Environmental Factors Regulating the Seaward Migration of European Silver Eels (Anguilla anguilla)

1986 ◽  
Vol 43 (10) ◽  
pp. 1909-1916 ◽  
Author(s):  
L. A. Vøllestad ◽  
B. Jonsson ◽  
N. A. Hvidsten ◽  
T. F. Næsje ◽  
Ø. Haraldstad ◽  
...  
Keyword(s):  
Water Temperature ◽  
Total Variation ◽  
Opposite Effect ◽  
Field Experiments ◽  
Water Discharge ◽  
Anguilla Anguilla ◽  
Silver Eel ◽  
Discharge Water ◽  
Silver Eels ◽  
Seaward Migration

We examined the connection between descent of European silver eels (Anguilla anguilla) and water discharge, water temperature, turbidity, photoperiod, light intensity, and moon phase based on 10 yr of field data and field experiments. The eel migration takes place between August and December, and mean water temperature for July–August and mean water discharge for August–October explained 91% of the total variation in the start of the yearly silver eel run. Low mean water temperatures during July–August and high mean water discharge during August–October resulted in an early start of the yearly silver eel run, whereas high mean water temperature and low water discharge gave the opposite effect. There was no significant correlation between the duration of the entire yearly silver eel run and the environmental variables investigated. But the duration of a part of the run, e.g. number of days from 5 to 25% cumulative eel descent, was significantly correlated with mean water temperature and increase in water discharge for the period in question (R2 = 0.84). Maximum silver eel descent was at a water temperature of 9 °C. Few silver eels descended at temperatures below 4 °C or above 18 °C. The migration speed of transplanted silver eels in the River Imsa was correlated with water discharge and water temperature (R2 = 0.88). Water discharge alone explained 85% of the total variation in migration speeds. The recapture rate of silver eels transplanted within the River Imsa was highest at 9 °C and decreased at higher and lower temperatures (R2 = 0.95). Illumination of 20 lx upon the river reduced the descent of silver eels.

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.

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

2009 ◽  
Keyword(s):  
Anguilla Anguilla ◽  
Small River ◽  
Biological Characteristics ◽  
Silver Eel ◽  
Biomass Density ◽  
Silver Eels ◽  
Close Proximity ◽  
Males And Females ◽  
Density And Biomass

<em>Abstract.—</em>This paper assesses potential production of premigrant European eels <em>Anguilla anguilla </em>based on analysis of sedentary eel populations in two small river systems in western France that are in close proximity. Abundance and biological characteristics were evaluated from electrofishing surveys conducted in three years in September and October, before the catadromous migration of silver eels. Mean density and biomass density of the eel population differed greatly between the systems (39 ± 6 ind.100 m<sup>–2</sup> [indivdual per 100 m<sup>2</sup>] and 1,352 ± 171 g.100 m–2 in the Frémur River and 3 ± 0.32 ind.100 m–2 and 385 ± 42 g.100 m<sup>–2</sup> in the Oir River). Premigrants were dominated by males in the Frémur (85.8%) and by females in the Oir (79.0%). Estimated premigrant biomass density was 4.5-fold higher in the Frémur (254.5 g.100 m<sup>–2</sup>/ year) than in the Oir (56.0 g.100 m–2/year). Mean Fulton’s K condition factor was significantly higher for both sexes in the Oir (0.20 ± 0.004 and 0.20 ± 0.003 for males and females, respectively) than in the Frémur (0.17 ± 0.002 and 0.17 ± 0.004, respectively). The large differences in densities and biological characteristics of eels from neighboring catchments suggest that huge variability of both quantity and quality of silver eel production can be expected at the scale of the European stock.

Plasma concentrations of ovarian steroids in the freshwater European silver eel (Anguilla anguilla L.): effects of hypophysectomy and transfer to sea water

1987 ◽  
Vol 114 (2) ◽  
pp. 289-294 ◽  
Author(s):  
B. Quérat ◽  
K. Nahoul ◽  
A. Hardy ◽  
Y. A. Fontaine ◽  
J. Leloup-Hâtey
Keyword(s):  
Plasma Levels ◽  
Sea Water ◽  
Plasma Concentrations ◽  
Anguilla Anguilla ◽  
Aromatase Activity ◽  
Silver Eel ◽  
Ovarian Steroids ◽  
Silver Eels ◽  
Dependent Inhibition ◽  
Stimulation Of

ABSTRACT Intact and hypophysectomized freshwater (FW) silver eels were transferred to tanks of FW or artificial sea water (SW; salinity = 0·60 osmol/l) which were simultaneously renewed twice a week. Fish were killed 2 months after transfer and plasma was assayed for ovarian steroids. In all fish, 5α-androstane-3β,17β-diol was present, while 5α-dihydrotestosterone and 5α-androstane-3α,17β-diol were undetectable. In intact FW eels, plasma levels of testosterone, 5α-androstane-3β,17β-diol and oestradiol-17β were approximately 0·15 nmol/l. In intact SW eels, no change in plasma levels of testosterone and 5α-androstane-3β,17β-diol was found, whereas the concentration of oestradiol-17β was increased significantly (P<0·01), indicating stimulation of aromatase activity. In hypophysectomized compared with intact FW fish, plasma levels of testosterone and 5α-androstane-3β,17β-diol were decreased (P<0·05) and there was a slight but significant (P<0·01) augmentation of the plasma concentration of oestradiol-17β which may have involved the removal of pituitary-dependent inhibition of aromatase activity, possibly by 5α-reduced compounds. In hypophysectomized compared with intact SW fish, plasma levels of testosterone, 5α-androstane-3β,17β-diol and oestradiol-17β were decreased (P<0·05); in the case of oestradiol-17β, this may have reflected the diminished ovarian synthesis of testosterone, its precursor. The plasma level of oestradiol-17β was, however, higher in SW than in FW fish, even in hypophysectomized eels. This suggests that extra-pituitary mechanisms mediate, at least partly, the effects of transfer to SW on aromatase activity. J. Endocr. (1987) 114, 289–294

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 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.

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 &lt; 0.0001) in the 1980s (breakpoints: Burrishoole 1982; Imsa 1988), in both catchments from &gt;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 Silver eel migration behaviour in the Baltic

2007 ◽  
Vol 64 (7) ◽  
pp. 1457-1462 ◽  
Author(s):  
Håkan Westerberg ◽  
Ingvar Lagenfelt ◽  
Henrik Svedäng
Keyword(s):  
Baltic Sea ◽  
Data Storage ◽  
Anguilla Anguilla ◽  
Light Level ◽  
Thermocline Depth ◽  
Silver Eel ◽  
Migration Behaviour ◽  
Swimming Depth ◽  
Silver Eels ◽  
The Baltic

Abstract Westerberg, H., Lagenfelt, I., and Svedäng, H. 2007. Silver eel migration behaviour in the Baltic. – ICES Journal of Marine Science, 64: 1457–1462 Female silver eels (Anguilla anguilla L.) were tagged with data storage tags and released in the Baltic Sea at the same time at a single site on the east coast of Sweden. Data on temperature, light, and depth were obtained from six eels, continuous records for 71 d at sea. The swimming behaviour was similar for all fish, almost stereotyped: swimming activity was between dusk and dawn, starting at a light level corresponding to civic twilight and ending in the morning at generally the same light level. During daylight, the eels rested on the seabed at depths of 2–36 m. Swimming depth was typically close to the surface: up to 95% of swimming time was spent within 0.5 m of the surface. Short dives at irregular intervals (some 1–2 h−1) were made down to the thermocline depth, or occasionally, to the seabed. The duration of such dives were typically 5–10 min. Although only a few days at liberty, the eels had migrated a considerable distance between recapture and release sites, indicating a mean rate of travel of ∼16 km d−1. The recapture positions suggested unidirectional movements towards the southwestern Baltic Sea, i.e. close to the straits leading to the ocean, supporting a belief that the recorded movements were related to eel spawning migratory behaviour.

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