scholarly journals Stocking is essential to meet the silver eel escapement target in a river system with currently low natural recruitment

2015 ◽  
Vol 73 (1) ◽  
pp. 91-100 ◽  
Author(s):  
Uwe Brämick ◽  
Erik Fladung ◽  
Janek Simon
Keyword(s):  
Anguilla Anguilla ◽  
River System ◽  
European Eel ◽  
Net Benefit ◽  
Silver Eel ◽  
Natural Habitats ◽  
Silver Eels ◽  
Management Plans ◽  
Approaches To Study ◽  
Study Population

Abstract Under the European Eel Regulation EG 1100/2007, Member States exhibiting natural habitats for the European eel (Anguilla anguilla) on their territory are obliged to prepare Eel Management Plans (EMP) containing appropriate measures to safeguard the escapement of a river system specific silver eel target biomass. Stocking is one management option to reach this target. We used various methodical approaches to study population parameters in a large lowland river under the application of a multi-annual intense stocking programme. The approaches were used to further enhance modelling of stock dynamics and silver eel escapement, in particular. Parameterizing the German Eel Model III (GEM III) with values and functions obtained for recruitment, growth, and mortality resulted in an annual escapement estimate of roughly 32 000–64 000 silver eels from 2010 to 2012. Escapement estimates based on a mark-recapture study conducted in parallel revealed somewhat lower values (11 000–25 000) for the same years. In view of the small number of natural recruits, such values are only contingent if stocking had a profound effect on silver eel production. Results from modelling annual silver eel escapement values indicate that escapement targets set in the EMP for this tributary cannot be reached without stocking. This constellation is likely to apply to other Eel Management Units with low current natural immigration values as well, and might be considered a key dilemma in eel management in such catchments due to the current confusion whether translocation of recruits yields a net benefit to the panmictic stock of the European eel.

On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn

2007 ◽  
Vol 64 (7) ◽  
pp. 1472-1482 ◽  
Author(s):  
M. W. Aprahamian ◽  
A. M. Walker ◽  
B. Williams ◽  
A. Bark ◽  
B. Knights
Keyword(s):  
Reference Conditions ◽  
Anguilla Anguilla ◽  
European Eel ◽  
Silver Eel ◽  
Management Scenarios ◽  
Test Management ◽  
Management Measures ◽  
Silver Eels ◽  
Management Plans ◽  
The Uk

Abstract Aprahamian, M. W., Walker, A. M., Williams, B., Bark, A., and Knights, B. 2007. On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn. – ICES Journal of Marine Science, 64: 1472–1482. The European eel stock has declined significantly since the 1980s, and the Eel Recovery Plan of the European Commission requires Member States to develop river basin Eel Management Plans (EMPs) that will achieve an escapement of silver eels that equals or exceeds 40% of the escapement biomass that would be produced in the absence of human activities. However, because silver eel escapement is not quantified within the UK, a modelling approach is required to estimate potential and actual escapement, and to assess the likely effects of management measures. We focus on two approaches developed in the UK, the Reference Condition Model (RCM) and the Scenario-based Model for Eel Populations (SMEP), and illustrate how such approaches can be used in EMPs using selected data from the River Severn. The RCM results indicate that the yellow eel population in the River Severn basin may be just 30–40% of the potential density indicated by reference conditions derived from a selection of rivers between the late 1970s and the early 1980s. The challenges of applying a model designed to be as realistic of eel production as possible, and the limited data on natural eel habitat and eel production in the Severn, preclude a SMEP analysis similar to that of the RCM, but simulations based on a simplified basin description and eel survey data from the early 1980s illustrate the potential of this model to assess compliance and test management scenarios.

scholarly journals Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge

2007 ◽  
Vol 64 (7) ◽  
pp. 1437-1443 ◽  
Author(s):  
Henrice M. Jansen ◽  
Hendrik V. Winter ◽  
Maarten C. M. Bruijs ◽  
Harry J. G. Polman
Keyword(s):  
River Discharge ◽  
Radio Telemetry ◽  
Anguilla Anguilla ◽  
Downstream Migration ◽  
European Eel ◽  
Route Selection ◽  
Migration Routes ◽  
Silver Eel ◽  
Silver Eels ◽  
The Impact

Abstract Jansen, H. M., Winter, H. V., Bruijs, M. C. M., and Polman, H. J. G. 2007. Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge. – ICES Journal of Marine Science, 64: 1437–1443. The European eel (Anguilla anguilla) has been in steep decline for several decades. Fisheries and hydropower-induced mortality presumably play an important role during the downstream migration of silver eels, and downstream-migrating silver eels must make various navigation and route-selection decisions to reach the sea. We examined the influence of river discharge on route selection of silver eels. To quantify the impact of hydropower and fisheries on silver eel mortality, radio-telemetry experiments were performed in the River Meuse in 2002 and 2004, surgically implanting 300 silver eels with Nedap-transponders. Route selection and passage behaviour near detection stations was assessed. Silver eels were distributed over the alternative migration routes in the river in proportion to the discharge until the silver eels reached the entrance to the turbines. The eels altered their behaviour when approaching the turbines of hydropower plants and showed stationary and recurrent behaviour. We discuss the consequences of this on route selection and mortality rates caused by hydropower facilities and fisheries.

PRESSOR ACTIVITY IN EXTRACTS OF THE CORPUSCLES OF STANNIUS FROM THE EUROPEAN EEL (ANGUILLA ANGUILLA L.)

1966 ◽  
Vol 34 (3) ◽  
pp. 393-408 ◽  
Author(s):  
I. CHESTER JONES ◽  
I. W. HENDERSON ◽  
D. K. O. CHAN ◽  
J. C. RANKIN ◽  
W. MOSLEY ◽  
...  
Keyword(s):  
Intravenous Injection ◽  
Renin Angiotensin System ◽  
Anguilla Anguilla ◽  
European Eel ◽  
Silver Eel ◽  
Renin Substrate ◽  
Angiotensin System ◽  
Corpuscles Of Stannius ◽  
Silver Eels ◽  
Pressor Activity

SUMMARY Extracts of corpuscles of Stannius from the silver eel have been shown to contain a substance with a powerful pressor action on intravenous injection into the rat. This material resembles mammalian renin in being non-diffusible through cellophane, heat-labile, and destroyed by acidification to pH 2. The effect in the rat differs, however, from that produced by mammalian renin in being more prolonged, and frequently biphasic. Pressor activity has also been demonstrated in extracts of kidneys from freshwater silver eels. Incubation of kidney extract with mammalian renin-substrate produced an angiotensin-like pressor substance. Both renal and corpuscular extracts had a prolonged pressor effect on intravenous injection into the eel. The identities of these pressor materials have not been finally established. Removal of the corpuscles of Stannius from freshwater silver eels was followed by a drop in blood pressure to levels normally found in eels adapted to seawater. The possible existence, in the eel, of a renin-angiotensin system analogous to that existing in mammals is discussed.

scholarly journals Long-term variation in numbers and biomass of silver eels being produced in two European river systems

2018 ◽  
Vol 75 (5) ◽  
pp. 1627-1637 ◽  
Author(s):  
W Russell Poole ◽  
Ola H Diserud ◽  
Eva B Thorstad ◽  
Caroline M Durif ◽  
Conor Dolan ◽  
...  
Keyword(s):  
Time Series ◽  
Life Stage ◽  
Early Period ◽  
Anguilla Anguilla ◽  
High Water ◽  
Water Levels ◽  
Annual Number ◽  
European Eel ◽  
Silver Eel ◽  
Silver Eels

Abstract The European eel (Anguilla anguilla) population has been in decline at least since the 1960s and reliable regional information, particularly on the spawner production and escapement (i.e. the silver eel life stage), is a requirement of the EU stock recovery regulation. Two comparable time series exist in Burrishoole (Ireland) and Imsa (Norway), with monitoring of total silver eel production since the early 1970s. Numbers of emigrating silver eels fell significantly (p < 0.0001) in the 1980s (breakpoints: Burrishoole 1982; Imsa 1988), in both catchments from >4000 eels per annum to ∼2000 eels per annum. The proportion of male eels dropped and the average size of female eels increased. Biomass of silver eels escaping has remained similar in Burrishoole (1.1/1.2 kg/ha), but not in Imsa (2.1/0.9 kg/ha) between the early period and the 2000s. Factors that govern the onset of eel maturation (silvering) and the annual production of silver eels are little understood. In this paper, the influence of time-lagged environmental variables on silver eel production is examined. Annual variation in the time series was partly (r2 Burrishoole = 0.43, Imsa = 0.46) explained by variation in water temperature and water level. Annual number of migrating eels in both catchments was positively related to summer temperature and summer water flow, negatively related to summer temperatures in the previous year, and in the Burrishoole, also negatively related to high water levels in September/October. The models did not transfer well between catchments, indicating likely catchment specific environmental factors impacting on eel production. The reduction in eel numbers observed in both catchments, accompanied by the change in sex ratio and mean weight of females that contribute to maintain biomass production, calls into question the advisability of basing a spawner escapement recovery target on biomass alone, while numbers and proportions of males decline.

scholarly journals Reproductive potential of silver European eels (Anguilla anguilla) migrating from Vistonis Lake (Northern Aegean Sea, Greece)

2014 ◽  
Vol 15 (3) ◽  
pp. 539 ◽  
Author(s):  
R. MACNAMARA ◽  
E. T. KOUTRAKIS ◽  
A. SAPOUNIDIS ◽  
D. LACHOUVARIS ◽  
F. ARAPOGLOU ◽  
...  
Keyword(s):  
Eastern Mediterranean ◽  
Reproductive Potential ◽  
Anguilla Anguilla ◽  
Aegean Sea ◽  
Oocyte Diameter ◽  
European Eel ◽  
Silver Eel ◽  
North West ◽  
Silver Eels ◽  
Spawning Stock

The European eel (Anguilla anguilla), once abundant throughout much of Europe and North Africa, has recently been classified as critically endangered. Information on its biology from the eastern Mediterranean is lacking, especially in relation to spawner quality. Therefore, silver eels were sampled during their seaward spawning migration from Vistonis Lake in Greece. Characteristics linked to reproductive output and success (i.e. body size and condition, sex ratio, silvering, Anguillicola crassus infection, fecundity and oocyte diameter) were examined. The lake produced large (687–1138 mm), exclusively female silver eels, 61.7% of which were infected by A. crassus. Silver eel fecundity, the first estimates from the southern part of the species range, was positively related to body length (R2 = 0.693; P < 0.001) and body weight (R2 = 0.731; P < 0.001). Fecundity did not differ between A. crassus infected and uninfected silver eels, but Greek silver eels were significantly more fecund than those in north-west Europe. The reproductive potential of Vistonis Lake silver eels and their contribution to the A. anguilla spawning stock is discussed.  

Sex-related growth of European eel, Anguilla anguilla, with focus on median silver eel age

1997 ◽  
Vol 54 (12) ◽  
pp. 2775-2781 ◽  
Author(s):  
K Holmgren ◽  
H Wickström ◽  
P Clevestam
Keyword(s):  
Growth Rate ◽  
Anguilla Anguilla ◽  
Growing Season ◽  
European Eel ◽  
Silver Eel ◽  
Glass Eel ◽  
Growing Seasons ◽  
Length Increase ◽  
Silver Eels

Silver eels, Anguilla anguilla, migrating downstream were monitored for 15 years since being stocked in 1980 as cultured fingerlings in a Swedish lake. Females were larger than males in each separate year, but mean growth rate was higher in males than in females. This discrepancy was due to different age distributions, with 64.5% of the males migrating after four or five growing seasons, while 86.3% of the females grew for 10-15 seasons before migration. Otolith length in the glass eel stage was independent of sex, and it was not correlated with year of migration. With focus on median year of migration, otolith lengths after one, two, and three growing seasons were significantly larger in males than in females. This was mainly due to a higher otolith length increase during the first growing season, i.e., before the eels were stocked in the lake. These results are inconsistent with the view that female eels generally grow faster than males.

scholarly journals Silver eel mortality during downstream migration in the River Meuse, from a population perspective

2007 ◽  
Vol 64 (7) ◽  
pp. 1444-1449 ◽  
Author(s):  
Hendrik V. Winter ◽  
Henrice M. Jansen ◽  
André W. Breukelaar
Keyword(s):  
Anguilla Anguilla ◽  
Mortality Rates ◽  
Downstream Migration ◽  
European Eel ◽  
Silver Eel ◽  
The Past ◽  
River Meuse ◽  
Silver Eels ◽  
The Impact ◽  
Insight Into

Abstract Winter, H. V., Jansen, H. M., and Breukelaar, A. W. 2007. Silver eel mortality during downstream migration in the River Meuse, from a population perspective. – ICES Journal of Marine Science, 64: 1444–1449. The European eel (Anguilla anguilla) population has decreased sharply over the past few decades owing to a combination of many factors. To determine the impact of hydropower and fisheries during the downstream migration of silver eel in the River Meuse, telemetry experiments were performed during the years 2002–2006, using 18 detection stations (NedapTrail-System®) in the river and two at the entrance to the hydropower turbines. Recaptures in fisheries were used to assess fisheries mortality. In all, 300 silver eels were surgically implanted with Nedap-transponders. For each stretch between subsequent stations, mortality rates were assessed and related to the different factors. However, to determine the overall effect on the escapement of silver eels from the River Meuse, insight into the distribution of silver eels in the entire catchment of the River Meuse is required. At two locations, mark-recapture experiments in 2002 revealed that the estimated number of migrating silver eels increased strongly in a downstream direction, suggesting that a large proportion of silver eels start their migration from the downstream stretches and tributaries of the River Meuse. Approaches and monitoring requirements that can be used to determine the impact on silver eel populations in a river basin are discussed.

scholarly journals Growth potential can affect timing of maturity in a long-lived semelparous fish

2018 ◽  
Vol 14 (7) ◽  
pp. 20180269 ◽  
Author(s):  
Kazuki Yokouchi ◽  
Françoise Daverat ◽  
Michael J. Miller ◽  
Nobuto Fukuda ◽  
Ryusuke Sudo ◽  
...  
Keyword(s):  
Anguilla Anguilla ◽  
Growth Conditions ◽  
Growth Potential ◽  
Individual Growth ◽  
Growth Trajectories ◽  
Female Size ◽  
European Eel ◽  
Growth And Survival ◽  
Silver Eels ◽  
Reproduction Cost

Many diadromous fishes such as salmon and eels that move between freshwater and the ocean have evolved semelparous reproductive strategies, but both groups display considerable plasticity in characteristics. Factors such as population density and growth, predation risk or reproduction cost have been found to influence timing of maturation. We investigated the relationship between female size at maturity and individual growth trajectories of the long-lived semelparous European eel, Anguilla anguilla . A Bayesian model was applied to 338 individual growth trajectories of maturing migration-stage female silver eels from France, Ireland, the Netherlands and Hungary. The results clearly showed that when growth rates declined, the onset of maturation was triggered, and the eels left their growth habitats and migrated to the spawning area. Therefore, female eels tended to attain larger body size when the growth conditions were good enough to risk spending extra time in their growth habitats. This flexible maturation strategy is likely related to the ability to use diverse habitats with widely ranging growth and survival potentials in the catadromous life-history across its wide species range.

scholarly journals Mortality of silver eel (Anguilla anguilla) migrating downstream through a small hydroelectric plant on the Drawa River in northern Poland

2016 ◽  
Vol 24 (2) ◽  
pp. 69-75 ◽  
Author(s):  
Piotr Dębowski ◽  
Rafał Bernaś ◽  
Michał Skóra ◽  
Jacek Morzuch
Keyword(s):  
Acoustic Telemetry ◽  
Hydroelectric Plant ◽  
Anguilla Anguilla ◽  
European Eel ◽  
Upstream Migration ◽  
Silver Eel ◽  
Passive Integrated Transponder ◽  
Northern Poland ◽  
Hydroelectric Plants ◽  
Pass Through

Abstract The European eel, Anguilla anguilla L., is an endangered species. Barriers to its downstream spawning migration are one of the greatest threats this species faces. There are hundreds of hydroelectric plants (HEP) on rivers in Poland (> 600), and thousands throughout Europe. Eel that pass through HEP turbines as they migrate downstream suffer high mortality, but this depends mainly on local and technical conditions. Silver eel mortality was estimated and the possibility of the fish bypassing the turbines was studied between November 2013 and June 2014 at a typical HEP in northern Poland. Two telemetry methods were used with 49 eel: passive integrated transponder (PIT) system and acoustic telemetry. Fifty five percent of eel migrated downstream in fall 2013, soon after their release, and 45% migrated the next spring. The eel did not use the fish passes designed for upstream migration; thus, they were forced to go through the turbines, which resulted in 55% mortality. HEPs cause interruptions and delays in eel spawning migrations and are responsible for high eel mortality. This can make implementing an eel restitution plan difficult or even impossible in river systems with many barriers.

Eels at the Edge: Science, Status, and Conservation Concerns

2009 ◽  
Keyword(s):  
Warning System ◽  
Anguilla Anguilla ◽  
Hydropower Station ◽  
European Eel ◽  
Mortality Factors ◽  
River Meuse ◽  
Silver Eels ◽  
Hydropower Stations ◽  
Integrated Study ◽  
Migration Event

<em>Abstract.—</em>To evaluate mortality factors and potential mitigative measures during downstream passage of silver European eel Anguilla anguilla in the River Meuse, an integrated study was performed consisting of: 1) telemetric experiments where the descent of 150 transpondered eels was tracked at 14 detection stations; 2) monitoring daily commercial catches; 3) testing a system to predict migration events from activity of captive eels in riverside tanks (the Migromat system); and 4) measuring turbine mortality by netting eels at turbine outlets. Migromat systems installed at two hydropower stations predicted 41 migration events. Most warnings (58.5%) were false positives, but Migromat failed to predict only one migration event. Mortality of eels passing through a turbine at the Linne Hydropower station was 24%. However, part of the downstream migrating population bypassed the turbines by descending over adjacent weirs. Descending eels moved primarily in brief pulses that were associated with increasing discharge and especially during the first half of the night. Thirty-seven percent of transpondered eels released at Ohé en Laak reached the sea. Because both fisheries mortality (estimated at 22–26% by fishing recoveries) and hydropower mortality (estimated at 16–26%) were considerable, a reduction in fisheries harvest and implementation of turbine management by application of the Migromat warning system would substantially increase escapement of silver eels from the River Meuse.

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