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The effects of artificial light on the distribution of catches of silver eel, Anguilla anguilla (L.), across the Killaloe eel weir in the Lower River Shannon


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Nocturnal migrations of silver eels were investigated daily at the Killaloe eel weir in the lower River Shannon during the silver eel fishing seasons of 1992-3 and 1993-4. Variations in the catches made in a series of coghill nets fished across the weir were analysed with regard to the total daily catch and the catch at different locations across the weir. The use of artificial lighting at a navigation arch in the eel weir was shown to reduce catches in nets placed at the arch.
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P. Cullen and T.K. McCarthy
Nocturnal migrations of silver eels were investigated daily at the Killaloe eel weir in the lower
River Shannon during the silver eel fishing seasons of 19923 and 19934. Variations in the
catches made in a series of coghill nets fished across the weir were analysed with regard to the total
daily catch and the catch at different locations across the weir. The use of artificial lighting at a
navigation arch in the eel weir was shown to reduce catches in nets placed at the arch.
P. Cullen (corresponding
author) and T.K.
McCarthy, Zoology
Department, National
University of Ireland,
Galway, Ireland.
Received 11 April
2000. Read 13
November 2000.
Published 29 December
The downstream migration of the silver eel,
Anguilla anguilla (L.), has been studied at a range
of European locations (Deelder 1954; Vøllestad et
al. 1986; Moriarty 1990). Extensive observations
made at eel fisheries and trapping stations have
also provided information on the effects of natural
environmental factors on the timing and extent
of the seaward silver eel migrations (Vøllestad et
al. 1986). Silver eel migrations in northern
Europe typically occur during autumn and
winter, and a lunar periodicity has also been
regularly observed, with many eels migrating
during the last quarter, or dark phase, of the
lunar cycle (Jens 1953). Artificial obstacles, river
regulation and other anthropogenic factors are
increasingly important determinants of the pat-
terns of eel migration evident in different river
In this short communication we present the
results of a series of observations made at the
Killaloe eel weir on the lower River Shannon
during the 19923 and 19934 silver eel fishing
seasons. Patterns in the distribution of eel catches
across the weir were investigated in relation to
the use of artificial lights mounted on the bridge
for navigation purposes.
The River Shannon (Fig. 1), the longest river
in Ireland, has a main channel length of 359km,
including the estuary, and drains a total area of
(Aalen et al. 1997). Ninety-four per cent
of the catchment area is upstream of Killaloe; this
includes a total surface water area of 414km
(McGrath et al. 1979). The River Shannon at
Killaloe is regulated for hydroelectricity generation
by the Parteen weir (Fig. 1), which diverts most of
the river’s waters via the headrace canal to the
Ardnacrusha power station. Annual discharge from
the Shannon system averages 176m
(A. Shaw,
pers. comm.).
Killaloe is directly downstream of the out-
flow from Lough Derg, 11km north of the
Ardnacrusha power station (Fig. 1). The river is
spanned by a bridge at this point, and a silver eel
fishing weir is attached to the downstream side of
the bridge (Fig. 2). This weir consists of a metal
walkway and the series of steel wattles and hydraulic
frames that it supports; these wattles and frames are
used to set and lift a series of coghill nets. The nets,
which are mounted on square frames, have a
circumference of 10m and a total length of 8m.
Up to 34 nets may be used across the width of
the river, although in times of low flow and/or
. 100B, N
. 3, 165– 169 (2000). © R
Fig. 1—Map of the River Shannon system showing the location of the Killaloe eel weir.
low catches it is more usual to sh seventeen to
twenty nets. The nets are divided into groups (Fig.
2), generally of three, and each group is shed on
the downstream side of an individual arch on the
bridge. The distribution of the nets across the weir,
the arches to which the nets are assigned and the
way in which the nets are set and lifted (manually
versus hydraulically) are indicated in Fig. 2. To
guide the downstream migrating eels into the set
nets, gridded metal frames have been placed be-
tween the bridge structure and the supports that
run from each bridge arch, with the exception of
Arch 3, the navigation arch. The nets at the
navigation arch are attached to a metal frame that
is suspended and lifted by a winch system, and the
net frame is held in place by the current alone.
Fig. 2Diagram of the bridge at Killaloe and the associated eel weir.
This arch is highly illuminated by two 400W
sodium spotlights mounted c. 4m above the waters
surface. These spotlights facilitate night-time boat
trafc, which is not impeded by the use of the
The weir is shed nightly throughout the
silver eel shing season, which generally runs from
SeptemberOctober until March. The nets are set
before dusk, usually at around 15.30 GMT, and
are lifted and emptied at 08.00 GMT. However,
when catches of silver eels are higher than 200
300kg it is sometimes necessary to empty some of
the nets several times during the night.
Fishing at the Killaloe eel weir was monitored
on a daily basis throughout the shing seasons of
19923 and 19934. The total weight of eels
captured on each date and the number of nets
shed were recorded, and the status of the naviga-
tion lights was noted. During most of the 19923
shing season and on occasions during the 19934
shing season the navigation lights were out of
operation due to technical failures. The distribu-
tion of silver eel catches across the weir was
determined on a number of occasions using two
methods: when catches were low, generally less
than 100kg, the catch from each set of nets was
placed in marked bins and weighed to give the
total catch for each arch; when catches were high,
the eel shery manager estimated the catch in each
group of nets at each arch whenever the nets were
lifted and emptied. His estimates were validated by
analysing the relationship between his estimation
of total daily catch and the exact records obtained
from the processing station. There was a very high
degree of correlation between actual and estimated
catch weights (p50.001 using the Spearman rank
correlation), with actual catches being on average
112%96.1% of those estimated.
The silver eel catches made at the Killaloe eel
weir during the 19923 and 19934shing sea-
sons amounted to 26.9 tonnes and 21.7 tonnes
respectively. In 19923shing began in early
September and continued until early March. In
19934 the shing season was similar in length but
ran from late September to late March.
The catch of silver eels in the net group
corresponding to each of the arches at the Killaloe
Fig. 3Average percentage (91 SD) of daily eel catch
recorded at each of the eleven arches fished at the
Killaloe weir during the 1992–3 fishing season (when
lights at the navigation arch were not in use) and the
1993–4 fishing season (when lights at the navigation arch
were in use). The navigation arch is marked ‘Nav.’ on
the x-axis of the graph.
Arch 4, while increases occurred at Arches 1, 2, 5
and 6 (Fig. 3). MannWhitney U tests indicated
that daily catches made at arch 3 when the light
was in use were signicantly lower than catches at
that arch on days when the light was not in use
A similar pattern was observed on two consec-
utive days during 1992. On 27 November 1992
the navigation lights were in use. A total catch of
878kg was recorded, with only 8.6% from Arch 3
(Fig. 4). However, on 28 November 1992 the
navigation lights were not in use and the distribu-
tion of catches shifted. On this occasion a catch of
1225kg of silver eels was recorded, with 25.4%
originating from Arch 3 (Fig. 4).
Silver eels are known to migrate in the faster-
owing reaches of rivers (Sinha and Jones 1975;
Tesch 1977). At Killaloe the fastest-owing water
occurs in the east of the river, in the vicinity of
Arches 26 (Figs 3, 4), where catches are higher.
Downstream, salmon smolt traps at Arches 4 and 7,
which are used after the silver eel season, show a
similar distribution of catches, with the numbers of
smolts trapped at Arch 4 being on average twice as
high as those at Arch 7 (ESB 1994).
Adult eels are nocturnal and are generally in-
active during daylight (Tesch 1992). Investigations
into the diel periodicity of the silver eel migration
on the Shannon carried out on three occasions
during the 19923 season showed no eels migrat-
ing (pers. obs.).
Light-avoidance behaviour is also demon-
strated by silver eels in relation to the use of lights
at the navigation arch, and this is reected in the
distribution of catches across the weir. Catches of
eels are markedly reduced at Arch 3 and are
generally higher at adjacent arches when these
lights are in use (Figs 3, 4). This inuence of
articial light on migrating silver eels is well docu-
mented by many authors (Deelder 1970; Sinha and
Jones 1975; Tesch 1977). Fires and torches have
been used in Camacchio (Italy) and in Ireland to
reduce the intensity of the silver eel runs while
nets were emptied and to calm down the trapped
silver eels (Went 1944). Sinha and Jones (1975)
cited Petersen (1906) and Lowe (1952), in which
both laboratory and eld experiments demon-
strated the deection of silver eels by articial light
sources. Hadderingh et al. (1992) listed other ex-
amples of the use of articial light in commercial
shery operations.
It can be concluded from this study that the
articial lights installed at the Killaloe eel weir for
navigation purposes inuence the downstream mi-
gration pattern of eels at that location and hence
Fig. 4—Percentage catch per arch at the Killaloe weir on
27 and 28 November 1992. The navigation lights were
used on 27 November 1992 and were not used on 28
November 1992.
eel weir was observed on numerous occasions
during the 19923 and 19934shing seasons.
When catches of eels were low and only seventeen
to twenty nets were shed, no clearly dened
pattern in the distribution of catches across the
width of the weir was apparent. However, in times
of high catches, when all 34 nets were shed, there
was a clear pattern. In 19923, when the naviga-
tion lights were out of use for most of the shing
season, the highest catches were recorded at the
navigation arch itself (Fig. 3). However, in the
19934 season the lights were in use and a shift in
the distribution of catches across the weir was
evident. Catches decreased considerably at the nav-
igation arch and decreased to a lesser extent at
affect the distribution of the catch of eels across the
eel weir. This nding could be used to further the
exploitation or conservation of the species. Lights
may be used to deect eels from harmful and
possibly lethal obstacles to their downstream mi-
gration, such as the intakes of power stations
(Hadderingh et al. 1992; Patrick et al. 2000), al-
though water depth and turbidity may affect the
efciency of such techniques.
This study is a contribution to the ESB Shannon
Eel Management Programme. The assistance of the
staff of the ESB eel shery at Killaloe, especially
Mr T. OBrien, is gratefully acknowledged. Mr
Alan Shaw, ESB Ardnacrusha, provided hydro-
metric data for the Shannon system.
Aalen, F.H.A., Whelan, K. and Stout, M. 1997 Atlas
of the Irish rural landscape. Cork University Press.
Deelder, C.L. 1954 Factors affecting the migration of
the silver eel in Dutch inland waters. Journal de
Conseil Permanent International pour l
Exploration de la
Mer 20, 17785.
Deelder, C.L. 1970 Synopsis of biological data on the eel
Anguilla anguilla,Linnaeus
FAO Fisheries
Synopsis no. 80 rev. 1. Rome. FAO.
ESB 1994 ESB Fisheries Conservation Annual Report.
Dublin. ESB.
Hadderingh, R.H., van der Stoep, J.W. and Habraken,
J.M.P.H. 1992 Deecting eels from water inlets
of power stations with light. Irish Fisheries Investigations
Series A (Freshwater) 36,809.
Jens, G. 1953 ber den lunaren Rhythmuss der
Blankaalwanderung. Archiv fu¨r Fischereiwissenschaft 4,
Lowe, R.H. 1952 The inuence of light and other
factors on the seaward migration of the silver eel
(A.anguilla, L.). Journal of Animal Ecology 21, 275
McGrath, C.J., OLeary, D.P., Sharkey, P.J. and Murphy,
D.F. 1979 An experimental electrical guid-
ance system for eels at Killaloe eel weir on the
River Shannon. In Eel research and management.
Rapports et Proce`s-Verbaux des Re´unions du
Conseil International pour lExploration de la Mer
Moriarty, C. 1990 Short note on the silver eel catch
on the lower River Shannon. Internationale Revue
der gesamten Hydrobiologie 75 (6), 81718.
Patrick, P.H., Poulton, S. and Brown, R. 2000
Using strobe lights and sound to protect American
eels. Hydro Review 19(6), 98102.
Petersen, C.G.J. 1906 The inuence of light on the
migrations of the eel. Report of the Danish Biological
Station 14,29.
Sinha, V.R.P. and Jones, J.W. 1975 The European
freshwater eel. Liverpool University Press.
Tesch, F.-W. 1977 The eel
biology and management of
anguillid eels. London. Chapman and Hall.
Tesch, F.-W. 1992 Anguillidae. In H. Hoestlandt
(ed.), The freshwater fishes of Europe,vol.
Anguillidae, 390437. Wiesbaden. AULAVerlag.
Vøllestad, L.A., Jonsson, B., Hvidsten, N.A., Haesj,
T.F., Haraldstad, O. and Ruud-Hansen, J. 1986
Environmental factors regulating the seaward mi-
gration of European silver eels. Canadian Journal of
Fisheries and Aquatic Sciences 43, 190916.
Went, A.E.J. 1944 The Galway Fishery. Proceedings of
the Royal Irish Academy 49C, 187219.
The subvention
granted by the
National University of
Ireland, Galway,
towards the cost of
publication of papers
by members of its
staff is gratefully
acknowledged by
the Royal Irish
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Weltweit nimmt die nächtliche Helligkeit durch künstliche Beleuchtung rasant zu. Je nach Wolkendichte und ‐höhe kann sie Nachtlandschaften weiträumig erhellen. Diese Lichtverschmutzung prägt Lebensräume in und um Gewässer nicht nur durch die veränderte Lichtstärke, sondern auch durch die Veränderung der spektralen Zusammensetzung, Beleuchtungshäufigkeit und ‐dauer. Künstliches Licht dringt besonders in die oberen Gewässerschichten ein und kann dadurch das komplexe Zusammenspiel zwischen Organismen und ihrer Umwelt aus dem Takt bringen. Es verändern sich dadurch nicht nur das Verhalten und der Tag‐Nacht‐Rhythmus nacht‐ und tagaktiver Arten, sondern auch die vielfältigen Wechselwirkungen zwischen den Arten. Angesichts vieler heute bereits nachgewiesener Auswirkungen der Lichtverschmutzung auf Organismen und ihre Lebensräume ist es dringend erforderlich, neue Licht‐ und Lichtschutzkonzepte zu entwickeln und anzuwenden. Durch Minimierung des Beleuchtungsniveaus, zeitweises Abschalten von Lichtquellen, Optimieren der Abstrahlungsgeometrie und Einsatz geeigneter Leuchtmittel können Umweltbeeinträchtigungen von Gewässern und des Gewässerumlands deutlich reduziert werden, ohne dadurch den Nutzen des nächtlichen Kunstlichts einzuschränken.
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Hovedmålene for denne rapporten er 1) å oppsummere kunnskapen om ålens utbredelse innen og mellom norske vassdrag, 2) vurdere vannkraftreguleringers påvirkning på ålens livssyklus og forekomst og 3) vurdere mulige avbøtende tiltak for å opprettholde og øke ålebestanden. Rapporten er basert på en litteraturoppsummering av norske og utenlandske undersøkelser, analyser og presentasjon av registreringer av ål i NINAs fiskedatabase samt analyser av elfiskeresultater fra 13 kalkede vassdrag i Agder og Rogaland.
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Eel migration takes place between August-December. 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 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; high mean water temperature and low water discharge gave the opposite effect. Duration of a part of the run, eg number of days from 5-25% cumulative eel descent, was significantly correlated with mean water temperature and increase in water discharge for the period in question. Maximum silver eel descent was at a water temperature of 9oC. Few silver eels descended at temperatures <4oC or >18oC. Migration speed was correlated with water discharge and water temperature. Water discharge alone explained 85% of the total variation in migration speeds. Illumination of 20 lx upon the river reduced descent of silver eels.-from Authors
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Behaviors that precede the daily migrations of mixed-species schools of juvenile grunts (Pomadasyidae), from patch reefs to grass beds at dusk and vice versa at dawn, are defined and utilized to ascertain the precision of the migrations. Although premigratory behaviors differ at dusk and dawn, the migrations are precise twilight events which occur at the same light intensities during dawn and dusk. Histological sections of the retina reveal that both cones and rods are fully exposed to ambient light during the migrations. Under the difficult photic conditions that prevail during migration, the retina is structured photomechanically to maximize the absorption of ambient light. Body colorations of the grunts, which consist mostly of intense colored stripes during the day, are replaced at night by cryptic melanic patterns. The precision of migration, the photomechanical movements in the retina, and the changes in body coloration are considered adaptive because they reduce predation on grunts when they migrate and are most vulnerable to attack. In support of this conclusion, the migrations take place just before the evening and just after the morning quiet period - thus they avoid that period during twilight when predation is highest in tropical fish communities.
1. Observations on the conditions, particularly floods and the phase of the moon, which affect the seaward migration of silver eels were collected for three years from six field sites in the English Lake District. 2. These observations confirmed that silver eel runs occur only between late summer and early winter. The higher upstream the eels the earlier in the season they are trapped. 3. The migration was confined to very few nights, on which large numbers of silver eels travelled seawards; eels ran on the same nights at widely separated sites. 4. Most eels migrated when floods coincided with moonless periods. Few eels ran when there was a full moon; the run could be checked experimentally by artificial light. The start of the eel season was delayed until there was a flood, after which some eels ran on dark nights when there was no flood. The greatest catches were made on rising floods. Weather conditions were not as important as floods in determining whether eel runs occurred. 5. Eels tended to run in the early hours of the night except on a rising flood. Evidence whether eels congregate near a lake outflow before migrating downstream was inconclusive. 6. Attempts to induce runs of eels and to re-trap marked eels were not very successful, but both contributed evidence that silver eels have to be stimulated into a `migration mood', a state of extreme activity, before they will migrate. This mood appeared to be related to conditions of flood, possibly fall in temperature and darkness of the night. Eels kept in tanks in the laboratory were particularly active the same nights that eels were migrating in the field. 7. Experiments in the field and laboratory have shown that the downstream migration of silver eels may be (a) checked, and (b) deflected by lights. At Cunsey Beck a series of underwater lights stopped the run of eels for half-hour periods. For the deflexion of eels into traps at the side of a river underwater lights must be thrown well upstream; lights shining vertically downward into the water are less effective. Upstream-shining lights of about 5 and 0.7 c.p. were effective in Cunsey Beck; evidence whether brighter lights would deflect more eels is conflicting. The extreme activity of the silver eel associated with migration appears to lower the threshold of response to light, artificial lights having most effect on the nights when the eels were most active. There is no reason to believe that the response to light differs according to the sex of the eel. 8. In an artificial river in the laboratory the downstream swim was delayed by light and eels chose a dark channel in preference to a lit one; there was evidence from laboratory experiments that once the shock effect of the light wore off the eels responded photokinetically instead of negatively phototactically. 9. In the artificial river the eels swam upstream against a flow of 1.1 ft./sec., but tired rapidly against a current of 1.7 ft./sec. 10. It was concluded that flood, or some associated factor, possibly fall in water temperature, acts as a trigger stimulus starting the downstream migration. Eels then run continuously, except when inhibited by light during the day and on nights with bright moonlight or starlight, until the effect of the activating stimulus wears off. Most eels will probably need restimulating several times before they reach the sea.
Catches of eel from the Shannon lake system of 35,000 ha have fluctuated from 10 to 46 t per annum over the period 1964 to 1988. Peak catches are observed at intervals from 3 to 5 years. Catches increased following improvement of fishing gear but otherwise remained steady in spite of a sustained increase in stocks. Migration was observed every month except June. The numbers of female eels>60cm decreased over the period. Male eels ranged from 39 to 44 cm, ages 5 to 15; females from 45 to 99 cm, ages 8 to 30 years.