by
B. Holmberg1
National Correspondents
I FISHERY BIOLOGY AND MANAGEMENT
1. Crayfish
The management, stocking procedure and intensive/extensive farming of the introduced crayfish species Pacifastacus leniusculus is intensely studied Several publications reporting on farming techniques were published in 1985 and early 1986, and the further development of stocked populations is being closely monitored annually. Dwindling eel stocks in many parts of the country account for the increase, on the other hand acidification is rapidly reducing the distribution of both species of crayfish, particularly in the southwestern and south-central parts of the country. Methods for crayfish farming are mainly developed on the island of Gothland in the Baltic Sea, where only Astacus astacus occur, and thus no crayfish plague outbreak has occurred, and concerning Pacifastacus mainly in the province of Scania and at the Institute of Freshwater Research at Drottningholm, west of Stockholm.
Recommendations so far emphasize that the development of Pacifastacus populations in the country is still promising and should be encouraged. In the recommendations published it is described how cannibalism acts and how it is affected by the fishing pressure. It is also discussed how the size-selective fishery could have negative long-term genetic effects on the populations. The two crayfish species coexist in certain lakes where introduced crayfish were not carriers of the plague and where the plague for some reason had “vanished” from the lake. These sympatric populations are of course to specific interest to science.
Heating of the water will prolong the growing season and will also make it possible to postulate the date of hatching.
2. Food organisms - crustaceans
Introductions of the three species of fish food organisms described in earlier reports have been halted, after it was shown that some of the crustaceans - mainly Mysis relicta - had deleterious effects on the plankton fauna, thereby effectively competing with the fish they were meant to provide a source of food for. All studies performed up to 1985 are presently being compiled and published, including recommendations for the use of these organisms in fisheries management and also with suggestions for future studies.
3. Tagging and stock assessment
Stocking of fish is still one of the main measures in fish management. In order to evaluate the results tagging is needed and in 1984 and 1985 35.000 and 20.000 fish were tagged, respectively, and released in freshwater. The dominant species are at present brown trout, land-locked salmon, char and lake-trout.
The results from tagging experiments are mainly evaluated locally but recently work has been started to analyse this data on a nation-wide basis.
Many of the larger lakes in Sweden sustain a commercial fishery partly built on stocking of trout or land-locked salmon. The main questions in this case are: How does the stocking effect the stocks of pelagic fish species, and thus, how much can be stocked before these pelagic stocks collapse. During 1985 equipment for stock assessment by echointegration was purchased and a sampling programme will start in 1986, covering mainly the four large lakes of southern Sweden.
1 Head of Section, Nstional Board of Fisheries, Box. 2565, S-40317 Göteborg.
4. Eel
The commercial catch of silver eel in the Baltic 1984 was 1.074 metric tonnes, which is very similar to the last 10 year mean. Preliminary data indicates a lower catch in 1985.
Immigration of glass eel was very low in 1984 and 1985.
About SEK 1.6 million was granted to the Board of Fisheries for stocking purposes in 1985. Mostly yellow eels of 28–54 cm in length are used as stocking material. Fingerlings kept for roughly a month in heated water are also used in some cases.
Due to fear of introducing viral diseases (e.g. Rhabdo- and IPN-virus) to salmonid cultures only elvers from the UK (Bristol Channel) are now allowed to be stocked in natural waters after quarantine procedures. Glass eel from France are, despite passage through quarantine, only allowed for consumption after culture.
The project with experimental lakes stocked with different sizes of eel is still going on. Eel stocks are monitored through test fishing with fyke nets, long lines and outlet traps.
Population studies on eel in productive lakes of southern Sweden have been conducted and the question of narrow- and broad-headed eels impact on bottomfauna has been raised.
Another population study on eel in shallow marine bays has also been commenced. In both studies the method of sub-cutaneous dye-marking with Alcianblue was adopted.
Another marking method was to “contaminate” elvers with an Europium isotope and subsequently measure the activity of potential recaptures.
Silver eels equipped with ultrasonic tags and Carlin tags have been tracked in the Baltic. In the experiments orientation of both blind and anosmic eels were tested.
II FISH CULTURE AND DISEASES
Swedish fish farms have remained clinically and virologically IPN-free since 1969, when there was a disease outbreak in southwestern Sweden. The disease was then eradicated by stamping out (Epizootic Diseases Act). In 1985 a sample of slaughtered fish, which were examined for the purpose of export licensing, was found to be IPN-positive. No clinical disease has been observed in the fish farms and no IPN-virus has been detected by sampling farmed fish extensively during 1985, except one case, detected in a fish farm on the Swedish west coast (coastal water supplied) during November 1985. No clinical disease was seen there
Proliferative kidney disease (PKD) is still prevalent in Swedish fish farms although no further spread has been observed. Mortality rates have usually been rather low in recent years.
There has been two viral recoveries from eel quarantine stations in the last two years: IPN (one rearing station) and Rhabdovirus anguillae (two rearing stations).
Ulcerative dermal necrosis (UDN), still appears among salmon and sea trout kept in holding ponds for spawning in Northern Baltic rivers.
Furunculosis of salmonids (inf with Aeromonas salmonicida v. salmonicida) is limited to a small number of fish farms, and an eradication plan for the disease includes a ban of stocking inland waters with fish from furunculosis infected fish farms.
Infections dermatitis caused by A. salmonicida v. achromogenes has a more common occurrence and is observed both in salmonid and nonsalmonid freshwater fish.
Bacterial kidney disease (BKD) was diagnosed for the first time in 1985 in 4 rainbow trout farms on the south and east coast of Sweden (Baltic area). A survey of the BKD situation in the Baltic area is underway.
III. FISH AND POLLUTED WATER
During the first part of this decade fish kills have been reported, where the fish obviously had been killed by choking. Different ways to explain the fish kill have not given an unequivocal answer.
In the eutrophic Lake Hjälmaren fish kills have been reported 1980, 1982 and 1985. In all cases the fish had died in fishing tackles.
The gills of the dead fish had a covering containing Fe, Mn as well as Al. These elements can all form precipitates on the fish gills thick enough to cause choking. At present it has not been possible to decide the importance of each individual element.
Low values of the oxygen content in the water have also been examined as a cause of the observed fish kill.
Another possibility examined is the exercation of ammonia from the fish in the fishing tackles. This possibility is of interest because of the high pH observed in Lake Hjälmaren during periods of algal blooms.
Recently fish kill has been reported from a fish breading (salmon) in southern Sweden which takes its water from a river with a pH ranging from 6–7 and a KMnO4-value ranging from 20–70 mg/l. The gills of the dead fish were covered with precipitates, which could cause choking. Even in this case there is no simple explanation to the observed fish kill.
In both cases we cannot exclude that soil processes can be the cause behind the observed fish kill.
IV. PRODUCTION STATISTICS
The yield of the total Swedish aquaculture in 1984 was 1.631 metric tons of fish for consumption, which when converted to round fresh weight is the equivalent of 1.925 tons. The dominating species was the rainbow trout (1.849 tons). The total value of this aquaculture production amounted to 52 million SEK.
The number of enterprises engaged in aquaculture was 637, of which 156 produced fish for consumption and 168 cultivated fry for stocking. For compensatory purposes 2,3 million fry of salmon and trout were released, mainly in rivers running to the Baltic.
For the inland fisheries there are no statistics.
V. BIBLIOGRAPHIC REFERENCES
Hamrin, S.F. 1985. Food and habitat resources of a slow growing eel population in an eutrophic lake. (In manuscript).
Hanson, M., L. Karlsson & H. Westerberg. 1984. Magnetic material in European eel (Anguilla anguilla). Comp. Biochem. Physiol. 77 A: 221–224
Hanson, M. & H. Westerberg 1986. Occurrence and property of magnetic material in European eel (Anguilla anguilla). J.Magn.Magn.Mater. 54–57:1467–1468.
Hanson, M., G. Wirmark, M. Öblad & L. Strid. 1984. Iron-rich particles in European eel (Anguilla anguilla). Comp.Biochem.Physiol. 79 A:311–316.
Karlsson. L. 1984. Migration of European silver eels, Anguilla anguilla. Orientation to the earth's magnetic field and pulpmill effluents particulary in the Baltic. Acta Univ.Ups. Abstracts of Uppsala Dissertations from the Faculty of Science 745. Uppsala. 30 p.
Wickström, H. 1985. Growth of cultured eel stocked in two Swedish lakes. Paper presented at the 1985 Meeting of the Working Party on Eel, Perpignan, France, 17–21 September 1985. 11 p.
