Institute of Freshwater Research
The drastic decline in catches of silver eel and in upstream-migrating yellow eel have stressed the need for restocking Swedish waters.
Traditionally, relatively large yellow eel (10–100 g each) caught in the sea or during their migration in rivers are used for stocking. Since 1976 elvers have been imported from France and stocked mostly in lakes and in the Baltic Sea. Some of the elvers have also been grown to a larger size in fish farms and subsequently used as stocking material or as seedfish for further culture.
Despite a long tradition of stocking lakes with eels (at least with the large specimens), the knowledge of stocking density, returns, economics etc., is poor. In order to increase that knowledge and to estimate the importance of eel stocking, a project was begun in 1977 within which the dynamics of a number of yellow eel populations along the Swedish coast and in some rivers were studied in order to determine the influence and effect of stocking. As part of the investigation a few lakes were stocked in 1979–80 with known numbers of large yellow eels, elvers and intermediate-sized eels from a fish farm, respectively. The eel populations of these lakes were then studied with respect to downstream migration, growth, sex ratios, predation on eels, etc.
La diminution considérable des captures d'anguilles argentées et d'anguilles jaunes remontant les fleuves a montré qu'il était indispensable de repeupler les eaux suédoises.
La méthode traditionnelle consiste à utiliser des anguilles jaunes d'assez grande taille (10–100 g chacune) capturées en mer ou dans les cours d'eau durant la migration. Depuis 1976, on importe de France des civelles que l'on emploie essentiellement pour repeupler les lacs et la Baltique. Des civelles sont également élevées dans des établissements piscicoles; lorsqu'elles ont atteint une taille suffisante, on s'en sert pour repeupler les eaux ou on les conserve comme géniteurs.
Le repeuplement des lacs en anguilles (du moins, avec de grandes anguilles) est une tradition déjà ancienne mais on manque de données sur la densité, la rentabilité, etc. de ces opérations. Pour remédier à ce problème et évaluer l'importance du repeuplement en anguilles, on a lancé en 1977 un projet consistant à étudier la dynamique d'un certain nombre de populations d'anguilles jaunes, le long des côtes suédoises et dans divers cours d'eau, afin de déterminer l'influence et l'effet du repeuplement. Dans le cadre de ces recherches, on a repeuplé quelques lacs en 1979/80 avec un nombre déterminé de grandes anguilles jaunes, de civelles et d'anguilles de taille intermédiaire provenant d'un établissement piscicole. On a ensuite commencé à étudier les populations d'anguilles de ces lacs: migration vers l'aval, croissance, taux de masculinité, prédateurs, etc.
The stocking of lakes with eels is an old tradition in Sweden. As early as the 18th century lakes were stocked with eels (Gyllenborg, 1770). Trybom (1893) recommended trapping and building of eel ladders in order to improve the fishery and to distribute eels into new waters.
The commercial catch of eel in Sweden has decreased rather drastically in the last 15 years (Fig. 1).
The decline of the Baltic eel population has been described in detail by Svardson (1976) and the ultimate cause seems to be a decrease in the immigration of elvers and young yellow eels. Svardson discussed changes in the North Sea as possible reasons for the reduced immigration of glass eels including a lower frequency of westerly winds, lower water temperature and a more Arctic predator fauna. All these factors resulted in more unfavourable conditions for the drifting glass eels. The decrease in immigration, which has become apparent along the coast of Sweden, is shown in Fig. 2. The decline of the Baltic eel population has also appeared as a diminished catch of eels per unit effort, both in Sweden and in Denmark (Hoffman et al., 1979; Neuman and Thoresson, 1981). The average weight of silver eels has increased, probably as an effect of better growth due to decreased intraspecific competition within this smaller population (Hoffman et al., 1979; Neuman and Thoresson, 1979; Neuman and Thoresson 1981).
In order to improve the low catch, large-scale introductions of elvers were proposed by Svärdson (1976) and some introductions of elvers have actually been undertaken since then. Elvers (about 1.5 t) were imported from France 1976–80 and stocked mostly along the coast and in lakes. Yellow eel, generally about 35–45 cm in length, have been sorted out from the commercial catch in the Straits of Öresund and on the west coast for stocking for many years. In laters years also intermediate-sized eels have been cultured for stocking, at a fish farm using heated water. Lake Vanern, the largest lake in Sweden, has been stocked since 1900 with elvers and small yellow eels trapped at the downstream dams of the lake as well as larger yellow eels.
The extent of the total stocking activity for the last three years has been about 250 kg of elvers, 14 t of yellow eels and 150 kg of cultured eel annually.
Since 1979 the Swedish Board of Fisheries has had about Sw.Cr. 425 000 available annually for actual eel stocking experiments. Other funds pay for additional stocking beyond this sum.
Although the stocking of lakes with eel has been practised for many years, not much is known about the economical and ecological results. An evaluation project was started in connexion with the more intensified stocking programme. Hopefully, this project will lead to an increase in knowledge about eels, with special emphasis on stocking and stocking results.
It has been suggested that large-scale stocking of Swedish waters be done with elvers, i.e., more or less pigmented small eels (≤10 cm) with at least outer pigment on the tail (EIFAC, 1981). As the Baltic eel stocks, except in the southernmost parts, are recruited from yellow eels which are larger and older than glass eels, elvers planted along the east coast would form well defined new year-classes in the population. These “artificial” year-classes are believed to change size and age distributions in favour of smaller and younger eels.
Svärdson (1976) suggested that sex ratios in eel populations could be used as an index of abundance. According to Svärdson, females have developed an effective dispersal mechanism but the males often stay and become overcrowded in coastal and estuarine waters. This difference in behaviour is thought to be the evolutionary reproduction strategy of this species, allowing females to grow big and produce high numbers of eggs.
To estimate the value of stocking the Baltic with eel, yellow eels were sampled annually from ten stations. Six of the stations are situated along the coast of Sweden and four of them are traps in rivers (Fig. 3). From each station 100 eels have been sampled annually, since 1977. The eels were delivered deep-frozen to the laboratory and when almost thawed they were analysed for weight, length and sex. Age was determined on ground otoliths in both transmitted and reflected light. Sexing was performed in the traditional way, i.e., gross morphological examination of the gonads. Histological slides were made according to Dolan and Power (1977) from all organs or Syrski and from all undifferentiated specimens longer than 200 mm.
As another attempt to increase knowledge on eels and eel stocking, six lakes were stocked with known numbers of the different sized material described earlier. Information about the lakes and the stocking material used are summarized in Table 1 and in Fig. 3. The populations in these lakes were studied with regard to downstream migration, growth, sex ratio, predation, etc.
Sub-cutaneous dye-marking with Alcian blue (Hart and Pitcher, 1969) has been tried in connexion with coast stockings with yellow eels (35–45 cm in length). Observations made by fishermen and from test fishing are expected to give more direct information about both the marking method and value of stocking.
As a very valuable supplement to the more practical work numbers of ascending eels from 22 rivers were collected continuously (Fig. 4).
As a preliminary to the evaluation project the literature on eel stocking was studied. Stocking densities used or recommended in Europe as well as the recorded recapture rates have been summarized by Wickström (1979) who recommends annual stocking with about 100 elvers or 20 yellow eel/ha in medium and high productive waters. In waters with poor nutrient supply 25 elvers or five yellow eels/ha/year may be sufficient. Intermediate-sized eel from fish farms or small yellow eel fall about half way between these values. The economics of the various stocking materials was also discussed and it may be concluded on the basis of prices and knowledge in 1979 that stocking with big yellow eels seemed to be the most economic. The difference in time until recapture played an important role in these calculations.
The otoliths sampled and the data on weight, length and sex have not been completely analysed yet. However, the size distributions from each sampling station are summarized in Figs. 5 and 6.
In the trial lakes a few eels have been trapped in the outlets. In Lake Ången, which was stocked with yellow eels (average length 370 mm) in autumn 1979, about 5 percent migrated downstream in the following spring. Of the cultured eels (average length 117 mm) stocked in Lake Fardume marsh in autumn 1980, only a few specimens were caught during 1980 and 1981.
Some results have also been obtained from lakes not directly included in the project. Fig. 7 presents the yield and numbers of eels stocked into three lakes. Important eel fisheries have built up in Lake Mälaren and Hjälmaren, which must be based mainly on stocking. Natural recruitment appears very low from the few ascending eels trapped in nearby rivers. Unfortunately, the stocking material varied much in size (and age) in these lakes and in many others, which makes comparisons and correlations difficult. In an attempt to facilitate comparisons “eel stocking units” were used in Fig. 7. Such units take into account our recommended stocking densities, which, in turn, take expected survival, sex ratio, etc., into consideration (Wickström, 1979). Expressed in these units about 7.5 elvers are comparable to about 2.5 10-g eels which, in turn, are equivalent to one 100-g eel. However, no correlations have been calculated as we do not yet know accurately the age of eels in the commercial catch from these lakes.
The decline of the Baltic eel population is believed to reflect some climatological changes in the North Sea (Svärdson, 1976). According to Hansen et al. (1981) temperature in the Northern Hemisphere decreased by about 0.5°C between 1940 and 1970 (despite a rapid CO2 build-up during the same time period). This is coincident with the negative trend in numbers of eels ascending Swedish rivers, reported by Svärdson (1976) (see also Figs. 2 and 9). Hansen et al. (1981) also described an increase in temperature of about 0.2°C since 1970 in the Northern Hemisphere. Whether this will create better conditions for survival of eel larvae and glass eels drifting in the Atlantic Ocean and in the North Sea, will not show up until several years after the inflexion in 1970. If this is indeed the case, immigration of elvers may have slowly increased from 1973 and an increase in commercial catch can be expected in 1983 at the earliest (assuming a silver eel age of ten years). We have no indications of such a development in immigration, possibly due to disturbance from annual fluctuations. Results of our stocking activities, however, could very well be “disturbed” by this climatological change.
As the Swedish eel fishery has already decreased considerably and is expected to decrease even further, recruitment must increase quickly. We must, therefore, use a combination of the different size of eel available for stocking, big yellow eels for recapture some few years later and elvers for a longer-term improvement. With their intermediate size and recapture time, cultured fingerlings play an important role in this plan. Unfortunately, the size (and age) of yellow eels used for stocking coincides with the natural immigrants in the Baltic area (Ask et al., 1971) which creates difficulties in interpretations of sampling results.
The numbers of immigrating elvers and yellow eels have decreased in the River Göta Älv, as depicted in Fig. 2. This is obvious if we, like Svärdson (1976) measure the amounts in kilogrammes. During this period of decline the average weight, calculated from observed numbers/kg, has also decreased (Fig. 8). This can be due to some technical changes of the eel ladder or changes in water-flow regimes, etc., allowing the smallest eels to ascend more easily. If this is the case, one might expect that ascending eels are smaller than but as numerous as formerly. However, in Fig. 9 the calculated numbers are presented and a negative trend is obvious. Thus, ascending eels have decreased in both weight and numbers. The proportion of older eels ought to increase when the inflow of elvers diminishes. Our observations are contradictory so we have to find another explanation. Formerly, when elvers and small yellow eels were more abundant in the River Göta Älv, predation and competition from large eels presumably reduced the proportion of elvers (Moriarty, 1978). Nowadays, the abundance of large eels is lower and the proportion of surviving small yellow eels and elvers can increase. If this is true, the average weight will increase if and when elver immigration increases in the future.
The import of elvers is essential for our stocking programme. Unfortunately, importation has not proceeded as expected, due mainly to a fear of introducing fish diseases.
Infectious pancreas necrosis (IPN) virus has been discovered in elvers both from France and the United Kingdom (Castric and Chastel, 1980; Hudson et al., 1981). Elvers may possibly act as carriers for IPN and may infect salmonids in both culture and natural waters. The former import of elvers from France was, therefore, prohibited by the Swedish authorities in 1981. A quarantine station has now been built in southern Sweden and in 1982 elvers will be imported from the River Severn in the United Kingdom. If the elvers are disease-free, after one month in quarantine and two months in isolation, they can be used for culture and stocking.
This station will simplify import procedures and perhaps elver sources other than the River Severn can be used in future. This will guarantee the supply of elvers and can extend the elver import season considerably. The eel stocking programme can then run without hindrance and if higher proportions of elvers and cultured fingerlings are used, resulting changes in the population will be easier to interpret.
Ask, L., K.-E. Berntsson aznd S.-O. Öhlund, 1971 Undersökningar om gulålens ålder, kön och tillväxt. Medd.havsfiskelab., Lysekil, (108):20 p. (in Swedish)
Castric, J. and C. Chastel, 1980 Isolation and characterization attempts of three viruses from European eel, Anguilla anguilla: preliminary results. Ann.Virol.Inst.Pasteur, 131E:435–48
Dolan, J.A. and G. Power, 1977 Sex ratio of America eels, Anguilla rostrata, from the Matamek River system, Quebec, with remarks on problems in sexual identification. J.Fish.Res.Board Can., (34):294–9
EIFAC (European Inland Fisheries Advisory Commission), 1981 Report of the 1981 meeting of the Working Party on Eel. Ferrara, 28–30 September 1981. Rome, FAO, EIFAC/XII/82/6:22 p. (mimeo)
Gyllenborg, J.G., 1770 Kort Afhandling om Insjö Fisket i Swea Riket. Stockholm, Wennberg and Nordström, 70 p. (in Swedish)
Hansen, J. et al., 1981 Climate impact of increasing atmospheric carbon dioxide. Science, Wash., 213(4511):957–66
Hart, P.J.B. and T.J. Pitcher, 1969 Field trials of fish marking using a jet inoculator. J.Fish Biol., 1(4):383–5
Hoffman, E., H. Hansen and P.M. Christensen, 1979 Changes in catch per unit effort and catch composition of silver eels in the south-eastern Denmark 1949–77. Anadromous and Catadromous Fish Committee. ICES. C.M. 1979/M:28:11 p. (mimeo)
Hudson, E.B., D. Bucke and A. Forrest, 1981 Isolation of infectious pancreatic necrosis virus from eels, Anguilla anguilla L. In the United Kingdom. J.Fish Dis., 4(5):429–31
Moriarty, C., 1978 Eels: a natural and unnatural history. London, David and Charles, 192 p.
Neuman, E. and G. Thoresson, 1979 Fisket efter blankål (Anguilla anguilla L.) kring Oskarshamnsverket åren 1962–77. Stockholm, National Swedish Environmental Protection Board (SNV PM 1162):45 p. (in Swedish)
Neuman, E., 1981 Fisket efter blankål (Anguilla anguilla L.) kring Barsebäcksverket åren 1972–79. Stockholm, National Swedish Environmental Protection Board (SNV PM 1428):51 p. (in Swedish)
Svärdson, G., 1976 The decline of the Baltic eel population. Rep.Inst.Freshwat.Res., Drottningholm, (55):136–43
Trybom, F., 1893 Fiskevård och fiskodling. Stockholm, Adolf Bonnier, Kongl. Hof- och Universitetsbokhandlare, 199 p. (in Swedish)
Wickström, H., 1979 Preliminary riktlinjer for ålutsättningar. (Preliminary recommendations for stocking with eels). Inf.Inst.Freshwat.Res., Drottningholm, (5):24 p. (in Swedish, English summary)
Table 1 Data on the trial lakes stocked with eel
|Condition||mesotrophic but acid||mesotrophic||mesotrophic||mesotrophic||mesotrophic||oligotrophic but shallow & “productive”|
|Fish species||perch, pike, burbot and roach||perch, ruffe, pike, burbot, bream and eel||perch, roach, bream, rudd tench and eel||perch, ruffe, roach, bream, tench, eel and crayfish||perch, ruffe, roach, bream, white bream, eel and crayfish||perch, ruffe, pike, roach, rudd, tench & crucian carp|
|Nos. stocked||about 6 000 (elvers)||about 5 000(yellow eels, average length 370 mm)||about 12 000(elvers)||about 28 000(elvers)||about 37 000|
(cultured eels, average weight 4 g)
|about 53 000|
(cultured eels, average length 117 mm)
Fig. 1 The commercial eel catch in Sweden 1930–80
Fig. 2 Ascent of elvers and yellow eels in five Swedish rivers. (Compared with the mean of the period 1977–81 = 1 unit)
|1||River Göta älv|
|8||River Motala ström|
Fig. 3 Locality of eel sampling stations and experimental lakes
Fig. 4 Swedish rivers where ascending elvers and yellow eels are trapped and recorded annually
Fig. 5 Size distributions of eels sampled at six coastal localities
Fig. 6 Size distributions of ascending eels sampled in four rivers
Fig. 7 Yield and numbers of stocked eels in three Swedish lakes
Fig. 8 Calculated average weight of ascending eels in River Göta älv
Fig. 9 Calculated numbers of ascending eels in River Göta älv