The production of healthy high quality female European eel in recycle systems is proposed as a means to secure sufficient numbers of silver eel for spawning migration in order to meet the requirements of the European Commission’s proposal for a Regulation for the recovery of the stock of the European eel. Main advantages besides checks for parasites and viral diseases and avoidance of elevated levels of specific pollutants are the easily controllable numbers of spawners to be released and a reduction of labour and costs that will occur when acting along the lines of the Commission’s proposal

Hilge, Volker

English

Aquatic Commons

Inf. Fischereiforsch. 53, 2006, 6–8 Online first: 8. Februar 2006 
© 2005 Bundesforschungsanstalt für Fischerei, Hamburg  DOI: 10.3220/Infn53_6-8_2006
e-ISSN 1861-2164
p-ISSN 1860-9902
Recovery of the stocks of the European eel (Anguilla 
anguilla) by spawner enhancement
Erholung des Bestandes des Europäischen Aals (Anguilla anguilla) durch 
Stützung der Laichfische
Volker Hilge
Federal Research Centre for Fisheries, Institute of Fishery Ecology, Wulfsdorfer Weg 204, 22926 Ahrensburg, Germany
volker.hilge@ifo.bfa-fisch.de
Abstract
Recovery of the stocks of the European eel (Anguilla anguilla) by spawner enhancement
The production of healthy high quality female European eel in recycle systems is proposed as a means to secure sufficient 
numbers of silver eel for spawning migration in order to meet the requirements of the European Commission’s proposal for a 
Regulation for the recovery of the stock of the European eel. Main advantages besides checks for parasites and viral diseases 
and avoidance of elevated levels of specific pollutants are the easily controllable numbers of spawners to be released and a 
reduction of labour and costs that will occur when acting along the lines of the Commission’s proposal. 
Kurzfassung 
Der Bestand des Europäischen Aales ist seit 3 Jahrzehnten rückläufig. Sorgen bereitet dabei der sehr starke Rückgang der in 
die Binnengewässer einwandernden Glasaale. Dem will die Europäische Kommission mit einer Erhöhung der in die Sargassosee 
abwandernden Blankaale begegnen und hat einen Vorschlag für eine Ratsverordnung zur Wiederauffüllung des Bestandes des 
Europäischen Aales vorgelegt. Danach muß die Abwanderung von 40 % derjenigen Biomasse der adulten Aale sichergestellt 
werden, die existierte, wenn es keine Eingriffe des Menschen gäbe, die sich auf die Fanggebiete oder den Bestand auswirken. 
Dazu werden verschiedene Einschränkungen für die Fischerei genannt wie eine Schließung an 15 Tagen pro Monat, um dieses 
Ziel zu erreichen. Eine alternative Möglichkeit die Abwanderung einer ausreichend großen Zahl von Laichern sicherzustellen 
ist ihre Aufzucht unter kontrollierten Bedingungen in geschlossenen Kreislaufanlagen mit anschließendem Aussetzen im Meer. 
Dieses könnte im besten Fall die von der Europäischen Kommission vorgeschlagenen Maßnahmen ersetzen. Sowohl die Zahl der 
so erzeugten Tiere wie auch ihre Qualität in Hinblick auf Krankheiten, Parasiten oder Schadstoffgehalte kann so leicht überwacht 
werden. Anhand einer überschlägigen Beispielrechnung wird gezeigt, dass dieser Vorschlag durchaus realisierbar ist. 
Introduction
Since three decades catches of the European eel have 
been declining as we know from catch statistics. Simi-
larly, the stocks of the European eel have been decreas-
ing as well. As we know since long there is nothing ex-
traordinary with variations in catches of eel in the long 
range. However, the unique and alarming very strong 
decrease of the number of glass eel arriving in the Eu-
ropean continental waters after their oceanic migration 
asks for precautionary measures to counteract this de-
velopment. So far we do not know the exact reasons for 
the decline. Besides a number of reasons, which will 
not be discussed here and which are primarily related 
to inland waters, we cannot exclude large-scale climatic 
events to be responsible for this. Strong variations in 
stock size coupled to long-term climatic events are simi-
larly known for e. g. small pelagics of the Pacific and 
Atlantic oceans. With eel, there is sometimes also the 
opinion that a decrease in number of spawners is due 
to high fishing pressure on yellow eel and even more on 
silver eel stages, which will finally result in lower num-
bers of spawners, and subsequently lead to low num-
bers of glass eel. In future we may probably have data 
from different fisheries that do not support a general-
ized hypothesis of overfishing. There may, however, 
be some other factors leading to increased mortality 
and reduced reproduction and, in fact, strongly influ-
ence the numbers of glass eel. 
To support a stock a sufficiently high number of spawn-
ers are necessary to produce enough offsprings. In the 
case of the eel, enough offspring means a sufficient 
number of glass eel. Prior to reaching the glass eelstage 
there could be events – during the Atlantic ocean cross-
ing of the larvae – that are the origin of the stock de-
V. Hilge: Recovery of the stocks of the European eel .......    Inf. Fischereiforsch. 53: 6–8, 2006
 Informationen aus der Fischereiforschung 7
cline. The question then remains whether an increased 
number of ready-to-migrate silver eel could counteract 
the glass eel decrease. The European Commission has 
assented to this thought and recently proposed a respec-
tive Council Regulation (CEC 2005). Even if we agree 
to this assumption the real effort to be undertaken to 
obtain measurable effects remains yet unknown. 
Again, it should be noted that some scientists believe 
that the stock variations can barely be compensated for, 
because they are largely due to oceanic events caused 
by human activities, but are outside of our immediate 
influence.
The Commission’s approach for a recovery
Despite the difficulties to answer the question of the 
importance of the oceanic events we could influence 
the number of spawners hoping that an increase in 
spawning capacity will also result in higher numbers 
of glass eel. This is the approach of the Commission’s 
proposal. The 40 % escapement target for silver eel 
stipulated there can be secured by different means. On 
the other hand the possibilities to secure that all the 
animals are of good reproductive quality are limited 
and may differ between river catchments. Pollutants, 
parasites and viral diseases can pose a severe problem 
with regard to the spawning migration and subsequent 
reproduction. This we know from the  recent the EU 
EELREP research project (EU 2005) (http://www.fish-
biology.net/EELREP_final_report.pdf ). 
The proposal of the Commission for a Council Regu-
lation for the recovery of the stock of the European eel 
mainly consists of the above mentioned 40 % goal for 
spawner escapement with reference “to the biomass of 
adult eel… in the absence of human activities affecting 
the fishing area or the stock”. This demand is linked to 
the threat of a 15 day closure per month of the fishery 
on all stages of eel or the request to develop national 
management plans for the eel. 
Three aspects are noteworthy with respect to this re-
quest:
1. The glass eel fishery is practically exempted from 
the 15 day regulation through some stipulations.
2. A natural stock size, on which the calculation of the 
40 % goal depends, is practically unknown due to 
stocking performed e. g. in Germany for more than 
a century. This interferes with the naturally occur-
ring variations in the stock size. 
3. There would be enormous expenditures for the use 
of the data collection programme established for 
marine fisheries for eel, which are caught mainly in 
freshwaters. Maybe there is a better use of resources. 
Spawner enhancement
With regard to this it must be questioned if there are 
other possibilities besides the Commission’s proposal to 
secure the return of a sufficient number of female spaw-
ners to the Sargasso Sea. The answer to this problem 
might be a controlled production of spawners, i. e. a real 
spawner enhancement. The idea is to produce a suffi-
cient high number of female spawners of adequate size 
under controlled conditions and to release them into the 
sea. This would be a way of aquaculture based stock en-
hancement directly in favour of the spawning stock. A 
really high number of female silver eel produced in this 
way could make the measures proposed by the Com-
mission, at the best, unnessecary. Furthermore it would 
be much more easily controllable than the 40 % of sea-
going eel scattered over such a huge area and difficult 
to detect in the waters. Such an enhancement would be 
based on experience gained over decades in Europe with 
the aquaculture of eel and it links with recent know-
ledge communicated in the above-mentioned EELREP 
report. Relevant results from this report could be used 
immediately or after a quick adaptation to practice. 
Main components are:
– the controlled rearing from the glass eel stage in a 
warmwater recycle system on low pollutant feeds (the 
use of estradiol during a short early life stage for the 
production of all-female stocks should be considered 
as these animals are not for consumption !),
– the medication of the animals before introduction 
into a recycle system against Anguillicola crassus, if 
necessary,
– the possibility to determine the silver eel stage cor-
rectly and to use stimuli resulting in the preparation 
for the spawning migration,
–  the transport of the animals by train/truck and ship 
into the sea for subsequent release.
There are further advantages besides the knowledge of 
the number of fishes. The migration could be examined 
by satellite detected swimming buoys fixed for some 
time to the back of a part of the animals. Feeding com-
mercial eel feeds of known low contamination levels 
will result in fairly low contaminated animals. This is 
in contrast to eel from a considerable number of natu-
ral water bodies whith sometimes extremely high con-
tamination burden, especially with dioxin-like PCBs 
and dioxins, that might result in a reduced reproducti-
ve capability (Palstra et al. 2006). Furthermore animals 
have to be examined for EVE (Eel Virus European) 
and EVEX (Eel Virus Europe X) and HVE (herpes vi-
rus of eel), which only recently have been identified as 
cause of lethal damage during migration. Therefore, a 
specific pathogen free (SPF) rearing needs to be aspired 
with regard to A. crassus, EVE, EVEX, etc. 
 8 Informationen aus der Fischereiforschung
V. Hilge: Recovery of the stocks of the European eel .......    Inf. Fischereiforsch. 53: 6–8, 2006
Some figures
What are the numbers and biomasses of silver eel to be 
reared with regard to the anticipated Council Regulati-
on for e. g. Germany? 
• A simplified calculation – just to get an idea of the 
order of magnitude – assumes 200 000 ha inland 
water surface area, that can be used by eel, suppor-
ting 2.5 kg/ha ready-to-depart eel per year, which 
probably reflects a rather good productivity. 40 % 
of this would then be 1 kg/ha. At an individual 
body weight of 500 g there are 400 000 animals 
of a mean body weight of 500 g or 200 t of eel to 
be produced per year. The amount of glass eel nee-
ded – allowing for 25 % of losses mainly during 
glass eel rearing – is 530 000 animals. With 3000 
glass eel/kg this means 180 kg at the beginning. 
At a mean price of 300,– €/kg glass eel they will 
cost 54 000 €. Depending on the action of the 
Commission perhaps a more favourable price can 
be achieved for the glass eel. It should be noted 
that prices for “big” eel from recycle systems are at 
present in the range of 11 to 12 €/kg. This means 
that a production at 10 €/kg seems reasonable for 
calculations. Total costs (incl. glass eel) for ready-
to-migrate silver eel should then be about 2 Mio € in 
the case of Germany. Although this is a very rough 
and simplified calculation it is probably not too 
far from reality. Again, it should be stressed that 
these calculations are based on rough assumpti-
ons for inland water areas well suited for eel and a 
stock of ready-to-migrate female eel of 2.5 kg/ha 
per year. 
• Other scenarios for such a female eel spawner 
enhancement are possible. Theoretically, it should 
be possible to accomplish the 40 % goal. The cor-
responding numbers of eel could be reared in a few 
systems while additionally natural migration conti-
nues as usual.
• In a scenario with continued fisheries activities the 40 % 
goal for silver eel might be missed by a certain por-
tion. This could be filled in with spawners from the 
enhancement recycle systems, i. e. basically the stock 
will not be protected by regulation of catch etc., but 
will be supported by enhancement as necessary. This 
way to help to recover the eel stock with less control 
and labour and financial input might attain more 
agreement among member states and fishermen than 
the measures proposed by the Commission.
Finally, it is important that the animals are released 
directly into the sea in order to avoid infestation with 
Anguillicola crassus. In any case, it seems possible to 
considerably reduce the burden that will otherwise be 
imposed upon the fishery, and it would at the same 
time support the stock in a more practical and cont-
rollable way. 
Nota bene
As the author favours the hypothesis that there are pri-
marily oceanic events at the origin of the recent decline 
of the eel stocks and that factors related to the inland 
waters have an additional effect. Therefore he wants to 
stress that much more research efforts should be direc-
ted into the controlled reproduction and rearing of the 
European eel. 
References
CEC (Commission of the European Communities), 2005: 
Proposal for a Council regulation establishing measures for the 
recovery of the stock of European Eel. COM(2005) 472 final, 
6. 10. 2005.
Palstra, A. P.; Ginneken, V. J. T. van; Murk, A. J.; Thillart, G. E. E. 
J. M. van den, 2006: Are dioxin-like contaminants responsible for 
the eel (Anguilla anguilla) drama? Naturwissenschaften (accepted 
for publication)
EU 2005: Estimation of the reproduction capacity of European 
eel. Final report, EU Project EELREP, Q5RS-2001-01836. 
Leiden University a. o. 272 p. URL: http://www.fishbiology.
net/EELREP_final_report.pdf, 6 Feb. 2006.


Recovery of the stocks of the European eel (Anguilla anguilla) by spawner enhancement

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Recovery of the stocks of the European eel (Anguilla anguilla) by spawner enhancement

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