Denmark

Country report

1986 – 87

(J. Dahl)

I. FISHERY BIOLOGY AND MANAGEMENT.

1. Fish stocking.

1.1. On the Budget (etc) for the financial year of 1987 an amount of 10 Mio. DKr. was earmarked for an extensive program of liberating trout, rainbow trout and eels in rivers and coastal waters all over the country. The greater part of this amount has been used for reimbursing those angling clubs manageing trout liberation schemes. A smaller part was reserved for the implementation of a series of research projects especially related to fish stocking problems, such as tagging techniques, stocking periods, and acclimatization of smolts and pre-smolts to salt water conditions. The experimental tagging and stocking program comprised 22,400 trout, 6,000 rainbow trout, 2,000,000 glass eels, and 77,000 young eels.

A further part of the amount was allotted for the elaboration of new and the revision of old trout liberation schemes, comprising 15 individual river systems and 6 geographically well defined groups of river systems.

2. Salmonids.

2.1. Studies were continued on the population dynamics of trout (sea-, brown-) in rivers, including migration, smolt production, spawning biology, homing, and energy flow.

2.2. Experiments were carried out to compare post-stocking mortality and growth, movements and smolt yield, effects of electrofishing and handling upon mortality, growth, movements, and catchability of two strains of 0+ and 1+ trout parr of wild and hatchery origin.

2.3. Investigations were carried out on the population dynamics of coexisting populations of trout and rainbow trout in a small tributary to the lake Hald sø, Jutland.

2.4. Investigations were carried out on the possibility of establishing artificial spawning grounds in streams with varying physical and hydrological conditions.

3. Coregonids.

3.1. Efforts are made to save the population of the threatened anadromous North Sea houting (Coregonus oxyrhynchus) which for the time being is restricted to one single river system, the Vidå, in south Jutland. The work comprises electrofishing and stripping of spawners, rearing of fry, stocking fry in the river, and restoring and improving natural spawning grounds. In 1987 66,000 fry were released in the river and in a few other south Jutland rivers from which the houting has disappeared since the beginning of the century.

4. Eels.

4.1. The long term studies on the daily ascent of glass eels and young yellow eels into a number of Jutland rivers are continued. The studies include observations of various hydrological and climatic parametres.

4.2. The analytical work on weekly samples of the glass eel ascent at the Vidå sluice (south west coast of Jutland) was continued through the biennium. The glass eels were analyzed for their elementary chemical condition (water, lipid and protein) in relation to time of ascent within the course of the total ascent, the pigmentation stage and the geographical distribution of the ascent, the latter including corresponding analyses of the glass eel ascent elsewhere in Europe.

4.3. The long term experiments on the artificial maturation of silver eels by means of hormonal treatment are continuing.

4.4. The studies on ageing of eels by means of otoliths of eels of known age originating from eel aquaculture are also continuing.

5. Grass carp.

5.1. Grass carp have now been stocked into about 200 small, closed water bodies. Artificial propagation of grass carp is still not performed in Denmark, and stocking material is being imported from Austria, Germany and Sweden under strict control by the veterinary and fisheries authorities.

II. FISH CULTURE AND DISEASES.

1. Feeds and feeding in trout farming.

1.1. Experiments have been started on trout growth, food conversion, and environmental effects in relation to the ratio between protein, fat and carbohydrate in trout feed.

1.2. Experiments have been carried out to elucidate a possible connection between the quality of raw fish for fish meal production and growth and food conversion in rainbow trout. It was shown that improved growth and food conversion and less discharge of nitrate and phosphorus was achieved when using fresh primary produce rather than using less fresh primary produce with a high content of TVN. Further-more it was shown that treating raw fish with formalin has a negative effect on the quality of fish meal for trout feed.

2. Trout growth studies.

2.1. Experiments were carried out to elucidate the validity of the previously published growth model for rainbow trout between 300 g and 2,000 g and for trout fry between 0.2 g and 3.0 g.

2.2. Experiments were carried out on trout quality in relation to growth rate, pond hygiene, and length of the fasting period before slaughtering.

3. Medical treatment.

3.1. Investigations have been carried out on residual concentrations after treatment of trout with oxytetracycline, oxylinic acid and trimetoprim.

3.2. Experiments on the possible growth inhibiting effect when trout are treated with sulphamerazine, oxylinic acid and furazolidon were carried out.

3.3. The retention time of malachite green after treatment of trout and trout eggs was investigated.

3.4. Some disinfection remedies alternative to malachite green to control Saprolegnia and Ichthyophthiriasis in trout and trout eggs have been tested, but so far no such alternative has been found.

4. Diseases.

4.1. A clinical test of vaccines against furunculosis has been performed.

4.2. Investigations were carried out on the biochemical reaction and sensitivity of Yersinia ruckeri to a number of antibiotics.

4.3. The possible transfer of ERM bacteria from generation to generation via the egg has been investigated in model experiments in which eggs were artificially infected during fertilization.

4.4. With regard to VHS ELISA methods for detection of virus and circulating antibody to the virus have been developed and applied. Monoclonal antibodies have been produced against the four major structural proteins of Egtved virus.

The antibodies have been immunologically characterized and biologically tested in-vitro as well as in-vivo systems with a view to their possible use in connection with identification of “protective” Egtved virus antigens.

In addition the monoclonal antibodies have been applied in diagnostic immunochemical techniques such as ELISA and IFAT.

4.5. Infection in elvers and trout by Rhabdovirus anguilla (EVX) has been studied under field and aquarium conditions. The virus infection has been revealed rather frequently in Danish eel farms as well as in elvers migrating into Danish streams. In aquarium experiments elvers were infected with EVX virus at high concentrations without developing disease signs, whereas rainbow trout fry under similar experimental conditions contracted disease and suffered from high mortality.

5. Veterinary services.

5.1. The services undertake registration of VHS- and IPN-free trout farms. Admission to the IPN-register is conditioned only by preceding virological controls performed by the National Veterinary Laboratory.

5.2. By the end of 1987 a total of 520 trout farms were in operation. Out of these 426 were admitted in the VHS-register. During 1986–87 stamping-out programs were conducted and approved for 54 trout farms. During the same period recurrence of VHS occurred in 64 farms.

5.3. By the end of 1987 50 trout rearing farms were admitted in the IPN-register.

III. FISH AND POLLUTED WATER.

1. Rivers.

1.1. Investigations are carried out on the development of biological structures in different types of running water, including evaluation of the oxygen balance.

1.2. Investigations are carried out on the discharge, decomposition and transport of nitrate, phosphorus and organic matter and their effect in streams.

1.3. A hydrological model and a model of substance transport in streams is being developed.

1.4. Investigations are carried out on the physical and biological conditions in streams aiming at elucidating the effects of modified and more pro-environmental stream maintenance and the effect of a changed utilization of the surrounding fields.

2. Lakes.

2.1. Investigations are carried out on the decomposition, biological structure and dynamics of phosphorus in a lake previously heavily polluted by urban sewage.

2.2. Investigations are carried out on the possibility of lake restoration partly by means of removal of sediment, partly by means of biomanipulation of the fish community in order to reduce the grazing effect of fish on zooplankton.

IV. PRODUCTION STATISTICS.

1. Trout pond farming

1.1. The production (export + home market) of farmed trout (mainly rainbow trout) during 1985–1987 amounted to

1.2. The export of eyed ova of trout (rainbow trout, brown trout) during the same period amounted to

The figures for 1987 only represent the period January – June.

2. Eel farming.

By the end of 1987 c. 30 eel farms were in operation. The production (export + home market) amounted to

3. Inland waters.

3.1. According to the official Danish fishery statistics the total catch in Danish inland waters in 1985 and 1986 amounted to 376 and 457 m.t. respectively. The percentage species distribution of the catch was as follows:
Bream 28%, eel 16%, roach 14%, trout 9%, pikeperch 6%, perch 5%, pike 4%, and other species 18%.

Bream, roach and “other species” are mainly considered as coarse fish which only to a very slight degree are used for human consumption.

The statistics for 1987 were not available at the time of reporting.

V. BIBLIOGRAPHIC REFERENCES.

1. Subcommission I.

Henriksen, P. W., 1987: Fiskedrab ved grødeskæring med mejekurv. (Fish-kills by weed cutting with reaping basket). - Vand & Miljø 4(3) : 139–140

Jensen, J. K., Rasmussen, E. K. & Mortensen P. B., 1986: Sørestaurering -en oversigt over metoder. (Lake restoration - a survey of methods). - Stads- & Havneing. 77(8) : 211–216

Larsen, K., 1987: Havørredopgangen i danske vandløb 1900–1960. II. Fyn samt Østjylland fra grænsen til og med Randers Fjord. (The sea trout spawning run into Danish streams 1900–1960. II. Funen, and eastern Jutland from the German border up to and including the Randers Fjord) - Medd. fra Ferskv.fisk.lab. 1/87, 69 pp.

Nielsen, J., 1986: Laksefiskene i Gudenåen og Randers fjord. (The salmonid fishes in the Gudenå river and the Randers fjord). - Vand & Miljø 3(6) : 251–254

Nielsen, J., 1987: Et nyt, bærbart elektrofiskeapparat. (A new portable electrofishing apparatus) - Vand & Miljø 4(5) : 249–250

Riemann, B., Jensen, H. J., Muller, J. P., 1987: Manipulation of søers fiskebestande. (Manipulation of the fish population in lakes). - Vand & Miljø 4(4) : 163–166

Thyssen, N., 1986: Vandindvindingens betydning for vandløbene. (Ground water extraction and its influence on streams). - Stads- & Havneing. 77(6–7) : 152–154

2. Subcommission II.

Bregnballe, F., 1986: åleopdræt som bierhverv til landbrug. (Eel culture as a sideline in agriculture). - Ugeskr. f. Landbrug, nr. 45 : 1331–1333

Bregnballe, F. & Jokumsen, A., 1986: Udviklingsmuligheder for dansk opdræt af store ørreder. (Possibilities for development of Danish culture of large trout). - Nord. Aquacult. 2(1) : 31–36

From, J., 1986: Nyt middel mod Octomitus (=Hexamita). (A new remedy against Octomitus (=Hexamita)). - Ferskv.fisk.bl. 84(9) : 194

Jeppesen, G., 1986: Krebs i akvakultur. (Crayfish in aquaculture). - Nord. Aquacult. 2(1) : 38–41

Jørgensen, P. E. V. & Olesen, N. J., 1987: Rødmundssyge (ERM) : Forsøg med kunstig infektion af nystrøgne ørredæg. (Red mouth disease (ERM) : experiments on artificial infection of freshly stripped trout eggs). - Ferskv.fisk.bl. 85(5) : 403–405

Larsen, J. L., 1987: Kombinationsvacciner til ørreder i havbrug. (Combined vaccines for trout in mariculture). - Ferskv.fisk.bl. 85(3) : 344–347

Larsen, K. & Rasmussen, H., 1986: Leversvulst - en alvorlig foderinduceret sygdom. (Liver tumour - a serious feed induced disease). -Ferskv.fisk.bl. 84(1) : 12–16

Lund, M., 1987: Rotter, Weils syge og dambrug. (Rats, Weil's disease and fish culture). - Ferskv.fisk.bl. 85(10) : 540–541

Mellergård, S., 1987: ålens svømmeblæreorm, en ny parasit i den europæiske ålebestand. (The swimbladderworm, a new parasite in the European eel populations). - Ferskv.fisk.bl. 85(9) : 526–531

3. Subcommission III.

Andersen, J. K., 1986: Løsning af nitratproblemer ved hjælp af nitratredox-metoden. (Solution to the nitrate problems by means of the nitrate-redox-method). - Vand & Miljø 3(6) : 255–258

Christensen, P. B. & Nielsen, L. P., 1987: Direkte målinger af denitrifikation i vandløb. (Direct measurements of denitrification in streams). - Vand & Miljø 4(3) : 117–120

Drabæk, J. & Madsen P. P., 1986: Kviksølv i danske ferskvandsøkosystemer. (Mercury in Danish freshwater ecosystems). - Miljøproj. nr. 74, Miljøministeriet, Miljøstyrelsen, 37 pp.

Geertz-Hansen, P., Rasmussen, G. & Skriver, J., 1986: Okkers indflydelse på vandløbenes fiske- og smådyrsfauna. (The influence of ochre on the fish and invertebrate fauna in streams). - Vand & Miljø 3(4) : 155–159

Helweg, A., 1986: Hvad sker der med pesticiderne i vort miljø ? (What happens to the pesticides in our environment ? ) - Vand & Miljø, spec. issue 2 : 22–24

Jeppesen, E., Thyssen, N., Prahl, C., Jensen, K. S. & Iversen, T. M., 1987: Kvælstofakkumulering og omsætning i vandløb med udgangspunkt i undersøgelser i Suså og Gryde å. (Accumulation and decomposition of nitrate in streams based on investigations in the rivers Suså and Gryde å). - Vand & Miljø 4(3) : 123–129

Kronvang, B. & Aub-Robinson, C., 1986: The transport of dissolved and suspended solids in the river århus å, Denmark. - Nord. Hydrol. Progr. Rep. No. 14 : 147–157

Kronvang, B. & Christiansen, C., 1986: The paths of the suspended particulate inorganic and organic matter in a small urban estuary -the århus harbour estuary. - Nord Hydrol. 17 : 31–46

Kronvang, B. & Thyssen, N., 1987: Transport af kvælstof i vandløb. (Transport of nitrate in streams). - Vand & Miljø 4(3) : 111–114

Kronvang, B., Kristensen, P. & Thyssen, N., 1987: Fosfor i vandløb. (Phosphorus in streams). - Vand & Miljø 4 (6) : 279–283

Lassen, H. H. & Wiberg-Larsen, P., 1986: Dambrugsbelastning - hvor meget må et dambrug forurene ? (Fish culture loading - what is the tolerable pollution from a fish culture ?). - Vand & Miljø 3(6) - 246–250

Madsen, H. B. & Kristiansen, H., 1987: Okkerforekomst i vandløb set i relation til potentielt svovlsure jorde. (The occurrence of ochre in streams in relation to potentially sulphate soils). -Vand & Miljø 4(4) : 178–181

Nielsen, N. E., 1986: Kvælstof, jordbrug, vand og miljø. (Nitrate, agriculture, water and environment). - Vand & Miljø, spec. issue 2 : 7–11

Nøddegaard, E., 1986: Pesticider og miljø. (Pesticides and environment). -Vand & Miljø, spec. issue 2 : 16–21

Olesen, M., 1986: Forureningsbekæmpelse med flydende bakteriekulturer. (Pollution control with floating cultures of bacteria). -Ferskv.fisk.bl. 84(6) : 124–125

Olesen, M., 1986: Anvendelse af flydende bakteriekulturer til begrænsning af forurening fra dambrug. (Use of floating cultures of bacteria to limit pollution from fish cultures). - Ferskv.fisk.bl. 84(9) - 182–192

Rebsdorf, Aa. & Thyssen, N., 1986: Evidence of stream acidification in Denmark as caused by acid deposition. - Proc. Workshop: “The reversability of acidification”, Grimstad, Norway, 9–11 June : 195–198

Rebsdorf, Aa. & Thyssen, N., 1986: Fosforudvaskning fra udyrkede og dyrkede arealer. (Leaching of phosphorus from uncultivated and cultivated areas). - Vand & Miljø 3(4) : 151–156

Schjørring, J. K. & Henze, M., 1986: Fjernelse af næringssalte fra byspildevand ved hjælp af planter i vandkultur. (Disposal of nutritive salts from urban sewage by means of plants in water culture). - Vand & Miljø 3(6) : 259–264

Sode, A., 1986: Okkereffekt på vandige økosystemer. (The effect of ochre on aqueous ecosystems). - Vand & Miljø 3(3) : 113–115