Fisheries Research, 4 (1986) 43--58 43 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

R I V E R I N E M I G R A T I O N O F Y O U N G E E L S A N G U I L L A A N G U I L L A (L.)

CHRISTOPHER MORIARTY

Department of Fisheries and Forestry, Fisheries Research Centre, Abbotstown, Castleknock, Dublin 15 (Ireland)

(Accepted for publication 18 October 1985)

ABSTRACT

Moriarty, C., 1986. Riverine migration of young eels AnguiUa anguilla (L.). Fish. Res., 4: 43--58.

Eels of length 6--50 cm, trapped in the course of up-stream migration in the River Shannon, were sampled during 10 seasons from 1973 to 1983. Migration normally began at the end of May at water temperatures of 13 or 14°C, but the extreme dates ranged from 17 May to 24 June. In five out of seven years, migration ended between 10 and 19 September; in the other two years it was on 29 July and 6 October. Positive correlations were observed between date of first migration and temperature between 15 and 21 May. No connection between temperature and date of end of migration was apparent.

Length measurements of 5008 eels showed that large individuals (~15 cm) migrated throughout the season, but that the majority of smaller eels ( ~ 10c m ) had a shorter migration period from mid-June to mid-August.

The ages of 158 specimens were determined. The maximum age was 10 +; the majority ranged from 1 + to 3 +. Elvers of 0 + were scarce or absent early in the season and were never plentiful. Numbers of eels caught ranged from 131000 to 417000 per year.

It was concluded that a number of factors influenced migration of the eels, the time of year perhaps being the strongest, with water temperature in May being secondary. Migration of the greatest numbers occurred in the year following the greatest immigration of elvers to the river. This suggests that migration may be influenced by population pressure in the lower reaches of the river.

INTRODUCTION

Migra t ion o f y o u n g eels f r o m t ida l wa te r s in to fresh w a t e r has been ex t ens ive ly s t u d i e d in G e r m a n y , The N e t h e r l a n d s and e l sewhere (Dee lde r , 1970; Tesch , 1977) . I n f o r m a t i o n on the u p - s t r e a m m i g r a t i o n o f eels wh ich have s p e n t some t i m e in f r e s h w a t e r h a b i t a t s is less r e a d i l y avai lable . In t he case o f Anguilla anguilla, Dahl (1983} and M o r i a r t y (1983} have p r e s e n t e d p r e l i m i n a r y resu l t s o f s tud ies at p o w e r s t a t i on dams . In N o r t h A m e r i c a , a c o m p a r a b l e s t u d y has been m a d e a t the Moses Saunde r s d a m on the St. L a w r e n c e River f o r A. rostrata (Liew, 1982) .

In I r e l and , o b s e r v a t i o n s on the y o u n g eels wh ich a scend elver l adder s a t Pa r t e en Weir on the River S h a n n o n have been in progress since 1973 , w h e n

0165-7836/86/$03.50 © 1986 Elsevier Science Publishers B.V.

44

measurements of the eels in samples taken th roughout the season began. Since 1977, a detailed record of the quantities of ascending eels has been kept, together with data on tempera ture and water flow. This paper de- scribes the migrating eels with reference to length, age and weight of samples and to the total quantities captured.

The Shannon, at 275 km, is the longest river in Ireland. It drains approxi- mately one-third of the land surface of the island, having a ca tchment area of 11 800 km 2. A weir at Parteen (Fig. 1) diverts most of the water to a ca- nal leading to a power station at Ardnacrusha at the head of the river estuary. The tailrace of the power station rejoins the river at a point 15 km down- stream of Parteen. A minimum flow of water is maintained in the natural bed of the river, which is also fed by tributaries. The crest of Parteen Weir stands 33.55 m above sea level and 9 m above the natural bed of the river. In the course of its 15 km, therefore, the river falls through rather less than 24 m and is a slow-flowing, meandering stream with a number of low weirs and islands.

Parteen Wei~

Fig. 1. The lower River Shannon. The position of the elver trap at Parteen Weir is marked by an arrow.

An overflow pool- type fish pass with a constant water flow of 0.5 m 3 s -1 allows salmon to surmount Parteen Weir. At the t ime of its construct ion, it was assumed that eels would also make use of the pass. When it was discovered that they failed to do so, elver ladders leading to holding boxes were installed on each side of the fish pass.

Both elver ladders are 120 cm long, with crest height 6 0 c m above the surface 6f the river. That on the left-hand side of the salmon pass is 80 cm wide and composed of strands of "sugaun", a rope made from hay. The right-hand ladder is 30 cm wide, and composed of tufts of nylon bristles

45

1 0 c m in length as described by O'Leary (1971). The tufts are set in a wooden board in staggered rows 15 cm apart.

Very large numbers of glass eels (up to 23 million in a year) enter the tailrace of the power station, where they are trapped for transport up-stream. Although there is no way of assessing the numbers of glass eels which enter the river, rather than the tailrace, it appears that they represent a relatively small proport ion of the total, and that the majority are attracted by the greater discharge of water through the power station.

MATERIALS AND METHODS

Samples for measurement were taken by hand-net from the holding boxes at irregular intervals from 1973 to 1975. Until Ju ly 1975, the mesh-size of the hand-net was 4 mm knot to knot; subsequently, a fine-meshed pond net was used. Sampling was renewed on a monthly basis beginning in June 1982. From this date onwards, the holding boxes were emptied the day before sampling took place so that none but the eels which had been trapped overnight were examined. When large numbers were present, from 200 to 650 specimens were measured and the remainder were counted, only those greater than 12 cm being measured. The latter were few and could be distin- guished easily f rom the smaller specimens. Dates and sizes of samples are given in Table II.

The samples were anaesthetised with chlorbutol, placed on a damp level surface, straightened by hand and measured with dial calipers of sensitivity 0 .05mm. Measurements were recorded to the nearest mm downwards. Recovery from the anaesthetic was satisfactory and nearly all the specimens were returned to the water in good condition. In 1974, otoliths were taken from a sample of 158 specimens and examined intact, immersed in creosotum. These specimens were also weighed using a balance of sensitivity 0.1g.

The usual management procedure was to remove eels from the holding boxes whenever substantial numbers had accumulated. Removal of the eels was therefore irregular, the frequency of operations depending on the numbers of eels migrating. Since 1977, records have been kept of the quantities of elvers taken from the traps and of the water temperature. The quantities were estimated by volume but recorded as weights.

RESULTS

Water temperature and flow

Water temperature was measured daily at Parteen, from 1977 onwards, by Electricity Supply Board fishery staff. Maximum and minimum temperatures for the months April--October, inclusive, are given in Table I. The years 1981 and 1982 had exceptionally warm conditions in April and cool in May,

46

with the April maxima being greater than the May minima. In the other 5 years, the April maximum was only once greater than the May minimum. The range of temperatures in June was relatively small, from a minimum of 13°C to a maximum of 18°C, and the highest temperature observed throughout the period was 22°C in July 1983.

TABLE I

Max imum and m i n i m u m mid-day water t empera tu re (°C) at Par teen

April May June Ju ly August September October

1977 9--6 16-- 9 1 8 - - 1 4 1 9 - - 1 6 1 8 - - 1 6 16--14 14--11 1978 9--7 17-- 9 1 6 - - 1 4 1 8 - - 1 4 1 8 - - 1 5 17--14 14--12 1979 9--5 12-- 8 1 7 - - 1 3 1 9 - - 1 6 1 8 - - 1 5 16--14 14--11 1980 1 1 - - 6 1 5 - - 1 1 1 6 - - 1 4 1 7 - - 1 5 1 8 - - 1 6 18--15 14-- 9 1981 1 3 - - 9 1 3 - - 1 0 1 6 - - 1 3 1 7 - - 1 5 1 9 - - 1 6 19--14 14-- 9 1982 1 3 - - 7 1 4 - - 1 0 1 7 - - 1 5 2 1 - - 1 6 2 1 - - 1 6 17--13 13--10 1983 9--7 13-- 9 1 8 - - 1 3 2 2 - - 1 7 2 1 - - 1 9 19--14 15--10

The water flow from the main River Shannon is maintained at a constant rate of 10 m 3 s -1, except on rare occasions when a surplus flow is "spilled" over the weir. The flow in the lower river is, however, influenced by a number of tributaries, in particular by the Kilmastulla River which joins the lower river a short distance down-stream of Parteen. Daily records of the flow from 1977 to 1982, inclusive, were made available by An Foras Forbartha (The National Institute for Physical Planning and Construct ion Research). The mean daily flow over the period was 2.3 m 3 s -1. Between October and March, flows were variable and generally high. From April to September, flows were lower and less variable, with minima in June and July. During the normal eel migrating period, May--September, flows greater than the annual average were observed on three occasions; May 1979, and August and September 1980. Mean monthly flows from April to September are shown in Fig. 2.

m 3 /sec

4 - -

3 - -

2 - -

1 - -

o

1 a 1977 • 1980 |

o 1978 ~ 1 9 8 1

• 1979

A M J J A

Fig. 2. Mean m o n t h l y f lows in the Kilmastul la River (in m 3 s -1 ) f rom 1977 to 1981.

47

Length

The eels caught in the sugaun trap were invariably small. Direct comparisons were made on 23 June and 11 August 1982. In the June sugaun sample, numbering 621, the biggest eel caught measured 13.9 cm and only 23 were greater than 11 cm. In the nylon-brush trap, 19 of the 27 eels caught were greater than 11 cm. In August, 28 eels were caught in the sugaun trap, the biggest measuring 11.5 cm. The nylon-brush sample numbered 567, of which 221 were greater than 11 cm and the biggest measured 50 cm.

On two occasions when large numbers of eels were present, two successive samples were caught by hand-net and measured separately. In both cases, the mean lengths of those in the second sample were greater than those in the first, resulting from the presence of higher proportions of specimens of more than 100 mm length. Apparently the larger eels were better able to escape from the net. It was considered likely that the use of the net to collect partial samples from the catching box led to a bias in favour of small specimens. The technique was therefore changed, and the 1982 samples consisted of the entire catch made in a single night. This ensured that all large specimens were measured. The use of a fine-meshed net in July 1975 was expected to result in the capture of a higher proportion of small eels, but in the event yielded fewer than in the previous month and in the pre- vious July.

Eels of less than 8 cm were very scarce, numbering only 52 out of the total sample of 5008 in the nylon-tuft trap. In the sugaun trap, which selected eels of less than 12 cm, 9 specimens out of 260 were less than 8 cm. Three specimens of less than 7 cm were collected; one of 68 mm being the smallest of all.

Monthly length-frequencies for 2 years, 1973 and 1974, and for June 1983 are shown in Fig. 3. In 1973 and 1974, the June and July distributions show a strong positive skew, which was also apparent in August 1974 but not in August 1973. As 1973 progressed, the mode shifted from 8--9 cm in June to 11--13 cm in July. Through August and September, the proportion of eels from 8 to 13 cm decreased, and in October a new mode appeared at 19 cm. In 1974, the distribution in May was flat-topped, with a weak mode at 20 cm. The 8--9-cm group continued to be dominant in July but yielded to the 10 cm group in August. The picture in June 1982 contrasted sharply with that for June 1973 and 1974. It resembled the distribution in May 1974, with its marked absence of small eels.

A summary of the length-frequencies for all samples from the nylon-tuf t trap is given in Table II. Eels of less than 10cm were scarce in May, September and October. They were also scarce in June 1983, when migration began exceptionally late. It appears therefore that larger eels, i.e. those greater than 15 cm, migrated throughout the season, but the smaller ones travelled mainly in June and July, failing to appear even in June in a late season.

48

%

%

20-_

June 1973 10-

,o i

August 1973

5 -

- [~1 ,m~ September 1973

9 10-

I I l l l I W l l l I l l l l I n i l I l l ' l | I 10 20 • 30

~ R

2010_ ~ __J_il11974

10

£ 20-

August 1974 10-

n V V V l , , v v I ~ , , V l , , , , i , , , , I ,

cm 10 20 > 30 cm

Fig. 3. Length-frequencies (to nearest cm downwards) of eels in samples from the nylon- tuft trap.

Frequencies of small elvers (< 120 mm), in length-groups of 2 mm from three samples, are shown in Fig. 4. Individuals of less than 80 mm were scarce, numbering only 30 from a total of 2332. The frequency distributions in all cases showed several weak modes. Unfortunately, in June 1982 very few eels were caught in the nylon-brush trap, and the sample was therefore taken from the sugaun trap, making a strict comparison impossible.

The mean lengths for the three samples were

Mean 8E July 1974 97.2 0.62 mm June 1982 93.5 0.58 August 1982 95.2 0.47

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% -- ~ J u n e

1 o - , , - , , ~ ~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ~ - X % ~ \ \ \ \ \ ~ % ~ - k ~

1 9 8 2

3 0 _ -- ~ i[k August 1982

10 -- ~ ~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ~ \ \ \ \ \ ~

1 0 - - ~ l y 974

I I I I 1~ m m 80 100 120

Fig. 4. Length-frequencies ( to nearest 2 m m downwards) of eels o f less than 120 ram.

Weigh t

Specimens in a sub-sample of 148 eels taken on 24 June were weighed to the nearest 0.1 g. The sub-sample was intentionally biased in favour of specimens of over 12 cm length, since these were relatively scarce in random

50

40

30

20

10

W = . 0 0 0 6 4 2 L 3"22

i i • /

/ /

/ /

/

] i •

/ /

/'

/

/ .

f ; r ] = T ~ I T = ~ , = I i ~ p

c m 10 20 30

Fig. 5. Weight-on-length curve for a sample of 85 eels of length >10 cm collected on 24 June 1974, with a scatter-plot of data for individuals.

51

samples. The scatter diagram in Fig. 5 shows the considerable variation in weight against length. A weight-on-length curve was fi t ted by least-squares for the 85 specimens of length greater than 10 cm. A value of 3.22 was calculated for the condit ion index b in the regression w = al b, where w = weight (g) and l = length (cm). This high value of the condition index in- dicated that girth increased at a relatively greater rate than length.

The calculated weights per length-group were used to make an estimate of the mean weight of eels migrating at each of the times of sampling. These are given in Table I in the form of number of eels kg -1 . The figures clearly show the relatively large size of eels migrating at the beginning and end of each season. Between years, the July figures showed the smallest variation, with a pattern of many small and few large eels being caught. August figures showed great variation, with the mean weight of eels sampled ranging from 3.1 to 12.8 g.

A g e

Age-for-length figures for the sample collected on 24 June 1974 are given in Table III. The age-frequency has no significance, since the sample was biased in favour of large specimens. Ten year-classes were present, and great variations in length with age were apparent. For example, eels of 3 years of age ranged in length from 9.3 to 18.4 cm.

T A B L E III

Leng th - f r equenc i e s accord ing to age group, Pa r t een ny lon -b rush t r ap 24 J u n e 1974

L e n g t h Age (years) Tota l ( m m )

1 2 3 4 5 6 7 8 9 10

7 0 - - 7 9 2 2 8 0 - - 8 9 16 12 28 9 0 - - 9 9 4 25 2 31

1 0 0 - - 1 0 9 7 6 13 1 1 0 - - 1 1 9 6 3 1 10 1 2 0 - - 1 2 9 6 6 1 3 0 - - 1 3 9 1 2 1 4 1 4 0 - - 1 4 9 1 4 1 6 1 5 0 - - 1 9 9 3 6 8 1 1 19 2 0 0 - - 2 4 9 1 5 7 6 1 1 21 2 5 0 - - 2 9 9 4 1 1 6 3 0 0 - - 3 4 9 1 1 2

Tota l 22 51 23 13 14 12 7 4 1 1 148

Mean l eng th 86 97 123 158 194 237 238 245 330 245

SE 1.2 1.4 5.1 6.7 7.2 8.3 3.6 3.4

52

No 0-group elvers were seen. The majori ty of specimens of less than 90 mm were 1 +, but more than one-third of their number were 2 +. Specimens of more than 4 years old nearly all measured more than 14 cm. Approximate length-for-age categories can therefore be drawn for the younger migrating eels.

Length (mm) Age (years) 80 1 + 80--89 1 + or 2 + 90--99 2 +

100--139 3 + 140--199 4 + or 5 + 200--300 5 + to 8 +

The mean length-at-age increased from 1 + to 6 +, but the annual increments were irregular. The figures for the eels of less than 20 cm may be compared with the length-frequencies shown in Fig. 4. The absence of any strong modes in the frequency diagrams is explained by the great overlap in length- for-age. The weak mode at 84--89 mm appears to represent the combinat ion of 1 + and 2 + eels, and the stronger one at 90--96 mm would represent the, 2 + alone.

In 1984, a sample of 12 of the smallest eels was collected on 25 July for age determination. This time, the smallest specimen, length 7.63 cm, was aged 0 +; the remaining 11 specimens measured from 8.52 to 11 .28cm. They were aged 1 + to 3 +, with length relative to age lying within the range of the 1974 sample.

Migration period

The traps were put into operation each year as soon as movement of the young eels was observed. The first and last dates of capture varied widely. Table IV gives the dates of earliest and latest capture and the temperature on those dates. The water temperature ranged from 11 to 22°C during migration, but was usually between 14 and 18°C. Temperatures greater than 18°C were recorded on only four occasions out of a total of 135 observations. Tem- peratures less than 14°C were recorded seven times.

The effect of temperature on the date of beginning of migration was examined by computing the coefficient of correlation in regressions of date of first capture on temperature measured in degree-days. Of 19 regressions, four gave significant values for correlation ( P ( 0.05), while one gave P ~ 0.10. The remainder were non-significant. These comprised total degree- days for each month, April, May and June, the 2-month period April--May, and also for 10-day periods 1--10 May and 21--30 May. Periods of 5, 30 and 60 days before the date of first migration (D 1) in each year and for periods of 6--10 days after 17 May were also non-significant. For the latter, the date

53

TABLE IV

Dates of first and last migrations and temperatures (°C) on given dates. Correlation coefficient (r) for date of first migration on temperature

Year First migration Temperature Last migration Temperature date date

1977 31 May 16 6 October 13 1978 28 May 14 14 September 16 1979 2 June 13 29 July 19 1980 17 May 14 19 September 16 1981 3 June 14 14 September 17 1982 1 June 14 10 September 16 1983 24 June 18 16 September 15

r 0.165

of 17 May was chosen as being the earliest on which migra t ion began in the 7-year per iod.

Da ta for the five per iods giving the highest corre la t ion coeff ic ients are given in Table V. I t appeared tha t warm water in mid-May induced early migra t ion, while cold cond i t ions at this date de layed it. The significant corre la t ions were r eco rded w h e n the high t empera tu res were measured in 5-day periods, the highest value for r being observed for 17- -22 May. An u n e x p e c t e d cor re la t ion be tween low t empera tu re and early migra t ion was observed for t empera tu res in degree-days fo r per iods be tween 10 and 20 days pr ior to first migrat ion.

TABLE V

Total degree-days for the periods indicated, where D 1 is the date of first migration (see Table IV). Correlation coefficient (r) for date of first migration on total degree-days

Date I0 days 20 days l l - -20May 15--19May 17--21May before D1 before D1

1977 150 270 115 60 64 1978 137 250 114 60 64 1979 123 234 108 55 55 1980 121 233 124 66 68 1981 129 249 120 59 59 1982 134 261 122 64 65 1983 167 307 103 53 54

r 0.798 0.854 0.744 0.786 0.800

P ~ 0.05 0.02 0.10 0.05 0.05

54

Numbers of migrants

The quantities of eels caught were first examined on the basis of weekly totals. This showed no apparent pattern, although the possible influence of the lunar cycle was examined. The weights on a twice-monthly basis are shown in Fig. 6. Numbers of migrants calculated from the weight data are given in Table VI. The migrating season ranged in length from 8 to 20 weeks. The longest season, that of 1977, yielded the lowest catch. The highest catch, nearly 2.5 tonnes, was made in 1980, when more than half the total migrated in the first 2 weeks of August. This run of 1483 kg in 2 weeks was greater than the entire catch in any of the other years. However, June was the peak month in 4 years out of the 7, and July in the remaining 2 years.

1 5 0 0 -

kg 1980

5 0 0 -

3 0 0 - 2

1 0 0 -

J J A S J J A S J J A S 5 0 0 -

3 0 0 - 1977 1978 79

-

,oo- 1 M J J A S 0 M J d A S J J M J J A S

Fig. 6. Weight (kg) of eels caught per 15 days.

TABLE VI

Estimate of numbers (thousands) of young eels caught per month (weight × mean of monthly values for numbers kg -1 from Table II)

1977 1978 1979 1980 1981 1982 1983 Total

May 11 26 87 124 June 39 89 143 62 110 117 15 575 July 36 65 55 70 60 119 114 519 August 21 19 191 21 22 16 290 September 4 1 7 2 3 4 21 October 1 1

Total 131 200 198 417 193 261 149 1530

55

No relationship was apparent between temperature and size of catch. The 1980 maximum occurred in the year following the greatest catch of glass eels made down-stream at Ardnacrusha. Regression of catch at Ardnacrusha on catch at Parteen the following year gave a fairly high correlation coefficient of 0.74 (p ~ 0.10).

DISCUSSION

The presence of large eels in the nylon-brush trap indicated that size did not impede ascent of the ladder. The longest eel sampled measured 50 cm. Specimens approaching this in size were regularly observed, and it is likely that longer ones were caught from time to time. It seems reasonable to believe that this trap caught a representative sample of all sizes of the migrants. The sugaun ladder, on the other hand, was almost impassable to eels greater than about 15 cm. In both traps the majority of eels were small, with as many as 70% in a sample being less than 10 cm. The proport ion of specimens of greater than 25 cm was never more than 24%.

On the basis of the age determinations made from one sample of 148 eels, about half the number migrating had spent one or two winters in the lower river, one-third had spent three winters, and the remainder between 3 and 10. A substantial number of the migrants measured more than 45 cm and were therefore definitive females. Females of as little as 45 cm are caught as silver eels on the River Shannon (C. Moriarty, unpublished data, 1982) and there is thus an overlap in length between yellow eels migrating up stream and silver eels on their outward migration. In general, the small eels have a shorter migration period than the large (~ 20 cm) ones.

In 5 of the 7 years 1977--1983, migration began between 28 May and 2 June, apparently without any immediate influence by the temperature, which ranged from 11 to 18°C on the first day of migration. The excep- tionally early date of 17 May occurred in the warmest May in the 7-year period, and the late date of 24 June was in the coldest May. A significant (P ~ 0.05) correlation was observed between date of migration and the total degree-days in mid-May. From this, it seems that the normal time for migration to begin is the end of May. Unusually warm conditions in mid-May can induce early migration (1980) but do not necessarily do so (1982).

From the final column in Table V, it appears that a total of 68 degree- days in a 5-day period are required to induce early migration, but that 60 degree-days may be sufficient at the end of May. In the warmest April, 1981, the highest total degree-days for a 5-day period was 60. It would seem that a warm spell which can induce migration at the end of May cannot do so in April. The negative correlation observed between date of first migration and temperature for the 10--20 preceding days may be explained by the dates in question. When migration is early, a period of 10 days or more before the first date includes low temperatures in April and early May, while a similar period for late migration is influenced by high temperatures in June and late May.

56

The annual catch usually showed relatively little variation; from 644 to 1391 kg, with evenly-spaced intermediate values. The sole exception was that of 1980, which was more than double the mean of the other six. This catch was made the year following an exceptionally large migration of glass eels. It seems possible that in this case the increased migration took place as a result of increased competi t ion for space in the lower river. The exceptional migration also coincided with an unusually high river flow; more than three times the month ly average.

A definite pat tern may be seen in the sizes of migrating eels. In the first month of migration, whether this begins early or late, large eels predominate and small ones are scarce. Small eels appear in numbers in June when migra- t ion begins in May, but remain scarce until July in late seasons. Specimens of less than 15 cm are abundant in July, and usually in August. When migration takes place in September and October, small eels are noticeably few.

Age and length data from Tables I and III show that the great majority of eels (from 63 to 88%) migrating in July were less than 15 cm in length and therefore less than 3 years old. In most years, similarly high proportions of small eels were also observed in June and August. Varying numbers of these (ranging from 10 to 82%) were less than 10 cm and therefore mostly 1 ÷ or 2 ÷. From a sample of 61 specimens of this size taken in June 1974, two were 3 ÷, 37 were 2 ÷ and the remaining 22 were 1 ÷. No 0 ÷ elvers were present. A sample: of 12 of the smallest eels, taken later in the year near the end of July 1984, contained one 0 ÷ elver, the others ranging from 1 ÷ to 3 ÷. It appears from this that the majori ty of migrants are 1 ÷ and 2 ÷ years, and that few elvers a t tempt the ascent of the ladder in the year of their arrival in fresh water. Active movement continues at least up to an age of 10 years, with as many as 24% of the migrants being greater than 25 cm in length and thus older than 6 years.

An estimate of the numbers of eels migrating is given in Table V. Based on mean month ly numbers per kg calculated from the length measurements of the eels sampled, the figures are approximations only. The greatest numbers of eels migrated in June and July, with the exception of August 1980 when an abnormal migration took place. Variations between years were considerable, and the smallest numbers were observed in 1977, the only year in which migration took place over 6 months, from May to October. A late start to the season has no effect in reducing the numbers migrating.

Dahl (1983) provided information on 3 years of studies on the River Guden~ in Denmark, located at a power dam 35 km up-stream from the mouth of the river. The total annual catches varied considerably, increasing from 93 kg in 1980 to 257 kg in 1982. Few, ff any, specimens of more than 23 cm length were seen, but the observation that a higher proport ion of larger eels was present early in the season was repeated. The migrating seasons were similar; from late May to August. Temperatures at the beginning of migration were a little higher in Denmark than in Ireland, ranging from 15 to 17°C, whereas in Ireland migration in 5 out of 7 years began at tem- peratures less than 15°C.

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As in Ireland, the majori ty of eels in Dahl's s tudy were 3 ÷ or less. The oldest specimens which he found in his sample of 450 specimens were two of 8 ÷, measuring 22.0 and 19.5 cm. The earliest date on which he collected specimens for age-determination was 27 June. This sample, numbering 171, contained three of 0 ÷. Samples taken in July and August contained as many as 18% of 0 ÷. It appeared, therefore, that 0 ÷ eels were scarce or absent early in the season, but that substantial numbers of them completed the 35- km journey to the dam in the course of 2 or 3 months. In all cases, however, at least 80% of the migrating eels had spent one winter or more in the river.

Migration of substantial numbers of eels in the river Guden~ ended in August in each of the 3 years of the study. Water-temperature data were only available in 1981 and 1982. In 1982, the decline in the daily eel catch coincided with falling temperature. However in 1981, the temperature remained high, above 15°C, throughout July and August, but the eel catch fell rapidly towards the end of July and was low in August, even when the temperature rose to 20°C. No clear correlations between lunar cycle and migration or between water-flow and migration were observed.

In spite of the fact that the eels of the St. Lawrence are very much larger than those of the Shannon, a similar pat tern may be observed. Liew (1982) showed that the high proport ion of small eels coincided with high tem- peratures in summer, and the high proport ion of large eels with low tem- peratures in spring and autumn. In the Shannon, however, August temperatures (Table I) were generally higher than in July, but the decline in numbers of small eels migrating was still apparent. The change thus appears to be seasonal rather than temperature-related.

The up-stream migration of pigmented eels is therefore seen to be influenced by season, temperature and the age or size of the eels. The migratory season appears to begin usually at the end of May, but will be advanced by exceptionally warm conditions and retarded by cold. Migration normally ends in September or earlier, but may persist to October. The date of the end of the migratory season does not appear to be influenced by the temperature. Large eels, those greater than 15 cm, migrate throughout the season. Small ones seldom begin to travel earlier than June, and relatively few of them migrate after the end of July. The great majority of the small eels had spent at least one winter in fresh water and no newly-metamorphosed elvers were observed earlier than July. Elvers which had entered fresh water in the same year (age 0 ÷) were few in number and were not seen earlier than July.

The wide range of ages and sizes of eels ascending the ladders shows that, for the majori ty of the eel populat ion of the lower Shannon, migration up- stream ceases shortly after entry to fresh water from the estuary. No firm conclusions on the cause of subsequent migration may be drawn in the absence of information on the eel populat ion density in the lower river. It seems reasonable to suppose that the later migration is a response by some individuals to increasing pressure brought about both by growth of the

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members of the resident populat ion and by increase in numbers following immigration of younger individuals f rom the sea.

ACKNOWLEDGEMENTS

I am most grateful to members of the staff of the Electricity Supply Board for their assistance, in particular to Noel Roycro f t and Patrick Barry for data on water temperature, to Edward Nolan for advice on river flow, and to Michael Fitzpatrick for supplying the details of eel catches. I also wish to pay tr ibute to the memory of the late Donal O'Leary, who developed the trapping system and who gave valuable advice and encourage- ment in the preparation of this paper.

REFERENCES

Dahl, J., 1983. Some observations on the ascent of young eels at the Tange Power dam, river Guden~. EIFAC Working Party on Eel, Drottningholm, mimeo., 11 pp.

Deelder, C.L., 1970. Synopsis of biological data on the eel Anguilla anguiUa (Linnaeus) 1758. FAO Fisheries Synopsis 80, 65 pp.

Liew, P.K.L., 1982. Impact of the eel ladder on the upstream migrating eel (Anguilla rostrata) populat ion in the St. Lawrence River at Cornwall: 1974--1978. In: K.H. Loftus (Editor), Proc. 1980 North American Eel Conference, Ontario Fish. Tech. Rep. Set. 4, pp. 17--22.

Moriarty, C., 1983. A study of ascending eels in the river Shannon, Ireland. EIFAC Working Party on Eel, Drottningholm, mimeo., 13 pp.

O'Leary, D., 1971. A low-head elver trap developed for use in Irish rivers. EIFAC Technical Paper 14, pp. 129--133.

Tesch, F.W., 1977. The Eel. Chapman and Hall, London, 434 pp.