ICES Journal of
Marine Science
ICES Journal of Marine Science (2016), 73(1), 142– 149. doi:10.1093/icesjms/fsv185
Contribution to the Symposium: ‘International Eel Symposium 2014’
Original Article
Estimates of escapement, exploitation rate, and number of
downstream migrating European eels Anguilla anguilla in Ichkeul
Lake (northern Tunisia)
Emna Derouiche 1*, Besma Hizem Habbechi 1, Med. Mejdeddine Kraı̈em 1, and Pierre Elie2,3
1
Laboratoire d’Aquaculture, INSTM, 28 rue 2 Mars 1934, 2025 Salammbô, Tunisie
Unité Ecosystèmes Estuariens et Poissons Migrateurs Amphihalins, Irstea, 50, avenue de Verdun, 33612 Gazinet Cestas cedex Bordeaux, France
3
Association Santé Poissons Sauvages, BP 9, 13790 Peynier, France
2
*Corresponding author: tel: +216 71730 420; fax: +21671732 622; e-mail: [email protected]
Derouiche, E., Hizem Habbechi, B., Kraı̈em, Med. M., and Elie, P. Estimates of escapement, exploitation rate, and number of
downstream migrating European eels Anguilla anguilla in Ichkeul Lake (northern Tunisia). – ICES Journal of Marine Science,
73: 142 – 149.
Received 1 December 2014; revised 15 September 2015; accepted 24 September 2015; advance access publication 20 October 2015.
The European eel (Anguilla anguilla) stock has been declining for the last three decades and today, it is currently listed as endangered. The objective
of this study was to quantify the escapement rate of silver eels to obtain an estimate of the future spawners migrating from the southern part of its
distribution area in Tunisia. A mark – recapture experiment was conducted in Ichkeul Lake (Tunisia) between December 2013 and February 2014,
covering the winter downstream run of eels. The size range of the downstream migrating eels was between 33 and 79 cm with 585.3 + 156.77 g
mean weight. The number of migrating silver eels was estimated to 342 221 (297 956 – 386 486), corresponding to a biomass of 200.2 (174.3– 226.1)
t, with a density of 23.55 (20.51– 26.6) kg ha21. The commercial fishing rate was estimated to be 18.8% (16.6 – 21.5%), which translates into an
escapement rate of 81.2% (78.5 – 83.4%). The ratio current/pristine escapement was 0.69 for the entire migration period.
Keywords: escapement estimation, European eel, Ichkeul Lake, management plan, mark –recapture, seaward migration, Tunisia.
Introduction
The European eel (Anguilla anguilla) stock is at a historical minimum
and is outside safe biological limits (ICES, 2014). Recruitment has
decreased dramatically across most of Europe since the early 1980s
(Elie and Fontenelle, 1982; Dekker, 2003; ICES, 2014) and has
declined by an estimated 50% in the past 10 years in northern
Africa (Azeroual, 2010). Various factors have been implicated such
as loss and degradation of habitat, contamination by POP and
heavy metals, parasitism by Anguillicola crassus, virus (Evex), predation, barriers to migration, and overexploitation by fishing at all
stages of the continental phase (from glass eels to silver eels). The
European eel is currently considered as an endangered species and
is consequently included in CITES Appendix II list (2007) and in
IUCN Red List (2008; Jacoby and Gollock, 2014). The European
Commission has adopted a regulation [(EC) No. 1100/2007], inviting member states to submit an Eel Management Plan for each river
basin which constitutes a natural habitat for this species.
The increase in silver eel escapement is a primary management
target within each management plan (Robinet et al., 2007). The
Council Regulation urged the escapement of 40% silver eels pristine
biomass (i.e. without any anthropogenic influence) as a minimum
for stock recovery.
Various methods have been set up and tested to estimate silver eel
escapement: directly by catching and/or counting silver eels, proxy
indicators based on knowledge of yellow eel populations, and model
predictions and extrapolations. ICES (2010) and Walker et al.
(2011) reviewed these approaches. Total escapement estimates,
where the whole run of silver eel is intercepted are rare and generally
estimates have to be derived from a mark–recapture study or direct
counts (ICES, 2010). Several studies on silver eels stock have been
carried out across the north side of the European eel distribution
area (Rosell et al., 2005; Aprahamian et al., 2007; Breteler et al.,
2007; Bilotta et al., 2011; Andersson et al, 2012; Prigge et al., 2013;
Marohn et al., 2014) and a few in the southern part (Bevacqua
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Estimates of silver eel number, exploitation, and escapement rates
et al., 2007; Amilhat et al., 2008; Charrier et al., 2012) but none in
North Africa. However, given the current fragility of the European
eel populations in the Atlantic and Mediterranean, a joint effort of
the countries bordering the Mediterranean appears as a necessary
condition for the proper management of this resource. Rarely
taken into account in the population functioning models at the
international level, the eel population fractions populating the
Mediterranean hydro-systems must now be integrated (Farrugio
and Elie, 2011). In this context, Tunisia chose to participate in the
collective effort by writing its own Eel Management Plan (DGPA,
2010a).
Before the current trade ban came into force, Tunisia was the
largest exporter of live eels to the EU, after Norway (Crook, 2010).
Export accounted for 80– 90% of the catches, as there is little tradition of eel consumption in this country (DGPA, 2010a). This work
aims to fill gaps regarding the state of the stock, the escapement rate,
and the exploitation rate of silver eels from Ichkeul Lake, an important hydro-system in Tunisia and one of the main sites of eel
exploitation.
Material and methods
Study area
The Ichkeul Lake is located in the southeast of Bizerta in northern
Tunisia on the central south Mediterranean coasts (Figure 1).
Covering an area of 85 km2, this lake is a part of the Ichkeul
National Park which includes a mountain “the Jebel” and
marshes. It is registered with three international conventions:
143
Biosphere Reserve in 1977 (UNESCO), World Heritage component
in 1979 (UNESCO), and Wetland of International Importance in
1980 (Ramsar Convention). The lake is indirectly connected to
the sea through the lagoon of Bizerta via Wadi Tinja. It has an
average depth of 1.5 m and receives freshwater from six wadies,
three of which are equipped with dams. During winter, those
wadies supply the lake with freshwater which once mixed with the
rainfall, causes a rise in water level, and flows towards the lagoon
of Bizerta. On the other hand, the combination of drought and
low water drop causes a reversal of water flow during summer
and, therefore, an increase in salinity. The salinity fluctuates thus
between 16 and 34 g l21 and temperatures are between 11.8 and
29.18C (Ramdani et al., 2001).
Ichkeul Lake is the subject of a traditional fishing based on fish
migration (Romdhane, 1998; Chaouachi and Ben Hassine, 2001).
Since 1998, this activity has been granted to a private operating
company (Tunisia Lagoons Company SLT). The exploitation is
carried out on a stock of marine origin, which depends on the
arrival of juveniles from the sea, basically mullet fry and glass eels
(Lemoalle and Vidy, 1983; Kraı̈em et al., 2003). Eels represented
26.1% of the total catches in 2009 (DGPA, 2010b). Only silver eels
are exploited during the migration period. Yellow eels are not particularly targeted but are subject to bycatch and glass eel fishing is
prohibited by a ministerial order (JORT, 1995).
The downstream migration season of A. anguilla occurs in
Tunisia between late October and early February of each year
(Hizem Habbechi, 2014). In addition to the traditional weir, and
after obtaining an authorization, fishers set up a net barrier to
Figure 1. The complex Ichkeul Lake-Wadi Tinja-Bizerta lagoon (ANPE, 2007; modified) showing the approximate location of the net barrier (black
bar) and the release location (star).
144
E. Derouiche et al.
Table 1. Number of silver eels marked and recaptured in Ichkeul Lake, and the total fishery catch (in kg and in number) from December 2013
to February 2014.
Recapture
Campaign (release date)
Green campaign (13 December 2013)
Red campaign (7 January 2014)
Total catch (silver + yellow) in kg
Silver eel catch in kg (number)
Total eels marked
701
284
December
88
0
20 200
19 168.6 (32 753)
January
38
36
14 400
13 664.7 (23 349)
February
7
15
5000
4 744.7 (8 107)
Total
133 (18.97%)
51 (17.96%)
39 600
37 578 (64 209)
catch the migrating eels (Figure 1). This net barrier is not completely
closed since it is divided into two halves to provide a passage for
small boats. It consists of a set of fykenets “capétchades” which
are passive gears with 10 mm mesh traditionally used in the
Mediterranean lagoons. Traps are emptied daily by fishers and
contain both silver and yellow eels.
Eel samples
Only the silver eels were considered for the colour-marking experiment, but the yellows were also counted to know their percentage in
total catch. To verify silver eel stage, we based on qualitative criteria:
a contrast in colour between a dark dorsal surface, and a silvery
ventral surface; the presence of a well-differentiated lateral line on
the flanks and large eyes (Acou et al., 2005).
Colour-marking and recapture experiment
The colour-marking experiment took place twice: a first campaign
from 9 to 12 December 2013 in which 701 silver eels were tagged
with green and a second campaign in 6 January 2014 in which 284
silver eels were tagged with red. To facilitate eel handling, they
were anaesthetized using Eugenol (diluted in ethanol) at a concentration of 6 ml l21. Each silver eel was weighed to the nearest
gramme and sized to the nearest millimetre. Sex was assigned
according to the size of individuals (i.e. eels with a total length
longer than 45 cm were considered female; De Leo and Gatto,
1995; Tesch, 2003; Durif et al., 2005). Eels were then tagged with
acrylic paint (“Van Gogh” permanent green light 618) or ink
(“Magic Color” process magenta MC 620) inside the base of the
dorsal fin with a syringe. This kind of paint was previously used
by Amilhat et al. (2008) and Acou et al. (2010). Cotton soaked in
iodine-based antiseptic (Betadinew) was applied on the sting’s
spot to prevent infection. Eels were subsequently placed in aerated
tanks until their total recovery. The release operations were made
after each campaign (13 December and 7 January, respectively) in
the lake upstream the net barrier (Table 1). Recaptured eels were
recorded and checked for marks by fishers every day until the end
of the migration period in February when they removed the net
barrier.
Data analysis
The number of migrating silver eels was estimated by the Stratified
Population Analysis System (SPAS software; Arnason et al., 1996)
using the Pooled Petersen estimator (Seber, 1982). This method
was used in similar studies to estimate silver eels stock (Caron
et al., 2003; Amilhat et al., 2008; Charrier et al., 2012). The confidence intervals (CI) were calculated at a ¼ 0.05. To have the total
silver eels catch, we eliminated yellow eels from the total catch
obtained from fishers. This total silver eel weight was converted
Figure 2. Annual eel catch (T) in the commercial fishery of Ichkeul
Lake from 1962 to 1979. Only the years between 1973 and 1979 were
taken into account to estimate the pristine biomass (data from General
Directorate for Fisheries and Aquaculture, Ministry of Agriculture
Tunisia).
into numbers of eels using the mean weight. The exploitation rate,
escapement, and escapement rate were calculated according to:
(i) Exploitation (%) ¼ silver eels total catch in number/pooled
Petersen estimate
(ii) Escapement ¼ pooled Petersen estimate 2 silver eels total
catch in number
(iii) Escapement (%) ¼ escapement/pooled Petersen estimate.
Pristine biomass estimate
The pristine biomass was estimated using the historical data of
Ichkeul Lake before the decline in 1980 and considered by the scientific community as the maximum historical recruitment (ICES,
2010). Only the years between 1973 and 1979 were taken into
account, since the fishing method used before that date did not
reflect the real potential of the lagoon (Figure 2).
Results
Silver eel characteristics
During the two colour-marking campaigns, 985 silver eels with a
mean weight of 585.3 + 156.8 g were caught and marked. Among
them, 97% were females (Table 2). Sizes ranged from 33 to 79 cm
and dominant sizes were 63 and 69.5 cm (Figure 3).
Estimates of silver eel number, exploitation,
and escapement rates
One hundred and thirty-three (19%) silver eels were recaptured
during the first campaign (green marks) and 51 (18%) of the
second one (red marks; Table 1). Furthermore, 56% of eels marked
in green and 71% for those in red were recovered within the same
145
Estimates of silver eel number, exploitation, and escapement rates
Table 2. Number and average weight of eels caught during the mark –recapture experiment in Ichkeul Lake.
Number
Average weight (g + SD)
Total eels caught
1038
Yellow eels
53
month of their release. However, a few eels lingered and left the lake
during the following months (29% in January and 5% in February
for eels with green marks and 29% in February for those with red
marks). This indicates that the crossing time in the lake is not the
same for all eels and may vary between 1 and 3 months.
Total catch was obtained from fishers, but it included both silver
and yellow eels. By removing the proportion of yellow eels, we
obtained 37 578 kg silver eels captured in total corresponding to
64 209 individual. Most of the catch (51%) was made in December.
The number of migrating silver eels was estimated at 342 221
(297 956–386 486) eels by the pooled Petersen estimator, corresponding to a biomass of 200.2 (174.3–226.1) t and a density of
23.55 (20.51–26.6) kg ha21. The rate of exploitation by the commercial fishery was estimated at 18.8% (16.6–21.5%), 4.42 kg ha21,
which corresponds to an escapement rate of 81.2% (78.5–83.4%)
or 19.13 kg ha21 (Table 3).
Pristine biomass estimation
The average catch between 1973 and 1979 in Ichkeul Lake was 71.6 T.
The current catch in 2014 was 39.6 T, which corresponds to 55%
of the historical one. As the fishing effort has not undergone any
major changes and remained relatively constant in this area since
1973, and with the current biomass of 23.6 kg ha21, this corresponds
to an average biomass of 42.6 kg ha21 for the period 1973–1979.
Discussion
Silver eels characteristics
Our results show that in Ichkeul Lake, the sex ratio is largely in
favour of females (over 97%) during the sampling period. These
observations are in agreement with BHH (unpublished data) who
has studied Ichkeul Lake’s eel population during 4 years (2004 –
2007). According to this author, the migratory fraction is composed
of 86% females in winter and the lake produces 70% females over the
whole migration period. The predominance of females was also
recorded in other North African and Mediterranean sites such as
the Sebou wadi and the mouth of the Moulouya in Morocco
(Yahyaoui, 1991) as well as in the lagoons of Porto Pino (Rossi
and Cannas, 1984) and Comacchio (De Leo and Gatto, 1995) in
Italy. According to Capoccioni et al. (2014), only silver females
from the westernmost and central part of the Mediterranean basin
would be able to reach the Sargasso Sea in time for spawning,
while just a small number of silver males could, which highlights
the importance of females in this area.
Estimates of silver eel number, exploitation,
and escapement rates
The migration peak in Ichkeul Lake occurred in December the year
of our study. According to the fishing data, 51% of the silver eel catch
was recorded in that month. The migration peak in Tunisia is shifted
relatively to what can be found elsewhere in the distribution area of
the species. The timing of migration is related to the geographical
location of the continental life phase and thus to the distance that
migrating eels have to travel to get the Sargasso Sea. Indeed, silver
eels mostly migrated in August in northern Norway (Bergersen
Silver eels
985
585.25 + 156.77
Female silver eels
957
599.07 + 136.19
Male silver eels
28
112.93 + 28.5
Figure 3. Size structure of silver eels used in the mark – recapture
experience at Ichkeul Lake.
and Klemetsen, 1988), in September and October in the River
Imsa, southern Norway (Vøllestad et al., 1986) as well as in the
Burrishoole River, Ireland (Poole et al., 1990), while it happens in
November in Bages-Sigean lagoon, France (Amilhat et al., 2008).
By leaving earlier, eels from northern latitudes which have more distance to cover will likely reach the spawning grounds at the same
time as other subpopulations (Bruijs and Durif, 2009).
Silver eel biomass was 23.55 kg ha21, according to our estimates.
This result is close to 30 kg ha21 mentioned by Amilhat et al. (2008)
in Bages-Sigean lagoon, France. However, the number of migrating
eels from Bages-Sigean was three times higher than that of Ichkeul
(respectively, 1 120 112 and 342 221 eels). This can be explained by
the difference in sex ratio between the two subpopulations, and consequently, the median weight. (Silver eels from Ichkeul are mainly
females with a median weight of 0.6 kg, while Bages-Sigean is mostly
composed of males with 0.1 kg.) Our estimate is also close to Rossi’s
(1979) in Comacchio lagoon, Italy (20 kg ha21) and higher than 4.8–
6.9 kg ha21 mentioned by Acou et al. (2009) in the Oir River, France,
which are also primarily composed of females and whose area is close
to that of Ichkeul (Comacchio, 100 km2; Oir, 87 km2; and Ichkeul
85 km2). Indeed, the lagoons are significantly more productive than
freshwater systems since growth rates are greater in habitat close to
the sea (Amilhat et al., 2008; Daverat et al., 2012).
In Ichkeul Lake, silver eel yield was estimated at 4.42 kg ha21. It is
higher than that reported in Imsa River (Norway) by Vøllestad and
Jonsson (1988) (2.27 kg ha21).
In addition, the escapement rate from Ichkeul Lake (81.2%) is
relatively high compared with the range of escapement estimates
reported in the literature (23 –87%) and mentioned by Amilhat
et al. (2008).
Our estimates are restricted to the winter run and exclude the
autumn one. According to earlier studies in this area, silver male
eels begin to leave the lake in late October with a peak in November,
but continue their migration until January. On the other hand, the
female migration peak occurs between December and January
(BHH, unpublished data). Fishers targeting females for their large
size set up the net barrier at this moment. Therefore, 30% of silver
eels had left Ichkeul Lake when the colour-marking experiment has
146
E. Derouiche et al.
Table 3. Estimates of migrating silver eel number, exploitation, and escapement rates in % and in kg ha21 during the winter run in Ichkeul
Lake.
Number (CI 95%)
342 221 (297 956–386 486) 23.55 kg ha21
Standard
error
22 584
Exploitation rate
18.8 % (16.6 –21.5) 4.42 kg ha21
Escapement rate
81.2 % (78.5–83.4) 19.13 kg ha21
Biomass
200.2 T (174.3 –226.1)
Table 4. Estimates of migrating silver eel number, exploitation, and escapement rates in % and in kg ha21during the autumn run
and the whole migration season in Ichkeul Lake.
Autumn run
Whole migration season
Number
146 666, 10.1 kg ha21
488 887, 33.65 kg ha21
begun. To rectify this issue, we have included the estimated autumn
run based on its percentage to have an overview of the whole migration
in the Lake. Thus, the number of migrating silver eels increased to 488
887 eels corresponding to 33.65 kg ha21. Similarly, the annual escapement rate rose to 86.9% (29.23 kg ha21), since no eel has been captured during the autumn run (Table 4).
Handling due to tagging may cause stress to the fish and, therefore, modify the eel behaviour which might delay its migration. The
colour-marking method was previously tested by Amilhat et al.
(2008) in a controlled environment over a 6 months period. These
authors reported that neither mark visibility loss nor induced mortality was observed. Most of the eels in our study were recaptured
within the same month in which they were marked and released.
Incomplete reporting of marked eel by fishers could have happened,
especially during the days of high catches.
Some eels may have migrated after the fishing closure, in late
February or March, or even the next year. Breteler et al. (2007)
reported that a few eels marked in 2004 in the Rhine River were
detected in summer 2005. Likewise, Feunteun et al. (2000) indicate
that among the eels candidate to emigration, 12% remains in the
watershed including 3.4% which regress to the yellow stage.
In addition, a false declaration of the total catch as well as the
poaching by illegal fishers, which increases during the eel migration
season according to the STL fishery manager, could lead to a lower
biomass, a higher exploitation rate, and therefore to a lower escapement rate.
Current vs. pristine biomass
The European Council Regulation (EC) No. 1100/2007 for the
European eel stock recovery imposes as a reference point the escapement of at least 40% of the silver eel biomass relative to the best possible escapement estimation that would have existed if the stock had
not had any anthropogenic influence (i.e. pristine situation). As this
objective is currently difficult to achieve, the regulation proposes the
use of approximations to skirt this problem, such as the historical
data before the decline in 1980, the production extrapolation by
habitat type, or the analogy with similar systems.
In Tunisia, as well as in many European countries, we unfortunately have no study concerning the eel stock status or its biomass
estimation. However, historical catch series are available from
1962, and until 1972 only 5 T year21 were registered on average
because fishing was done in an archaic way with a small number
of artisanal traps. Since 1973, an Italian company has been exploiting eels in Tunisia and the production has increased (Hizem
Habbechi, 2014). For this reason, only the years between 1973 and
Exploitation rate
0%, 0 kg ha21
13.1%, 4.42 kg ha21
Escapement rate
100%, 10.1 kg ha21
86.9%, 29.23 kg ha21
Biomass
85.8 T
286 T
1979 were taken into account to get an estimate of the pristine
biomass in Ichkeul Lake (Figure 2).
To meet the Eel Regulation reference point of a 40% escapement
of potential spawners, 17.04 kg ha21 silver eels should leave the system every year. In the light of our results, 19.13 kg ha21 corresponding to 45% (38 –52%) of the pristine biomass, mostly females,
escaped from Ichkeul Lake during the winter run. The escapement
increases to 29.2 kg ha21 (69% of the pristine biomass) if we extrapolate to the entire migration season of 2013/2014.
Using a population dynamic model, Aalto et al. (2016) suggested
that current escapement is 35% of the pristine biomass levels across
the Mediterranean basin. Other studies, with different approaches
for calculating the pristine biomass, show current/pristine ratios
lower or equal to the reference point (Table 5). Therefore, the
ratio obtained in our study appears superior to those available in
the literature.
Models based on management scenarios have been developed to
maintain a sustainable fishery (Bevacqua et al., 2007; Charrier et al.,
2010). The current escapement of 36.2% in the Or lagoon relative
to the pristine biomass would reach 40% from a 10 mm mesh,
45.4% from a minimum size capture of 23 cm and 45.8 and 50.6%,
respectively, if a fishery closure was applied, respectively, during
(April, May, June) and (July, August, September) (Charrier et al.,
2010). In Tunisia, the law allows the use of fykenets of 10 and
15 mm mesh for eel fishing and 30 cm is the minimum catch size
(JORT, 1995). In addition, the fishery in Ichkeul Lake focuses on
the migration period of female eels which starts in December, allowing the escapement of 10.1 kg ha21 of silver eels that left the lake with
the first flood, before the start of the fishing season. During the rest of
the year, the eel is a bycatch and fishers catch other fish like mullet or
sea bass. Our study has demonstrated that the practiced law in Tunisia
as well as the management policy in Ichkeul Lake are effective to meet
the reference point of the European regulation and preserve the
species.
Nevertheless, the estimated number of escaping eels from
Ichkeul Lake will not necessarily reach the Sargasso Sea because
various factors can hinder their migration. Moreover, low muscle
fat content (Svedang and Wickström, 1997; Belpaire et al., 2009),
parasitosis by A. crassus (Palstra et al., 2007; Sjöberg et al., 2009),
chemical contaminations (Palstra et al., 2006; Pierron et al., 2008;
Elie and Girard, 2009, 2014; Geeraerts and Belpaire, 2010), viruses
such as EVEX (van Ginneken et al., 2005) could also have a
harmful impact on the eel migration. In Tunisia, some information
is available on the eel health status. Anguillicola crassus was reported
for the first time in 1995 (Maamouri et al., 1999). The prevalence of
147
Estimates of silver eel number, exploitation, and escapement rates
Table 5. Available studies on silver eel escapement vs. pristine biomass.
Year of
the study
***
Reference
Study area
Bevacqua in Amilhat Camargue lagoon (France)
et al. (2008)
Amilhat et al. (2008) Bages-Sigean lagoon (France) 2007
Charrier et al. (2010)
Bilotta et al. (2011)
Or lagoon (France)
Huntspill River (UK)
Prigge et al. (2013)
Schwentine River (Germany)
This study (2014)
Ichkeul Lake (Tunisia)
Method used to estimate the
pristine biomass
Modelling
Historical data (Loste and
Dusserre, 1996)
2008/2009 Modelling
2009
Escapement values from similar
environments (Breteler,
2008)
2009/2010 Escapement values from
German EMP
2013/2014 Historical data (DGPA, 2010b)
Current
escapement
(kg ha21)
1 –3.3
Pristine
biomass
(kg ha21)
25
Ratio
current/
pristine
0.04 –0.132
State
Below
24
60
0.4
Limit
10.13
6
28
17.5
0.36
0.35
Below
Below
0.0620.13
1.5
3.8
42.6
0.04 –0.087
0.016 –0.034
0.45 –0.69
Below
19.13 –29.23
Above
The ratio current/pristine should be 0.4 or above to be satisfactory.
this parasite in Ichkeul Lake reaches 32 and 56.3% for silver males
and females, respectively (Hizem Habbechi et al., 2012).
Conclusion
Our work has allowed establishing an initial assessment on the
escapement rate and the number of silver eels in Ichkeul Lake.
However, it is based on an estimate of the current state and cannot
be extrapolated to future years. In the Loire River (France), the escapement estimated by mark –recapture was different over the
years ranging from 352 000 potential spawners in 2001/2002 to
150 000 in 2008/2009 (Acou et al., 2010). Similarly, the results
cannot be extrapolated to the other main eel production sites in
Tunisia (Ghar El Melh lagoon and Tunis Lake) because each has its
own peculiarities. Although they are all in the north of the country, and silver eels are mostly females during the fishing season,
the fishing effort is not the same. Tunis Lake is subject to massive
poaching since 2011, while fishing is controlled in Ghar El Melh
lagoon where independent fishers renew their permits from the
local authorities every year. Besides, unlike Ichkeul Lake, these
two lagoons have direct access to the sea.
The Ichkeul Lake contributes significantly to the renewal of the
eel stock with some 342 221 individuals of silver eels leaving the
watershed during the winter run, and 488 887 for the entire migration season in 2013/2014.
Furthermore, to get a better idea on the migratory stock, researches must combine quantitative and qualitative aspects. In this
line, an European Eel Quality Database started to collect recent data
of contaminants and diseases over the distribution area of the eel
since 2007 (Belpaire et al., 2011). In addition to the information available about muscle fat content and the contamination by A. crassus,
heavy metals assessment is being analysed, and investigations concerning the other pollutants (PCB. . .) and viruses have to be initiated
for a global idea about the Tunisian eel population capacity to undertake the seaward migration in favourable conditions.
Acknowledgements
The authors would like to thank the fishers of Ichkeul Lake to have
actively participated in this study as well as Messrs. Riadh Ben Nsir
and Fathi Lazzez the managers of the SLT Company for providing
free eels. All our gratitude to Dr Elsa Amilhat from the Center of
Education and Research on Mediterranean Environments (University
of Perpignan Via Domitia, France) who explained to us the eels
tagging methodology and the use of SPAS software, and to the
Irstea (Bordeaux, France) staff for their material and scientific assistance. We are very grateful to the INSTM technical staff and to the
voluntary students Hela Jaziri, Wafa Cherif, and Boutheina Ziadi
for their help during the tagging operations. Finally, we gratefully acknowledge the editor Dr Caroline Durif and two anonymous
reviewers who provided useful and constructive comments that
greatly improved the quality of our manuscript. The research was
financed by the PRO-EEL Project (FP7).
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