P.N. Markmann
National Agency of Environmental Protection
Copenhagen, Denmark
ABSTRACT
Grass carp (Ctenopharyngodon idella Val.) was stocked into Danish natural waters in Denmark for the first time in 1968 but only in very few numbers in a single locality.
In 1978 a second and larger series of stocking experiments were begun. Stockings were for weed control and only closed water bodies or localities situated within water systems with no fish farms have been stocked.
Stocking material has been imported from Malaysia, the Netherlands, Sweden and Austria.
Until now, grass carp has been stocked in 14 separate ponds and lakes which range in area from .04 to 35 ha in densities ranging from 2 to 500 kg/ha.
Overwinter survival has generally been high in several localities, nearly 100 percent. Growth has mostly been fast with yearly weight increments of up to 150 percent. Weed density has been markedly reduced in 4 of the 14 localities. No spreading of fish diseases has occurred.
Undesirable side effects have only been reported from two localities where a reduction in light penetration due to increased phytoplankton growth has been observed. This has lead to supplementary stocking of silver carp (Hypophthalmichthys molitrix Val.) in one locality. This first stocking of silver carp into a Danish lake was made in December 1981.
RESUME
En 1968, on a pour la première fois déversé des carpes de roseau (Ctenopharyngodon idella Val.) dans les eaux libres du Danemark mais en très petites quantités et en un seul endroit.
En 1978, des expériences ont été entreprises sur une plus grande échelle. Ces opérations visaient uniquement à limiter la prolifération des herbes aquatiques. Le repeuplement de même que les importations restent soumis à des restrictions. On n'a repeuplé que des pièces d'eau fermés ou des endroits situés sur des réseaux hydrographiques où il n'y a pas d'établissement piscicole.
Le matériel utilisé à été importé de Malaisie, des Pays-Bas, de Suéde et d'Autriche.
Jusqu'ici, on a repeuplé avec la carpe de roseau 14 étangs et lacs de 0,04 à 35 hectares à des densités initiales de 2 à 500 kg/ha.
La survie à l'hiver a en général été bonne, avoisinant 100 pour cent en plusieurs endroits. La croissance a la plupart du temps été rapide avec des augmentations annuelles de poids pouvant atteindre 150 pour cent. La densité des herbes aquatiques s'est déjà nettement réduite dans 4 des 14 sites. Aucune maladie transmissible aux poissons n'a été propagée.
Des conséquences néfastes n'ont été observées que dans deux endroits où l'accélération de la croissance du phytoplancton a entraîné une réduction de la visibilité. Dans l'un de ces endroits, on a procédé à un repeuplement supplémentaire avec des carpes argentées (Hypophthalmichthys molitrix Val.). Ce premier repeuplement d'un lac danois avec des carpes argentées a eu lieu en décembre 1981.
Grass carp (Ctenopharyngodon idella Val.) was introduced into Denmark for the first time in 1965 with 12 adult fish from U.S.S.R. Only one year later the potential of grass carp for weed control in Denmark was reviewed by Christensen (1966).
In June 1968 a 2-km long, 5-m wide reach of the River Esrum a in northern Zealand was stocked with about 100 small grass carp (mean length 9 cm) originating from Malaysia. Predatory fish (pike and perch) were removed from the reach by electro-fishing shortly before stocking. In November only about 10 percent of the stocked fish were still alive. These fish had reached a mean length of only 15 cm and no weed reduction could be found (Denmark, County of Frederiksborg, 1966, 1971; Hansen, 1969).
As a result of these somewhat discouraging results no new stocking experiments with grass carp were attempted until 1978.
The introduction of fish species not indigenous to the Danish fauna implies certain risks including the spreading of fish diseases, unforeseen ecosystem responses and uncontrolled spreading of the new species due to natural reproduction. Although natural reproduction is not likely in Denmark, where the June isotherm is below 20°C, import and stocking into Danish waters have been highly restricted.
Permission for import and stocking is granted by the Danish Veterinary Service and the Ministry of Fisheries.
Permission to import is only granted for fish showing no clinical signs of diseases when dispatched and which originate from fish farms which are subjected to official sanitary control. Furthermore, the fish farm must be guaranteed free of spring viraemia of carp for a period of at least two years prior to shipment. The consignment should be accompanied by a certificate from the veterinary authorities conducting the official sanitary control in the exporting country.
Until now stocking has only been permitted into closed water bodies or localities situated within water systems with no fish farms.
In certain cases permission to stock has had to wait for the results of Danish veterinary spot tests on the imported fish and the total consignment has been kept in quarantine until the results are available.
The present import regulations and stocking procedures are overly bureaucratic and are to be changed during 1982 on the basis of results given in this paper and in Mortensen (1982).
In 1978 a series of rather comprehensive stocking experiments was started when the Inland Fisheries Laboratory, the Water Quality Institute, the County of northern Jutland and the municipalities of Albertslund and Pandrup cooperated in a project to stock grass carp into Lake Nols So in northern Jutland and into four artificial waters in Albertslund Municipality west of Copenhagen (Markmann, 1979; Markmann and Warrer-Hansen, 1980; Denmark, County of Northern Jutland, 1980). The activities in Lake Nols So came to an end during the spring of 1979 and since then both monitoring and new stockings have been managed solely by the Water Quality Institute.
Since 1978 a total of about 3 600 kg of grass carp has been imported into Denmark from the Netherlands, Sweden and Austria (Table 1).
The stocking material has generally consisted of fish with mean weights of about 0.5 kg but in 1979 three different size groups with mean weights of about 0.2, 0.4 and 0.8 kg, respectively, were stocked.
2 900 kg of the imported fish were stocked before the spring of 1982. About 225 kg still remain at the Water Quality Institute awaiting stocking during the spring of 1982 into two new localities (compare Table 2) where the status of the ecosystem just before stocking has been investigated in more detail than in previous localities. The rest of the imported fish have been used in aquacultural experiments and veterinary investigations or died during transport or quarantine.
Due to lack of money an intensive control programme for periodically measuring water quality parameters as pH, alkalinity, N- and P-compounds, phytoplantkon primary production and weeddensity has only been conducted in one locality (Nols So). In this locality, however, the experiments came to a sudden end after a few months as the subject fish died during the winter 1978/79.
Generally the increase in biomass of the stocked grass carp has been used as a measure of the weed reduction. Rough estimates of fish biomass have been calculated from growth data and from mortalities reported in the period between two estimates. The fish were sampled once or twice a year to check growth in length and weight as well as general health.
Grass carp have been stocked into 14 separate localities between 1978 and April 1982 and two additional localities are to be stocked during this spring with fish imported from Austria later in 1981.
Initial stocking densities have ranged from 2 to 500 kg/ha but densities of about 100 kg/ha have been more common (Table 2).
Generally stockings have been one-time plantings of densities believed to ensure significant weed reduction shortly after stocking (i.e., in the second or third summer). In one locality (Vallensbaek Mose) the numbers of grass carp have been built up slowly during successive stockings (Table 2).
All available data on growth are given in Table 3. More detailed information on growth, survival and weed reduction are given below for individual localities.
3.3.1. Nols So
In May 1978 about 830 grass carp were stocked to control dense aquatic vegetation dominated by Myriophyllum spicatum L., Ceratophyllum demersum L. and filamentous green algae. The fish did well during summer and autumn and increased their weight by about 50 percent until November 1978.
The winter 1978/79 was very severe and the lake was covered with ice for more than three months. Four hundred and eighty grass carp were found dead but fresh in March when the ice disappeared and these fish had a mean weight increase of 51 percent indicating no loss in weight through the winter. No grass carp were found despite all the intensive fishings and it is assumed that fish stocked died during this period.
Although this experiment was very short-lived there were signs of effective grazing on the filamentous algae but no significant trends in weed density could be monitored.
3.3.2 Albertslund Kanal sections I and II
This canal runs through a dense urban district and is surrounded on both sides by pavement, bicycle tracks and low buildings. Sections I and II were originally separated by wire netting barriers, which broke down shortly after stocking. The results from these sections are, therefore, treated together.
There were heavy losses through the whole summer and autumn of 1978, presumably due to stress caused by the heavy traffic in the surroundings. All the remaining fish died during the winter of 1978/79 when this stretch of the canal was nearly frozen solid leaving only 3–5 cm of muddy anoxic water below.
No substantial weed reduction could be observed although the weed was dominated by Myriophyllum spicatum L. and Potamogeton natans L., which are both soft plants known to be esteemed by grass carp elsewhere.
3.3.3 Albertslund Kanal section III
Section III is upstream of sections I and II, with similar surroundings but with less traffic. The ice-cover of section III during the winter 1978/79 was not as thick as that of the downstream sections due to a groundwater inlet at the upstream end of the section.
Some fish died during the summer and autumn but losses were significantly smaller than in the downstream sections and the remaining fish survived the winter. The growth during the first growing season was however slow, only 19 percent, presumably due to stress caused by the traffic along the banks of the canal.
In September 1979 the remaining grass carp were transferred to locality 4 (Albertslund Radhusbassin). When transferred, the grass carp had reached a mean weight of 1 400 g.
Any weed reduction was difficult to quantity as the grass carp were found in small shoals confined to totally grazed areas, leaving the rest of the vegetation nearly untouched.
3.3.4 Albertslund Radhusbassin
This artificial lake constitutes an integrated part of the town hall of Albertslund. It has concrete walls and is completely surrounded by pavements and low buildings. The water is up to 2.5 m deep. The bottom consists of muddy sand as the lake receives urban storm water run-off as well as pumped ground water.
The grass carp stocked in May 1978 increased their weight by 129 percent during the first growing season. This rapid growth continued in the following years and the total weight increase during the 3.5 growing seasons from stocking until September 1981 was 320 percent. These growth data reflect rapid growth which decreases with increasing size. The decrease could be due to normal physiological growth-regulating mechanisms although shortness of food seems to be the more likely explanation as intense grazing during 1981 resulted in the extermination of the submersed macrophytic vegetation in the late summer of 1981. As a consequence the Water Quality Institute plans to transfer these grass carp to the nearby locality 5 (Vallensbaek Mose) where food is abundant.
Before stocking in May 1978, the lake water had high transparency and a dense macrophyte vegetation dominated by Myriophyllum spicatum L., Potamogeton pectinatus L. and macro algae belonging to the family Characeae. During 1981 the transparency declined markedly due to phytoplanktonic blooms and silver carp (Hypophthalmichthys molitrix Val.) were stocked in December 1981 for the first time in Denmark. One hundred kilogrammes (about 350 fish with a mean weight of about 300 g) was stocked and observations in April 1982 indicate that the silver carp have had a very limited weight-loss during the winter of 1981/82.
3.3.5 Vallensbaek Mose
Vallensbaek Mose is part of a recreational area southeast of Copenhagen. Vallensbaek Mose consists of two lakes of nearly equal size connected by a narrow canal. The lakes are used for sailing, canoeing and waterskiing, which uses were impeded by dense aquatic vegetation dominated by Elodea canadensis Rich., Potamogeton natans L. and Potamogeton pectinatus L. It was decided to try to reduce the weeds by means of herbivorous fish and in May 1978 about 115 grass carp were stocked. These were supplemented in August 1980 with about 475 additional fish. The fish stocked in 1978 and 1980 increased their mean weights by 114 and 55 percent, respectively, during the first growing season. As the fish stocked in 1980 had a short growing season due to the late stocking, the growth of the two groups are of the same order of magnitude.
Dead grass carp have not been reported even during or after this latest very severe winter of 1981/82.
It has been impossible to quantify the extent of weed reduction as the weed was reduced mechanically in the summer of 1980. The weed reduction caused by grass carp must, however, have been considerable as indicated by the very fast growth reported for the fish from this locality, where the mean weight of the earliest fish stocked is expected to exceed 3 kg in the spring of 1982.
3.3.6 Avnso
Eleven grass carp were stocked in 1979 and were all found dead in the spring of 1980. This shallow lake receives road surface run-off and has a thick organic sediment. Although the ice-cover lasted for only about 1.5 months in the winter of 1979/80 the water has presumably become anoxic due to the high oxygen demand of the sediment. The 3–5 cm thick layer of Lemna minor L., which covered all the pond surface when the fish were stocked showed no signs at all of being grazed by the grass carp.
3.3.7 Pennesoen
Four of eleven grass carp stocked in 1979 were found dead in the spring 1980. The owners of this shallow pond believe that the fish have been of some use but growth of the surviving fish is uncertain and cannot confirm any possible reduction in weed.
3.3.8 Duevej So
Five of eleven grass carp stocked in 1979 were found dead in the spring 1980. The owners believe that the fish had been of some use but growth of the surviving fish is uncertain and cannot confirm the presumed reduction in weeds.
3.3.9 Krenkerup Voldgrav
In September 1979 this rather shallow most was stocked with about 245 grass carp of medium size (mean weight about 412 g). The growth has been very fast. In the first and second year after stocking the fish increased their mean weight by 73 and 106 percent, respectively. No dead grass carp have been reported during this period. But in the winter of 1981/82, when the ice-cover lasted for about 3.5 months, approximately 150 grass carp died showing clear signs of asphyxiation as they all crowded around a little opening in the ice at the water inlet to the most. Whether some of the originally stocked 245 grass carp are still alive is yet uncertain.
The reduction in weeds in this locality is reported to be very satisfactory. When the grass carp were stocked, Ceratophyllum demersum L. made up nearly 100 percent of the dense weed, which covered about 70 percent of the lake surface. In the autumn of 1981, however, only a thin layer of this species was left on the lake bottom.
3.3.10 Fuglsang Park Voldgrav
This moat was stocked in September 1979 with about 200 grass carp of the smallest size group used (mean weight 174 g). Due to a very early ice-covering the first winter after stocking (1979/80) the growth has not been measured. More than 50 percent of the stocked fish were found dead when the ice melted, and presumably all grass carp must have died during this winter, as no grass carp at all were caught during two experimental fishings in 1980 and 1981, respectively.
The grass carp had no obvious effect on the dense vegetation of mainly Ceratophyllum demersum L. and Elodea canadensis Rich., which covered the total lake surface.
3.3.11 Glorup Park Fontainedam
In this artificial fountain lake about 160 grass carp of the largest size group used (mean weight 815 g) were stocked in September 1979. The lake had at this time clear water (visibility more than 2 m (to the bottom)). The vegetation was very dense. A thick layer of Ceratophyllum demersum L. covered the bottom nearly totally, and Potamogeton crispus L., Potamogeton natans L. and Lemna minor L. were abundant together with green epiphytic filamentous algae.
Growth of fish has been rather slow in this locality - only 23 percent during the first growing season. As food seems abundant the slow growth could be a result of low summer temperatures resulting from shading by trees which surround the lake on nearly all sides.
3.3.12 Holstenshuus Voldgrav
In September 1979 about 105 grass carp with a mean weight of 815 g were stocked to graze Phragmites communis L. and Typha latifolia L. The lake had no submersed vegetation and only a narrow zone of other macrophytes along the banks.
The growth has been rather slow in this locality which could be a consequence of the food items present. It is uncertain whether the grass carp have grazed on the Phragmites but as the soft species present has not been significantly reduced this is a possibility. Only two dead grass carp have been reported from this locality. These fish have been injured during transport or stocking.
3.3.13 Shell Reservoiredam
In September 1979 about 75 grass carp with a mean weight of 412 g were stocked in this artificial reservoir. The water was clear (visibility about 3 m) and the vegetation very sparse consisting mainly of spots of Potamogeton natans L. and Potamogeton pectinatus L.
Intensive fishings in 1981 showed no grass carp at all which was quite inexplicable as only a limited number of fish were reported dead - mainly shortly after stocking. The grass carp must either have died during the winter of 1979/80 when the lake was ice-covered for more than 2.5 months or have served as prey for a resident population of rainbow trout (Salmo gairdneri Rich.) which had been stocked for angling. Very large scales have been found in the stomachs of large rainbow trout (2–3 kg).
The grass carp have had no effect on the sparse macrophytic vegetation.
3.3.14 Oster Molle Soen
In this shallow lake about 475 grass carp were stocked in August 1980 where the water surface was nearly covered by dense coherent vegetation consisting of Elodea canadensis Rich. with epiphytic filamentous green algae. The transparency was about 1 m.
The grass carp have grown very fast in this locality as the increase in mean weight during the first growing season has been 146 percent. In spite of this, no significant trend in the density of weeds could be detected. On the other hand there seemed to be a slight decrease in the transparency presumably caused by an increased phytoplankton growth.
In 10 of 14 localities weed density has been reduced to a varying extent, from marked reductions to reductions appearing to be only just perceptible.
The localities where no weed reduction have been recorded include waters where the stocked population has been completely exterminated during the first year or winter after stocking (localities 3, 6, 10 and 13).
The localities where the grass carp have had only limited effect include some localities where the stock has been depleted or completely exterminated during the first year or winter after stocking (localities 1, 2, 7 and 8) and localities where growth due to other reasons has been slow to moderate (localities 11 and 12).
The localities where the grass carp have had a substantial effect on the density of aquatic weeds include waters where the stocked fish have displayed yearly weight increments between 70 and 150 percent (localities 4, 5, 9 and 14).
There have been no reports of any outbreak of fish disease and the stocked fish have shown no signs of being more vulnerable to normally occurring fish pathogens than the indigenous Danish fish species.
This paper could not have been written without the help of several persons. I, therefore, wish to express my deep gratitude to my old colleagues Ivar Warrer-Hansen and Sverre Mohr Mortensen at the Water Quality Institute, Horsholm for supplying parts of the latest data, to Hans Heidemann-Lassen at the Water Authority of the County of Northern Jutland for supplying data concerning weed density in Lake Oster Molle Soen. Further, I am indebted to the staff of the Inland Fisheries Laboratory, Silkeborg, especially to Jorgen Dahl (Head of the Laboratory) and Gorm Rasmussen for supplying investigational equipment. Finally, I wish to express deepest gratitude to my wife, Susse Mohr, for invaluable practical help during stocking and monitoring activities and linguistic and professional criticism of this paper.
Christensen, J., 1966 Are grass carp the solving of our weed problems? Fiskeriundersogelser 1965. Pap.Dan.Inst.Fish.Mar.Res., (26): 72–4 (in Danish)
Denmark, County of Frederiksborg, 1966 Recipient waters in the County of Frederiksborg 1963–66. Report. County of Frederiksborg, Water Authority, pp. 8–9 (in Danish)
Denmark, 1971 Report from the water authority of the County of Frederiksborg concerning the period 1967–71. County of Frederiksborg, Water Authority, 4 p. (in Danish)
Denmark, County of Northern Jutland, 1980 Report concerning experimental weed-control with grass carp in Lake Nols So 1978/79. Report. County of Northern Jutland, Water Authority, June 1980, 52 p. (in Danish)
Hansen, A., 1969 A note concerning grass carp. Yearb.Soc.Water Auth.Inspectors, (41): 19 p. (in Danish)
Markmann, P.N., 1979 Grass carp and other herbivorous fishes. Vand, 1979 (3): 64–74 (in Danish)
Markmann, P.N. and I. Warrer-Hansen, 1980 Experimental stocking of grass carp in small water-bodies in Albertslund Municipality, May 1978. Report. Horsholm, Denmark, Water Quality Institute, January 1980, 30 p. (in Danish)
Mortensen, S.M., 1982 State of affairs report to the Danish Veterinary Services and the Ministry of Fisheries concerning grass carp in Danish lakes. Report. Horsholm, Denmark, Water Quality Institute, May 1982 (in Danish)
Table 1 Danish grass carp imports 1978–81 and their origin
| Year | Biomass | Origin |
| 1978 | 1 000 kg | The Netherlands |
| 1979 | 400 kg | Sweden |
| 1980 | 500 kg | Sweden |
| 1981 | 1 700 kg | Austria |
| 1978–82 | 3 600 kg |
Table 2 Stocking localities in Denmark 1978–82 together with data on stocking densities and size of stocked fish. Data in brackets represent estimates of total biomass after supplementary stockings
| Locality | Stocking date | Stocking localities | Area | Biomass stocked | Mean weight of fish | Stocking density |
| No. | ha | kg | g | kg/ha | ||
| 1 | 5/5/78 | Nols So | 2.80 | 440 | 530 | 160 |
| 2 | 5/5/78 | Albertslund Kanal I | 0.26 | 80 | 498 | 300 |
| 2 | 5/5/78 | Albertslund Kanal II | 0.34 | 170 | 498 | 500 |
| 3 | 5/5/78 | Albertslund Kanal III | 0.65 | 64 | 498 | 100 |
| 4 | 5/5/78 | Albertslund Radhusbas. | 1.00 | 100 | 498 | 100 |
| 5 | 5/5/78 | Vallensbaek Mose | 35.00 | 57 | 498 | 2 |
| 5 | 29/8/80 | Vallensbaek Mose | - | 280 | 587 | 8(15) |
| 5 | 21/12/81 | Vallensbaek Mose | - | 1 200 | 645 | 35(60) |
| 6 | 13/9/79 | Avnso | 0.05 | 5 | 446 | 100 |
| 7 | 13/9/79 | Pennsoen | 0.04 | 5 | 446 | 125 |
| 8 | 14/9/79 | Duevej So | 0.04 | 5 | 446 | 140 |
| 9 | 18/9/79 | Krenkerup Voldgrav | 1.00 | 100 | 412 | 100 |
| 10 | 18/9/79 | Fuglsang Park Voldgrav | 0.40 | 35 | 174 | 90 |
| 11 | 22/9/79 | Glorup Fontainedam | 1.30 | 130 | 815 | 100 |
| 12 | 24/9/79 | Holstenshuus Voldgrav | 0.85 | 85 | 815 | 100 |
| 13 | 24/9/79 | Shell Reservoiredam | 1.00 | 30 | 412 | 30 |
| 14 | 30/8/80 | Oster Molle Soen | 2.20 | 220 | 587 | 100 |
| 15 | Spring 82 | Agerso | 2.00 | 150 | - | - |
| 16 | Spring 82 | Naesbyholm Voldgrav | 0.70 | 75 | - | - |
Table 3 Mean weight in g of grass carp on stocking date and biomass-estimate days. In brackets the weight increase in percentage of the weight at stocking are given
| Year of stocking | Locality | Mean weighting and in brackets weight increase in percentage since stocking | |||||||||||
| 1978 | 1979 | 1980 | 1981 | ||||||||||
| No. | May | Nov. | May | Sept. | Aug. | Oct. | Apr. | Jul. | Aug. | Sept. | Oct. | Dec. | |
| 1 | 520 | 8001 | |||||||||||
| 2 | 498 | - | 5972 | ||||||||||
| (19) | |||||||||||||
| 3 | 498 | 1 | |||||||||||
| 1978 | 4 | - | - | - | - | - | 1 400 | - | - | ||||
| (180) | |||||||||||||
| 4 | 498 | - | 1 143 | - | - | - | - | - | - | 2 100 | - | - | |
| (129) | (320) | ||||||||||||
| 5 | 498 | - | 1 068 | - | - | - | 2 225 | - | - | - | - | - | |
| (114) | (347) | ||||||||||||
| 6 | 4463 | ||||||||||||
| 7 | 446 | - | - | - | - | - | - | - | - | ||||
| 8 | 446 | - | - | - | - | - | - | - | - | ||||
| 9 | 412 | - | 714 | - | - | - | 1 472 | - | - | ||||
| (73) | (257) | ||||||||||||
| 1979 | 10 | 1743 | |||||||||||
| 11 | 815 | - | 1 004 | - | 1 417 | - | - | - | - | ||||
| (23) | (73) | ||||||||||||
| 12 | 815 | - | 1 068 | - | 1 276 | - | - | - | - | ||||
| (31) | (56) | ||||||||||||
| 13 | 4123 | ||||||||||||
| 5 | 587 | - | 910 | - | - | - | - | - | |||||
| (55) | |||||||||||||
| 1980 | 14 | 587 | - | - | - | 1 449 | - | 1 690 | - | ||||
| (146) | (187) | ||||||||||||
| 1981 | 5 | 645 | |||||||||||
1 All fish died during the winter 1978/79
2 All fish transferred to locality 4 in autumn 1979
3 All fish died during the winter 1979/80
