4 knots. The other factor is that big individuals of horse mackerel are
underestimated by R/V Al Awam because its maximum fishing depth is 100 m.
The main conclusion from these intercalibration exercises is that local vessels
have the ability to jointly conduct regional surveys to complement and eventually
FIGURE 34
Morocco 2005 fourth intercalibration. Correlation between
the nautical mile values recorded by R/V Al Amir and
R/V Dr. Fridtjof Nansen
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Méthodes et modèles pour l’évaluation des stocks de petits poissons pélagiques

Science and Management of Small Pelagics/Science et aménagement des petits pélagiques
284
replace the R/V Dr. Fridtjof Nansen. The total comparability between the local vessels
may be reached after more exercises, readjustments and improvements in trawling
performance. Standardization in methods use for scrutinizing echosounder records,
installation of blister on the keels to reduce weather effect, regularly performing
intercalibrations betweens vessels of the subregion are all factors that will improve
comparability between local vessels. The efforts of acoustic scientists in the region are
oriented to these issues to finally have reliable coordinated acoustic surveys.
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