Environmental Biology of Fishes 67: 349–357, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
Hydrometric and meteorological factors affecting the seaward migration of
silver eels (Anguilla anguilla, L.) in the lower River Shannon
Paula Cullen & T. Kieran McCarthy
Zoology Department, National University of Ireland, Galway (NUI, Galway), Galway, Ireland
(e-mail: pauline.cullen@nuigalway.ie)
Received 1 August 2002
Accepted 15 April 2003
Key words: spawning migration, environmental influence
Synopsis
European eels, Anguilla anguilla, L., are captured in coghill nets at three commercial weirs on the Irish River
Shannon during their autumn/winter seaward migration. The variations in daily silver eel catches were analysed
in relation to environmental conditions, especially hydrometric and meteorological factors. Three multivariate
environmental axes were distinguished with which daily eel catches could be correlated. The relative importance
of various hydrometric, meteorological and temporal (seasonality, lunar phase) factors was identified, showing
how hydrological and meteorological factors generally obscure the underlying lunar periodicity of the silver eel
migrations at Killaloe.
Introduction
The nocturnal, downstream, seaward, migration of the
European silver eel, Anguilla anguilla, L., usually
begins in the latter half of the year (Deelder 1970,
Tesch 1977). Investigations have often been made on
the effect of various environmental factors on silver
eels. Among the best documented of these is the influence of the lunar cycle on silver eel runs with the highest
catches of silver eels usually occurring during the last
quarter of the lunar cycle (Deelder 1954, 1970, Tesch
1977). However, the effect of the moon does not appear
to be the same everywhere. In the Upper Rhine peaks
tended to occur before the moon’s last quarter, whereas
in Dutch inland waters the peaks occurred after the
last quarter (Deelder 1954). A series of experiments
on captive eels showed that they exhibited the greatest tendency to escape during the last quarter of the
lunar cycle (Boetius 1967). Because the experiments
took place in darkness at all times, it was concluded
that the migratory behaviour of eels was independent
of light, daylight or moonlight. Indeed Deelder (1970)
also stated that runs will still occur if the moon is
obscured by cloud, and that eels may have an internal
rhythm related to the lunar cycle, which operates independent of the presence or absence of moonlight. River
discharge, temperature, wind speed and direction, and
the occurrence of atmospheric depression generated
microseisms have also been investigated in relation to
the patterns observed in silver eel migrations (Deelder
1954, Vøllestad et al. 1986).
The tendency of silver eels to migrate in one direction, and in large numbers makes them relatively
easy to trap, by creating a barrier to their migration
(Moriarty 1988). In the case of the River Shannon,
the largest river system in Ireland, a number of weirs
operate commercially on this principle. As part of
an overall study on the biology and management of
eels in the River Shannon system (McCarthy et al.
1994 a,b, McCarthy & Cullen 2000 a,b), studies on
the migratory behaviour of silver eels captured at
the Killaloe eel weir were carried out during the
1992–1993 and 1993–1994 silver eel fishing seasons. The variations in daily catches for both of
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these years were analysed in relation to a series of
environmental factors including water flow, air and
water temperature, wind speed and direction, mean sea
level (msl) pressure and lunar phase. Records for the
daily catches obtained during the fishing seasons from
1981–1982 to 1991–1992 were obtained and similarly
analysed.
This study investigated the environmental factors
which affect the migratory patterns of silver eels in the
lower River Shannon, at the Killaloe eel weir in particular. Understanding the factors that influence rates
of silver eel migration is important in respect of two
objectives of Shannon eel fishery management: (a) the
optimisation of the commercial eel weir operations;
and (b) the facilitation of downstream eel migration
in a regulated river system utilised for hydroelectric
power generation.
Study area
The River Shannon (Figure 1) is the longest river in
Ireland, with a total main channel length of 359 km and
drains a total catchment area of about 14 000 km2 . In the
mid 1920s, the river was regulated to generate hydroelectric power. This involved the construction of the
Parteen Regulating Weir, which directs the main body
of the river, via a headrace canal, to the power station
at Ardnacrusha (mean annual discharge: 176 m3 s−1 ;
summer mean: 99 m3 s−1 ; winter mean: 274 m3 s−1 ;
maximum: 400 m3 s−1 ) while still allowing a statutory
10 m3 s−1 down the original river channel. However,
at times it is necessary to ‘spill’ volumes of water in
excess of the usual 10 m3 s−1 at the Parteen weir to alleviate flooding or high lake levels up river. During the
period 1981–1994 the highest rate of spillage recorded
was in 1990 with an average of 350 m3 s−1 between
26 January and 9 February.
The Killaloe eel weir is located 11 km upstream of
the Ardnacrusha power station, about 3 km upstream
of the regulating weir (Cullen & McCarthy 2001). The
silver eel fishing season at this location usually starts in
August or September and ends in February or March.
To determine the start of the fishing season the manager of the weir would set one or two nets as indicators, when he feels that discharge and other conditions
are favourable to silver eel migration. He would then
gauge the catches made in these nets to assess when
to start fishing for the season. Once fishing has started
it generally continues until February or March, or until
catches cease.
Materials and methods
Silver eels caught at the Killaloe eel weir are boxed
manually, weighed and frozen for export at a nearby
processing station, on a daily basis. Monitoring of
this operation made it possible to obtain exact records
on the daily catches of eels for both the 1992–1993
and 1993–1994 fishing seasons. However, such exact
records of daily weights were not available for the
catches of the 1981–1982 to 1991–1992 fishing seasons. For these years the fishery manager’s records
of daily catch were used. These records are an estimate of the daily catches, made by the fishery manager,
and based on the volume of eels captured (T. O’Brien
pers comm.). Spearman correlation analyses showed
that the estimated catch weights for both the 1992–
1993 and 1993–1994 fishing seasons were significantly
correlated (p ≤ 0.001) to the exact catch weights
obtained from the processing station observations, with
no overall difference between the observed and estimated values (paired samples t-test, p > 0.05). It was
thus decided that the estimated catch weights from
the previous years were a valid record of the daily
catches. Data on flow through Killaloe and the volume
of water passed down the original river channel (used
as a measure for flooding/high water levels in Lough
Derg) were obtained from Ardnacrusha power station.
Records of meteorological data were obtained from the
Irish Meteorological Office.
The factors included in all analyses were: lunar
phase, air and water temperatures, flow at Killaloe,
spillage rates at the Parteen Weir, wind speed and
direction and atmospheric pressure. Because of the
complex nature of the interrelationships among all the
environmental variables and in order to obtain an integrated overview for the full 1981–1994 period a principal components analysis (PCA) was carried out using
SPSS for Windows (version 10) for all years combined.
PCA was chosen as the main analytical method because
it is a useful statistical method for summarising information from many variables in reduced multivariate
space, based on the covariance or correlation among
the variables.
Results
The patterns of capture of silver eels at Killaloe in
1992–1993 and 1993–1994 were similar, although the
1992–1993 season, which started in early September,
began almost a month earlier than that in 1993–1994.
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Figure 1. Maps of (a) Ireland with the Shannon catchment area highlighted, (b) the River Shannon system and (c) the lower River Shannon
area showing the location of the Killaloe eel weir.
352
Figure 2. Daily catches of silver eels made at the Killaloe silver eel weir during the 1992/1993 (top) and 1993/1994 (bottom) fishing
seasons in relation to river discharge rates at that point. Also indicated is the lunar ‘dark’ i.e. the period during the lunar cycle from the
start of the last lunar quarter to the start of the 1st lunar quarter.
The main run of eels started about 70 days into the fishing season, in both years (Figure 2), with approximately
85% of the season total catch (26.9 t in 1992–1993 and
21.7 t in 1993–1994) captured within the first month
after the start of the run. However, when the data on
daily catches for prior seasons was examined greater
between year variations were noted. Figure 3 shows the
pattern of daily catches for the 1981–1982, 1986–1987
and 1991–1992 fishing seasons, where more than one
run was observed.
In both the 1992–1993 and 1993–1994 the start of
the main eel run coincided with an almost doubling of
the mean daily discharge through Killaloe (Figure 2).
Similarly, increases in catches occurred at the start of
periods of spillage at the Parteen regulating weir in
both years. Peak catches for each season were also
recorded directly following high winds (stormy conditions). In 1992–1993 high catches, relative to the
runs at the period they were recorded, of 1099 kg on
31 November 1992 and 106 kg on 21 January 1993
353
Figure 3. Daily catches of silver eels made at the Killaloe silver eel weir during the 1981/1982 (top) 1986/1987 (middle) and 1991/1992
(bottom) fishing seasons in relation to river discharge rates. Also indicated is the lunar ‘dark’ i.e. the period during the lunar cycle from
the start of the last lunar quarter to the start of the 1st lunar quarter.
occurred following high winds of 18.88 and 22.96
knots, respectively. Similarly, the exceptional catch
of 4121 kg on 9 December 1993 occurred following
high winds, measuring 24.54 knots, on 8 December
1993. Figure 4 illustrates the relationship between wind
direction and the catch at Killaloe for 1992–1993 and
1993–1994. As can be seen, in both years the majority
of the eels were captured when the wind was from a
westerly direction. The relationship between the daily
catch and the lunar cycle is illustrated in Figure 5. In
1992–1993 the largest portion of the catch was made
during the full moon period of the lunar cycle. In
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Figure 4. Graphs illustrating the relationship between wind direction and the catches of silver eels made at the Killaloe eel weir during
the 1992/1993 and 1993/1994 fishing seasons. In each case the radius of each wedge represents the % of the overall catch made.
Figure 5. Graphs illustrating the relationship between lunar phase and the catches of silver eels made at the Killaloe eel weir during the
1992/1993 and 1993/1994 fishing seasons. In each case the radius of each wedge represents the % of the overall catch made.
1993–1994 the largest portion was recorded during the
last lunar quarter.
Because the overall relationship between environmental variables and catch appeared complex, a PCA
analysis was carried out to summarise the information from all the variables in a reduced multivariate
space. In Figure 6(a) and (b) the factor loadings for
each of the environmental variables included in the
355
the PCA factors in determining the silver eel catches at
Killaloe.
Discussion
Figure 6. Ordination plots of (a) factor 2 vs factor 1 and (b) factor 3 vs factor 2: results of a PCA analysis carried out on the environmental data for the 1981/1982–1993/1994 fishing seasons.
analysis are plotted for the three main factors identified by the PCA. As can be seen in Figure 6(a), high
flow associated with low air and water temperatures
are the main environmental variables associated with
factor 1 (which accounted for 29.4% of the variance).
High wind speed (indicative of stormy conditions) and
low air pressure were identified as the environmental
variables that were associated with the second factor
(which accounted for 20.8% of the variance). The third
factor, as illustrated in Figure 6(b), related to lunar
phase and accounted for a further 12.5% of the variance
in the overall data set. Low scores were assigned to
the dark phase of the lunar cycle, starting with the
last quarter, with the brighter, full moon, phase being
coded for by higher values. The relationship of variations in daily silver eel catches to the three PCA
axes was investigated by means of correlation analyses. Daily catch was shown to be positively correlated
with factor 1 (rS = 0.148, p ≤ 0.001) and factor 2
(rS = 0.186, p ≤ 0.001) and inversely correlated with
the third PCA factor (rS = −0.106, p ≤ 0.001). These
correlations highlighted the importance of the environmental variables that were most associated with each of
The effect of variations in river discharge on the run
of eels is well documented (Deelder 1954, Tesch 1977,
Vøllestad et al. 1986). An increase in water flow tends
to encourage eels to migrate, water discharge seeming to be the key environmental variable determining
the migration speeds of European eels (Vøllestad et al.
1986). Studies on the Dutch polders, where water levels
remains steady, showed that eel migrations increased
in relation to increased flows in the polder canals, suggesting that water flow rather than water level is the
key factor (Deelder 1954). Similar studies carried out
on migrating silver shortfin eel, Anguilla australis, and
New Zealand longfin eel, Anguilla dieffenbachii, in
New Zealand by Todd (1981) found that heavy rainfall resulting in increased stream flow was commonly
associated with the largest runs of silver eels.
In association with discharge rates, water temperature is also thought to influence the migration of
silver eels. Vøllestad et al. (1986) found that low
water temperatures in conjunction with high increases
in discharge increased the rate of descent of eels in
early seasons. The range of temperatures over which
most silver eels were found to descend the Swedish
River Imsa was 9–12◦ C, with the maximum descent
of eels occurring at 9◦ C. Their findings indicated that
temperature may control the timing of the transformation of yellow eels to migrating silver eels and the start
of their descent. It was also suggested that low summer temperatures led to an early start of the run on the
Imsa. Similarly migration of silver eels in the Elbe was
delayed by prolonged warm weather, suggesting that
certain minimum temperatures are necessary to initiate migration (Tesch 1977). The converse was found
in Poland (Sweirzowski unpublished data) where an
increase in temperature, in conjunction with a rise in
the water level, initiated the run. A temperature drop
stopped or limited the run, with a subsequent temperature rise bringing about an intensification in the
migration. Although no sharp increases or decreases in
water temperature occurred in conjunction with silver
eel runs at Killaloe, most of the main runs occurred
when the water temperature reached and dropped
below 10◦ C.The fishing season tended to start earlier
in years that had cold summers, e.g. 1985 and 1986.
The correlation between PCA factor 1 and catch would
356
indicate that in the case of the Killaloe eel weir, catches
of eels generally increase as air and water temperatures
drop, and flow conditions increase.
Wind speed and direction are known to influence
the silver eel migration in many locations, especially
in open water, (Deelder 1970). Frost (1950) observed
that southerly winds blow across Lough Neagh, causing water to be pushed in the direction of the mouth of
the River Bann. This stimulates the silver eels to move
in that direction and hence good catches are made on
the Bann. On Lake Windermere the converse holds true
(Sinha and Jones 1975). In the case of eels in the River
Shannon Moriarty (1990) concluded that the effect of
northerly winds was noticeable especially in conjunction with high flows. Using the Killaloe daily eel catch
data it was possible to show correlations between catch
and both the wind speed and direction for some of
the years analysed. In 1992–1993 and 1993–1994 data
the highest percentage of the catch could be attributed
to nights on which there were westerly winds. These
westerlies, often north westerlies, occurred on many
of the nights on which the peak catches occurred. For
example, the peak catch for the 1993–1994 season
was recorded on the morning of 9 December 1993 following stormy weather, associated with high winds.
Overall the influence of high wind speeds on silver eel
catches at Killaloe was demonstrated by the correlation
between PCA factor 2 and catch levels.
Although the aforementioned correlation between
catch and PCA factor 2 also indicated that catch
levels increased in relation to low pressure levels,
atmospheric pressure itself is generally not thought
to greatly influence silver eel runs. However, the
occurrence of depression generated microseisms may
do so. Deelder (1954) found that the intensity of the
migration of silver eels in Holland rose sharply under
weather conditions which were associated with the passage of depressions over the English channel and North
Sea. These depressions in turn generated microseisms,
and it was found that those with a period of about three
seconds preceded increases in the silver eel catches.
Unlike many smaller European river systems where
the migratory patterns of silver eels can be linked to
one or two environmental factors, the overall effect
of environmental variables on the run of silver eels
at the Killaloe eel weir is complex. Interrelationships
occur between many of the environmental variables
which influence the catch. However, despite these
complexities, it was possible to show, by means of
PCA analyses, the influence of all the environmental variables included in the analysis on the catches
of silver eels at the Killaloe eel weir. Although initial
analyses appeared to highlight the importance of flow
in determining silver eel runs at this location, the PCA
demonstrated the influence of many other environmental factors, including an obscured lunar periodicity to
the silver eel migrations.
With a decline in the panmictic population of the
European eel recorded since the 1980s there is an
increased importance for understanding of the dynamics of migratory silver eel populations, on both local
and Europe-wide bases (Feunteun 2002). The present
study has highlighted the need for not only local
but also long-term monitoring of eel migratory patterns. The determinants of daily silver eel catches,
or sizes of migratory populations, include natural
environmental variables as well as the regulation of
flow for power generation. Like many large river
systems the Shannon catchment is an extensive and
complex network of lakes, which leads to a prolonged accumulation of eels in the lower reaches.
As a result, the final phase of the riverine migration of the eels from this system is influenced by
both environmental conditions and the impact the seasonal variation in electricity demand can have on flow
conditions.
Acknowledgements
This project was carried out as part of the River
Shannon Eel Management Programme 1992–1994,
initiated and funded by Electricity Supply Board
(ESB) Fisheries and Conservation. The assistance
of the staff of the Salmara eel fishery at Killaloe,
especially Mr. T. O’Brien, is gratefully acknowledged.
Mr. Alan Shaw, ESB Ardnacrusha, provided hydrometric data for the Shannon system.
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