3. PANEL REPORTS AND SUMMARIES

3.1 Panel 1 - Appraisal of present status of eel fisheries

(a) Trends in catches

Official statistics for eel underestimate the amount taken in all countries. Freshwater statistics are generally lacking or are very incomplete. Many fishermen do not report their catch or do not sell via public auctions and only part of the catch is therefore recorded.

The following statistics are subject to the above-mentioned reservations. Where no catch data can be quoted from the papers presented to the Symposium, the FAO Yearbook of Fishery Statistics has been used for comparing the fisheries in the different countries. Data for the last 15 years for the total area where Anguilla anguilla are caught show some fluctuations in catch and a slight tendency for the total amount caught to decrease over the last three years.

Examination of the data for individual areas indicates that there have been greater fluctuations than are indicated by the total catches. In the Baltic, Swedish catches have been declining since 1964. The catches of Poland, the German Democratic Republic and the Federal Republic of Germany have also decreased in the area since 1969. Danish Baltic catches, however, show no decrease and, according to Danish official statistics, have been relatively constant over the last 15 years.

From the ICES Working Group Report (Paper No. 53), it may be seen that the Swedish c.p.u.e. fluctuates, but this seems to be more an effect of variation in total catches than in effort. The Danish c.p.u.e. has been decreasing for at least 30 years. This is due to a steady increase in effort and is probably the reason for the constant Danish catch figures in the area. No further effort data are available.

Svärdson (Paper No. 28) confirms that eel catches in the Baltic have been decreasing since the early 1960s. Swedish silver eel catches which were about 1 600 t/yr at that time are now only 800 t/yr. Catch per unit effort has also declined. The same trend is true of the Danish silver eel fisheries in the Baltic. He claims that this reduction of catch is due to a decrease in the numbers of elvers and small eels entering the Baltic with a resulting diminution in the stock. Statistics from elver collecting stations on the Swedish coast show a decrease of more than 50 percent since the 1950s. According to Svärdson this decline had already started in the 1930s, and the ICES Working Group Report (Paper No. 53) shows the same trend for the annual catches of Swedish elvers and small eels over at least 20 years. Renström (Paper No. 51) finds a relation between wind direction and eel catches in the Hanö Bight on the east coast of Sweden. If this is generally true, the decrease in the frequency of westerlies, which has taken place during the last few decades and reached a minimum in the 1960s, should also diminish catches.

The drift of elvers that passes north of Scotland and part of which goes into the Skagerrak must be the same drift that reaches the Norwegian coast. Norwegian catches of eel are small and according to Gundersen (Paper No. 40) it is not possible to say whether fluctuations in catches are due to changes in stock density or fishing effort.

The catch from the eel fishery of the Danish region of the North Sea is low (500 to 600 t/yr) and has not changed much in the last 15 years. The Dutch fishery on the contrary has shown a pronounced decrease since 1969 according to official statistics.

The North Sea fishery of the Federal Republic of Germany has increased considerably since 1964 when a trawl fishery was started. This now comprises 30 to 40 percent of the total German North Sea catch. Unfortunately, Aker and Koops (Paper No. 1) have no effort data for the total period, but some data quoted in kg/h from experimental catches in 1973, 1974 and 1975, seem to indicate a decreasing c.p.u.e.

According to FAO Statistics there was no French Atlantic fishery for eel before 1971, but eel catches have been increasing in this area since that year. Spanish catches of eel on the Atlantic coast have also increased in recent years. In Portugal eel fishing is carried out mainly in the estuary of the major rivers and rice culture irrigation channels. Elvers are caught mainly in the northern part of the country. In the Mediterranean French, Spanish and Italian catches have shown a decrease over the last few years (FAO Statistics).

There are considerable differences between eel fisheries in the Republic of Ireland and England and Wales. Most strikingly, more information is available on the Irish eel fisheries than those of Great Britain even though their mean catch is only of the order of 137 tons, which is about half of the estimated England and Wales catch (for comparison the Northern Ireland catch is about 630 tons per year).

These figures should be treated with caution (Moriarty, Paper No. 20, and Stott, Paper No. 21) for they are known to be inexact, and no reliable catch data are available. However, it does seem that catches have declined due to economic forces in England and Wales and because of the effect of hydro-electric dams in the Republic. This latter led to the adoption of an elver stocking programme in Ireland.

In England and Wales the abundance of elvers seems satisfactory for stock maintenance and an increase in the exploitation of silver eels would seem to be both possible and desirable in view of the apparent decline in catches in mainland Europe. At present, the British Isles probably contribute a considerable proportion of the eel spawning stock.

(b) Composition of catches

Aker and Koops (Paper No. 1) give the length distribution for experimental catches in 1974 and 1975 from the German Bight, and Saint-Paul (Paper No. 58) gives the length distribution of young eels fished for stocking purposes in the estuaries of the North Sea.

In the ICES Working Group Report (Paper No. 53) an attempt has been made to determine the stock size and fishing mortality of a population. As the analysis is based on only one sample originating from the German trawl fishery, it cannot be considered reliable, and the results should be taken with some reservation. The calculations do indicate one possible way to determine stock size and fishing mortality if adequate data are available.

(c) Production and productivity

Assessments in Normandy rivers using Petersen's method indicate standing stocks of between 1 500 and 2 000 kg/ha. Large stocks also exist in Algeria where in some confined areas several tons per hectare were present. This patchy distribution of eels indicates their gregarious nature. In terms of production, the Etang de Berre (France) produced more than 100 kg/ha for several years, but this stock has since been exhausted. Productivity estimates are difficult as not only must the area of the actual water body be taken into account but also that of the whole drainage basin. It would appear that eels concentrate in certain areas of the system during their seasonal migrations.

(d) Stocking

Very little or no stocking takes place in coastal and estuarine areas in Europe, and only Leopold (Paper No. 14) and Rossi and Colombo (Paper No. 63) mention stocking in the estuarine areas of Poland and Italy respectively.

In Italy the brackish lagoons of the north western Adriatic are stocked. Stocking with both elvers and small eels (10–20 g) is very irregular, the stocking material originating from the Thyrrenian Sea in the east to the French Atlantic coast in the west. The Italian lagoons give high productions of 30 to 40 kg eels per ha.

The inland waters of most countries are stocked with elvers and small eels to a certain degree, because the natural ascent has been obstructed in many ways, for instance by power stations or pollution. Only very few countries, however, seem to be investigating the impact of their stocking programmes.

Although the eel fishery in Ireland is small, a lot of investigations have been made. Moriarty (Paper No. 20) has given catch and effort data for a period of more than a hundred years. Over the last 15 years the number of fixed gear has increased and the number of longlines has decreased. Total catches have decreased. A more thorough investigation of two areas with very different conditions has been described. The first area is estuarine and contains the highest density of eels in Irish waters. Nevertheless, one year of netting is able to exhaust the stock which only needs five years to recover. In the second area, a river and lake district, the construction of a power station in 1928 blocked the ascent of elvers, but a real decrease in the fisheries did not show up until approximately 20 years later. Late in the 1950s active stocking of the closed areas began and has continued ever since, and now about 15 years later, the catches have started to increase again.

Deufel and Strubelt (Paper No. 5) give results obtained in the adjoining upper and lower Lake Constance in southern Germany. From 1953 onwards regular stocking with elvers has taken place in both lakes with increasing intensity. Initially the quantity stocked was only about 5 g of elvers per ha, but more recently stocking has been at about 30 to 40 g of elvers per ha. From the beginning of the 1960s catches rose from about 0.6 kg/ha/yr to about 2.2 kg/ha. According to the authors, this increase is due to both stocking and increasing fishing effort but unfortunately they give no data for either effort or c.p.u.e.

Heermann (Paper No. 8) describes the eel fisheries of the Federal Republic of Germany both as a whole and by district. These statistics are only available for 1961 and 1971. In the Federal Republic of Germany both the total water area and the total catch of eel measured by weight have decreased by 30 percent, and the average yield of 3 kg/ha remains unchanged. Considered separately, the area of rivers has decreased by 51 percent but the eel catches have only gone down 39 percent, which gives an increase in the catch per ha from 3.6 to 4.5 kg/ha; lake areas have decreased only 9 percent but eel catches have gone down 19 percent giving a decrease from 2.7 to 2.4 kg/ha. This apparent decline may be illusory as the numbers of sportfishermen have increased considerably during the same decade and their catches are not registered. Most of the areas seem to have been stocked, but data are only available for the inland waters of Schleswig-Holstein. Figures for young eels which are preferred for stocking are given from 1952 to 1975. Initially only 5 000–6 000 kg were stocked per year but the numbers stocked have increased steadily up to 25 000–28 000 kg in the last three years. Despite these stockings, catches have decreased by 31 percent between 1961 and 1971 although the water area has only decreased by ten percent.

According to Leopold (Paper Nos. 13, 14, 15, 17) management in Poland has been carried out since the latter half of the 1940s and now includes about 2 000 lakes covering about 300 000 ha. Stocking started on a small scale in 1948 and has continued until today with a fluctuating, but still increasing, trend. Catches, which are only given from 1951, remain rather constant during the 1950s, but the catches have doubled since the 1960s. It is anticipated that the increase could last for some years more. The coastal and western districts of Poland have an ascent of elvers or small eels but are also stocked, while the eastern districts are entirely dependent on stocking.

(e) Assessing the stocks and demand for eels

Serene (Paper No. 25) predicts that the demand for eels for consumption in Europe in 1985 will be about 30 000 tons per year. If we are going to fulfill this demand, investigations of exploitation and assessment of our eel stocks are necessary. Until now, very little has been done in this field and although so many interesting papers have been submitted to the Symposium, they do not supply the data necessary for a more rational exploitation of the eel stocks.

We therefore need to improve the catch statistics. It is particularly necessary to find a way of expressing effort, though this is one of the most difficult tasks. Many different kinds of fishing gear are involved in eel fisheries and these are often especially adapted to local conditions.

Because data on catch composition are few and scattered and the procedures for ageing eels are also uncertain, data on natural and fishing mortality are lacking. In fact, we do not know if we are under or over-exploiting the available stocks of eel at the present time. On the other hand, even if there are abundant and detailed statistics, as is the case with Poland, it is very difficult to assess the state of eel fisheries in fresh waters.

(f) Points arising from discussion

A general lack of good catch and other statistical data was apparent but no simple solution to this problem could be suggested.

Reports from northern Europe and the Baltic area suggest a decrease in the abundance of elvers but was not seen to be the case in The Netherlands, France, the British Isles and the Mediterranean area. No simple reason for the decline was given but changes in climate may be of major importance.

The artificial stocking of elvers was discussed and it was agreed that stocking could be a valuable management technique but careful analysis of data was necessary to establish the effect of stocking.

Also mortality rates were discussed; it was suggested that the rates for male and female should be determined separately.

Minimum legal takable sizes for each in various countries were reported by participants in:

3.2 Panel 2 - Measures for the improvement and the maintenance of eel fisheries

(a) Improvement and management of eel stocks in inland waters

In recent years increases in eel catches have come mainly from fresh waters, particularly lakes. The continuing existence of eel fisheries on inland waters is almost totally dependent on human activity. Potentially the whole cycle of the eel in fresh water is controllable, although this is not actually done at the moment. Basically, present-day eel management consists of stocking (see Panel 1 for discussion) and catching, two processes which in themselves are only slightly understood. The intervening interval, when the eel is in the natural water body, remain virtually unstudied. The greatest possibilities for the improvement of eel fisheries are probably associated with an understanding of the processes going on in this period and their eventual manipulation to secure greater production. In Poland, comprehensive studies are being carried out on the management of lakes. Data are available from about 600 lakes over the last 20 years and results of the preliminary analysis of these are given by Leopold (Paper Nos. 13 to 17). Such studies support the view that the greatest improvement in catch can be achieved by human intervention during the productional phase between stocking and catching. Catches are normally well correlated with stocking density, exploitation rate and a roughly assessed lake fertility. There are, however, groups of lakes in which eel catches are either much lower or much higher than one could expect from such relationships. Hopefully, multifactorial analysis of the various individual conditions in such lakes will indicate the management strategies appropriate to such water bodies and their stocks.

(b) Electric fishing

The use of electricity for eel fishing is described by Lamarque (Paper No. 9), Dembinski and Swerzowski (Paper No. 34), Dembinski et al. (Paper No. 35) and Swerzowski (Paper No. 46). Traditional electric fishing methods or the electrification of standard fishing gears such as trawls and seines are recommended for capturing eels. Specific selectivity for eels is high; they comprise 51 percent of the total catch by traditional electric fishing methods and 45 percent of the catch in electrified trawls. There is a seasonal variation in c.p.u.e. and the hourly catch rate is maximal in spring for traditional electric fishing gear (electrified dip nets) and in autumn for electrified trawls. There is also a correlation between fishing efficiency and the size of the water body fished. Efficiency is maximal in lakes of less than 100 ha surface area. Catches using electrified gear are sufficient to ensure the rapid amortization of the purchase price of the equipment and the increase in the quantities caught by electrification of the gear allows the fishing season to be shortened.

In Polish inland lakes the recommended mesh size for electrified seines is 20 mm, corresponding to a legal size of eel of 50 cm. In coastal lakes, the high proportion of males has resulted in a lowering of the legal size to 40 cm. Fish of this length are captured by 12 mm meshes.

Studies in France indicate that for a power source of 3'kW the following types of current are appropriate.

A for classic electric fishing gear

-   constant, continous current for waters with a conductivity of less than 800 μs/cm

-   constant current in wave form in waters with conductivities of between 800 and 1 900 μs/cm

-   rectangular impulses at a frequency of 400 Hz in waters with conductivities in excess of 1 900 μs/cm (including sea water)

B for electrified trawls

-   rectangular impulses at a frequency of 100 Hz where waters are very conductive and it is necessary to economize on energy

In France, a galvanonarcotic basin using constant continuous current has been developed for measuring and weighing eels. This can replace narcotics satisfactorily, is rapid to use and is without danger either to the eel or to the operator.

(c) Eel diseases

Disease is obviously significant in the cultivation of eels; for example, Japan produced about 23 000 t in 1968 to help to meet a consumption of about 27 000 t. Production fell in 1970 due to shortage of elvers and 200 t of European eel elvers were imported for culture. Subsequently, the development of a gill and kidney disease caused losses of 600 t. “Cauliflower disease”, a skin tumor in the mouth area, has been estimated to result in a 5 percent loss in catch value in the River Elbe. The disease develops rapidly at higher temperatures and stagnates or even regresses below 8°C. The cause of the disease is unknown but pollution is not blamed.

The so-called “red disease” is also important in the German Bight with an incidence of 15–20 percent. The disease can be controlled when it occurs in fish farms.

(d) Points arising from discussion

Poor statistics are hampering the assessment of management policies with, perhaps, the exception of the Poles whose state eel fisheries appear to have adequate date. Possible improvements were suggested including a change in the national reporting which, if made in a uniform and regular way, might result in better collection from the fishermen. An example of such improvement came from the setting-up of a working Group on the Atlantic salmon. Future improvements should include more information on catches/ha, mortality figures for both sexes separately, and the age or size. Composition of catches should also be given. A proposal to examine the efficiency of elver stocking in virgin lakes was thought to be of limited value from the point of looking at improvement in sustained yields.

Mention was made of a well illustrated Japanese reference work on eel diseases and which may, possibly, be translated and published in Europe. The idea of issuing identification sheets for diseases similar to those issued by FAO for species was also advanced.

The spread of disease through trade in live eels was mentioned, particularly in connexion with live eels coming from New Zealand, some of which had, allegedly, been released in European waters.

Electro-narcosis was thought to be valuable for measuring and even weighing eels as an alternative to the use of anaesthetics. An electric fishing technique suitable for sampling in shallow marine situations was described an electrified marine trawl is being developed. The competition of eels with other valuable resources, notably crayfish and lobsters, was raised but little information was available on the subject.

Marking methods for eels was clearly a problem. Floy tags, internal tags with or without external streamers, magnetic tags, the injection of sub-cutaneous dyes and silver nitrate branding have all been tried, but there is a need for development work.

3.3 Panel 3 - Age and growth under natural and artificial conditions

(a) Age

The determination of the age and growth of eels needs to be approached with great caution (Charlon, Paper No. 4, and Deelder, Paper No. 18). Charlon has dealt with this problem in quite a new way. Starting with pond-reared elvers the otolith development has been been studied in subsequent periods. The results obtained prompted the author to conclude that she could observe the appearance of supernumary rings, some of which were well defined under the microscope. The process of age determination by simply tracing the rings is, therefore, by no means absolute, which explains the disparity in results obtained with relative simple methods usually employed by fishery biologists.

Deelder refers to previous communications about pond-reared eels, which contained otoliths with 1–3 bands per year. Also an instance is given where there were different numbers of bands in the two halves of the otoliths.

The warning about correct age determination expressed in these two papers, and already mentioned during the meeting of the EIFAC Workshop on Age Determination of Eels (Paper No. 56) is emphasized by Moriarty and Steinmetz (Paper No. 23) who say that the reading of eel otoliths of expected known age by four research workers did not result in a general agreement about the average age of the samples. Rasmussen and Therkildsen (Paper No. 43) and Boetius (Paper No. 53) indicate similar findings. From these papers a rather embarrassing comparison can be made: Rasmussen and Therkildsen state that a Bertalanffy growth curve fitted their data very well, but Boetius found either that it was impossible to calculate Bertalanffy parameters or that Bertalanffy parameters were meaningless.

The age-determination problem is a serious one and its solution will be difficult. Experiments with animals of known age are in this respect suspect, as the problem is not to determine an age already known but to study animals of unknown ages, a task of a completely different nature.

(b) Summary of discussion on methods of age determination

Several different methods are used and until now ageing has involved considerable subjectivity. Ageing of otoliths from eels of known age, for instance, has resulted in differences of interpretation by several investigators using the same material. The scanning method may reduce personal error as the graphs do give objective criteria which allow comparison and mathematical treatment. However, such graphs still leave room for individual interpretation as up to three growth periods may appear in one year. These are possibly correlated with temperature fluctuations or with the amount of food available. Crystalline structures are identical for real year rings and for secondary rings. Further basic studies should be carried out on otolith development to identify changes induced by environmental factors (for example temperature) or by nutritional changes on their fine structure. Studies using electronic microscopes allow identification of rings laid down daily and by other short period fluctuations. Otoliths from eels of known age are in fact urgently needed to improve the interpretation of the graphs from scanning and to cross-check different methods used in ageing Records of physical, chemical and biological conditions are also required.

(c) Growth under natural conditions

Data on the natural growth of eels are given by Moriarty (Paper No. 21) for the Shannon River in Ireland, and by Rasmussen and Therkildsen (Paper No. 43) for a Danish brook. Growth under brackish-water conditions in the lagoons near Comacchio (Italy) is discussed by Rossi and Colombo (Paper No. 63). Ciepielewski (Paper No. 33) refers to the age of migrating silver eels descending from the Masurian lakes, and also gives data on the size, weight and sex composition in comparison with eels from other areas.

(d) Summary of discussions on growth under natural conditions

Growth studies depend largely on the reliability of ageing. Males may turn silver 2–3 years earlier than females. The percentage of females increases with the distance from the sea and the most distant inland waters hold females only. However, nearly all eels in the German Bight and off the Dutch coast are females. The time for the metamorphosis to silver eels is more related to size than to age. The size of female silver eels increases with distance from the sea. Thus, for example, silver eels in northern Scandinavia are considerably larger than those from southern areas. Possibly a higher effort needed for migration is an explanation for the higher energy store laid down by these fish. The age of silver eels depends on water conditions and may vary between 4 and 5 years in some Mediterranean lakes and between 12 and 18 years in some Irish and Norwegian rivers.

(e) Growth under artificial conditions

A group of papers by Bienarz et al. (Paper No. 2). Descamps et al. (Paper No. 6), Koops (Paper No. 11), and Kuhlmann (Paper No. 12) produces a vast amount of data and gives a fair idea of what may be expected when eels are raised under artificial conditions. The influence of various types of food, vitamins, stock density, water temperature, growth differentiation, sexual manipulation and the origin of elvers has been examined. The combined body of data provides a valuable supplement to the knowledge already existing in this field. However, several important problems remain to be solved. These include the mortality due to lack of initial food intake, differences in individual growth rates and how to obtain satisfactory growth rates at moderate water temperatures.

(f) Summary of discussions on growth under artificial conditions

Eel culture is increasing in Europe and commercially operated farming has been started in Italy and southern France. The use of thermal effluents is expected to contribute to the rapid progress of eel farming in the next few years. Feeding of glass eels for the first weeks should be done using natural wet food such as ground spleen or liver. Thereafter, chopped fish or industrially manufactured dry feed can be fed. Knowledge of the nutritional requirements of the European eel is limited. The water-soluble vitamins (vitamin C, vitamin B complex) seem more important than the fat-soluble vitamins, although vitamins A and D seem to be essential. Optimum growth rate is related to an optimum stocking density, low stocking gives poor results. Agressive behaviour which occurs at low density can be depressed by stocking greater numbers. Feeding activity also increases under these circumstances. Pond or tank cultured eels show large differences in growth and only about 20 percent are fast growing. Grading the eels improves results. The optimum temperature is reported to be 26°C for elvers and about 22°C for bigger eels, but further studies seem to be necessary. Growth improves 3–4 times in constantly warm water compared with natural waters where temperatures fluctuate. Artificial feeding increases the percentage of males in both heated water and natural waters where temperatures fluctuate. Artificial feeding increases the percentage of males in both heated water and natural waters with temperature fluctuating. Both temperature and the type of food seem to influence the sex ratio. Glass eels may be fed in order to get better stocking material with respect to the time of release, size and mortality. The economics of commercial farming of marketable eels for consumption need more study.

3.4 Panel 4 - Migration (all stages) and reproductive phase

(a) Migration

Lindquist (Paper No. 48) offers some views on the possible mechanism of migration of elvers. It is suggested that elvers coming from the North Sea are carried into the Baltic by the Jutland current. This suggestion is especially important in interpreting the decrease of the Baltic eel stocks described by Svärdson (Paper No. 28).

Tagging is a valuable method for tracking migrating eels. Gundersen (Paper No. 40) used tags which last longer than those previously used. Krüger (Paper No. 49) proposes a method for labelling eels with radionucleids which lasts up to 140 days and allows the marking of small eels down to a size of 20 cm in length. Tesch (Paper No. 29), Westin and Nyman (Paper No. 31) and Westerberg (Paper No. 52) all demonstrate that tagging of eel with ultrasonic transmitters for tracking purposes is easily possible.

Results of experiments on silvel eel migrations are available for the Baltic based both on conventional tagging as well as tracking experiments. In addition, Renström (Paper No. 51) presents results of coastal silver eel catches in relation to wind and current which might give some indication of the migratory behaviour of the eel. Lindroth (Paper No. 10) analysed recapture data of tagged silver eel as related to moon phase. He found that eels released before full moon moved over a smaller distance than specimens liberated after full moon. Tesch's (Paper No. 29) tracking results are also compared with moon phase and also give information on directional preference and speed of migration in the Baltic and other areas. These confirm that in the North Sea there is a tendency for a westerly to northerly course in all areas of the North-East Atlantic shelf. Similar tracking results were presented by Westerberg (Paper No. 52) who presumed that the current might give cues for orientation. There is now a hint that temperature gradient in the Baltic might also provide a signpost for orientation (Nyman, Paper No. 31).

(b) Activity

Catch statistics - In paper No. 10, Lindroth reported on eels catch and lunar cycles on the Swedish east coast. Silver eels of unknown sex and length were tagged conventionally and released. Recaptures showed that catches varied with the phase of the moon, being highest between the third and fourth quarter and lowest at full moon. Three interesting hypotheses explaining the phenomenon are presented. The first is that eels would be more apt to avoid nets. This implies that eels undergo sensory system improvement or achieve thresholds of stimuli. This is not likely. The second hypothesis suggests eels after their route, choosing deeper water or a more direct path. This is possible, but experimentally unproven. The third hypothesis is that eels stop migrating (or slow down) around the time of the full moon. This implies chronometric or orientation change and is the hypothesis most strongly confirmed by the data.

Swierzowski (Paper No. 45) reported that activity of silver eels as deduced from catch statistics is highest on darker nights usually between the fourth and first quarters of the lunar cycle. Additionally, catches were effected by air temperature and water level with the former being most significant during a large spring migration of silver eels out of the Narew River drainage basin. Over many years, the spring peak of migration fell between 5 and 10 May when water temperatures reached an average of 15°C. Finally, the author found the migration intensity to be a function of the population of eels at a particular site.

A decline of silver eel catches since 1959 has caused fishermen to blame the paper mill built in the area. Nine to 14 years catch data from 9 sites showed that catches were related to wind direction (Renström, Paper No. 51). Winds from the west through the northeast resulted in poorer catches. Wind from other directions gave good catches. The number of days per season of “bad winds” have increased over recent years. When the wind direction was good, wind speeds between six and 13 m/second gave the best catches. No correlation between current direction and catches was shown. No effect of the paper mill was shown.

Laboratory investigations - Edel (Paper No. 7) - reported that manipulation of shelter affected the locomotor activity patterns of silver American eels held under controlled light-dark conditions. Eels were always more active in darkness than in light. Eels were easily entrained to light-dark cycles of different periods. Eels in constant light or darkness showed indication of a circadian rhythm. The removal of shelter caused the levels of activity to increase independently from the light-dark cycle. A conservation of energy was suggested by aquaculturists supplying eels with shelter and keeping them in constant light.

Sixty gramme yellow eels remained dark active even after blinding and pinealectomy or a combination of treatments (Van Veen et al., Paper No. 30). Covering the head of blinded, pinealectomized eels with opaque aluminium foil caused the dark active rhythm to disappear. Control eels with heads covered with transparent material remained dark active. The authors concluded that an extra-optical-photoreceptor in the brain of eels is possible.

According to Westin and Nyman (Paper No. 31), both yellow and silver eels are nocturnally active but the time of the highest activity peak varied with the season. In summer the highest peak was around midnight; in autumn the peak was highest at dusk and a second peak occurred at dawn. Silver eels remained more active in autumn than yellow eels under identical temperature conditions. Silver eels were highly active at full moon and most active at third quarter, results which contradict with most other workers. Exceptionally interesting was the activity response of silver eels to experimentally manipulated temperatures. When temperature was rhythmically changed silver eels would show a burst of high activity as temperature began to decline. This suggests temperature may be a cue triggering the emigration of silver eels. Yellow eels responded with activity parallelling temperature; therefore a decline in temperature caused a decline in activity. The authors emphasize the importance of interactions between temperature, moon phase and season.

(c) Metabolism

Holmberg and Saunders (Paper No. 50) have studied the oxygen consumption of yellow eels and silver eels at different swimming speeds. Differences in respiratory rates between yellow and silver eels were insignificant, but in yellow eels an increasing amount of lactate was demonstrated. In silver eels, no accumulation of such an oxygen debt was found, and this was taken by the authors as an indication of the silver eels' better adaptation for long distance swimming.

The silver eel needs energy for both swimming activity and spawning. In paper No. 55 (Boetius and Boetius) the establishment of an energy budget for migration and spawning is attempted. Estimates of standard oxygen consumption of silver eels were made. It was calculated that the amount of energy available for migration would equal about 5–6 times the standard metabolism over 150 days, the time an eel will possibly take for the migration.

The respiratory rate of an eel swimming at a constant speed of about 1.5 km/h had an activity rate which was about three times above the standard, a result very similar to that stated by Holmberg and Saunders.

Thus the silver eel has an energy reserve sufficient to cover both the long distance migration and spawning.

(d) Reproduction

Passakas (Paper No. 60) described the morphology of female and male eel chromosomes on the basis of gill epithelium cells. She identified sex chromosomes and quoted earlier results which gave rise to the assumption that A. anguilla and A. rostrata differ in the morphology of their chromosomes. A paper by Kuhlmann (No. 12) presents results on sex relationships under experimental conditions such as temperature and food; although the origin of the elvers used for experiments is also of importance. Feeding experiments of Descamps et al. (Paper No. 6) showed that eels cultivated under normal water conditions gave rise to fewer females than eels raised in heated water basins. Saint-Paul (Paper No. 58) found that the populations of small eels in North Sea estuaries exhibited a great percentage of undifferentiated specimens and populations in the open sea mostly differentiated females. From the figures of Ciepielewski (Paper No. 33) it is obvious that in the Mazurian Lakes seaward-migrating silver eel females are more numerous than males. This is also true in Adriatic lagoons where the percentage of females is even greater as found by Rossi and Colombo (Paper No. 63). Svärdson (Paper No. 28) notes that on the Swedish coast a change of the sex distribution took place during the last century, there being more females now than in 1890.

(e) Maturation and fecundity

Mature males and females have now been produced in all three northern temperate Anguilla species and Todd (Paper No. 47) adds the New Zealand species: Anguilla australis and A. diffenbachii. Todd treated his female eels of both species with i) MCG + oestradiol, and ii) with carp pituitaries. He got remarkably high GSI-values (up to 44 in A. australis) by his MCG experiments and obtained spawning specimens by carp pituitary treatment in both species.

Fertilization attempts, however, failed. Todd ascribes this to the circumstance, that the mobility of sperm cells in MCG-treated males was very low.

Boetius and Boetius (Paper No. 55) agree with this conclusion, and only a few months ago achieved their best result to date. Female silver eels treated in February with carp pituitary and MCG in combination became strip-ripe in about 40 days in seawater at 23°C. The maximum GSI amounted to about 46. Had the eels not been stripped, they would have spawned in the tank.

Females treated with carp pituitary only arrived at GSI values of about 44, but did not turn strip-ripe. Bienarz et al. (Paper No. 2) also obtained their best results with CP + MCG.

Males were ripened by combinations of carp pituitary and MCG as well, but the sperm motibility was extremely low. Fertilization, however, was obtained in two cases.

3.5 Panel 5 - Conclusions and recommendations

Concern has been expressed about the state of eel stocks particularly in the Baltic. While eels are commercially important at present, their importance can be anticipated to increase further in the future. International bodies and individual governments will, therefore, concern themselves increasingly with the state of the stocks and the management of the fisheries. There are two approaches. Firstly stocking, to the effect of which no satisfactory conclusions have been drawn at this Symposium and secondly the regulation of the natural resources, for which methodologies have to be developed.

(a) Growth

(i) Ageing techniques

The ageing of fish is an important prerequisite to most methods of stock assessment. Two aspects have emerged as promising during the Symposium, namely (i) the comparative reading of otoliths by different techniques and (ii) the analysis of structures in otoliths of known age. In view of the extreme importance of the development of appropriate ageing techniques, it is recommended that the EIFAC and ICES Working Group on Eels be asked to continue their work and to cooperate on the improvement of ageing technique by:

-   Comparative reading of identical otoliths of known age by different techniques;

-   Analysis of otolith structure development from otoliths of known age.

The results of the Working Group should be submitted to council meetings of EIFAC and ICES.

(ii) Age at first capture and age at spawning migration

It is not possible to generally define the age at first capture or age at spawning migration. Length is therefore used in the meantime and a size of silver eels at migration of 55–60 cm appears in many reports. This figure is considered as of the right order for many countries but in some regions eels probably migrate at smaller sizes.

(iii) Length-weight relationship

Two models of length-weight relationship are commonly used: where both k and d are computed, or where a simple fraction of L³ is assumed. It was generally felt that the first type of model was more useful and more accurately expressed the type of growth of eels.

(iv) Shape of the growth curve

Two basic growth models have been applied to eels. The Ricker type equation, in which growth is always a multiple of a starting weight, is particularly applicable to the data that are available at present. The von Bertalanffy type of equation, in which the weight is always a fraction of an estimated final weight has not so far been generally applicable.

(b) Fishery

(i) Management units

European eels are usually assumed to be one stock and all genetic physiological data so far support this. It is however useful for a single stock covering such a wide geographic and climatic area to be subdivided into management units. On the basis of the information available, the Symposium tentatively identified the following management units:

-   The Baltic

-   The North Sea

-   The Atlantic coast from the English Channel to Gibraltar

-   The Mediterranean,

no further breakdown for the other areas of distribution being possible at present.

Within management units the area should be considered as extending inland from the coast to include other catchment areas where eels are found.

(ii) Abundance

Catch per unit effort is considered as a possible measure of population density in eels, but little information exists in overall abundance.

(iii) Yield

Data on eel landings are inadequate and are urgently needed for assessment of the stock.

(iv) Overfishing of yellow eels

There are indications of declining catches in some areas, for instance in the Baltic management unit, but this can at present not be attributed to overfishing. The Symposium identified economic overfishing (where any increases in catch with increasing F values are not paid for by the value of the extra fish caught).

(v) Recruitment

Yield assessments do not consider any stock recruitment relationship. Monitoring of the recruitment is therefore vital and should be done by sampling elver runs to the extent that an assessment of the total run in relative or absolute terms is possible.

(vi) Conclusions

Data for the assessment of European eel stocks are at present insufficient to permit any analysis of their state of exploitation. Considering that such an analysis is essential for all of the management units identified, the Symposium recommends that the following data be made available to both the EIFAC and ICES Working Groups:

-   Age and length composition of landings

-   Length and weight at age

-   Catch and effort statistics for yellow and silver eels separately from both marine and fresh waters

-   Catch and effort statistics for elvers and young eels.

In view of the urgent need for the assessment of stocks, such work should proceed with the data available. However, a need was identified for improved methods for the collection of statistical and biological data and it was further recommended that member countries should intensify their studies on how to collect reliable data particularly to quantify the ascent of elvers in order to supply the Working Groups with suitable material.

(c) Migration and reproduction

Insufficient information exists on actual elver potential, and it was suggested that a survey of known existing concentration of elvers be made.

Furthermore, noting that the present knowledge of the spawning places of the European eel is still incomplete, the Symposium recommends to ICES and EIFAC that they encourage the organization of an international expedition to the supposed spawning areas of the European eel and requests Dr. Tesch, in collaboration with interested colleagues, to present a detailed proposal to the next Working Party meetings of ICES and EIFAC.

(d) Publication of papers

A large body of valuable information was presented to the Symposium in about 60 papers. This information should appear in a more formal publication and it was recommended that ICES and EIFAC be asked to negotiate on the publication of selected papers submitted to the Symposium on Eel Research and Management.