D. Barthelmes
Institut fur Binnenfischerei
Berlin, German Democratic Republic
ABSTRACT
On the basis of stocking experiments with up to 10 000 silver carp/ha in ponds and eutrophic lakes, heavy stocking may be defined as that level required to establish metabolic silver carp biomasses of more than 1 t/ha. There have been very few experiments in lakes to date. Ther was no evidence for massive alteration of phytoplankton biomass by a stock of 10 000 individuals of age 2/ha in a highly eutrophied, weakly stratified lake nor did H2S accumulation in the “hypolimnion” alter significantly. Conditions for bream Abramis brama (L.)) and eel (Anguilla anguilla L.) remained relatively good. In contrast zooplankton declined sharply, experimental stocking of pike-perch (Stizostedion lucioperca (L.)) fry was unsuccessful and survival of perch fry was reduced (Perca fluviatilis L.)). Since there was high zooplankton production in the littoral, growth and probably survival of cyprinid fry was good. As heavy silver carp stocking renders seine fishing impossible overcrowding with indigenous cyprinids may result with a consequent slowing of growth of individual silver carp. At present optimization of total yield in small, highly eutrophic lakes in the German Democratic Republic is attempted by stocking 1 000 two-year old individuals/ha with total harvesting every four years. An alternative might be stocking smaller numbers each year and catching the silver carp by gill netting at greater age. The long-term effects of both management strategies are still obscure.
RESUME
Sur la base des expériences de repeuplement de lacs eutrophes et d'étangs à carpes avec des quantités de carpes argentées pouvant aller jusqu'à 10 000 individus à l'hectare, le repeuplement intensif en carpes argentées peut être défini comme l'établissement de biomasses métaboliques supérieures à l t à l'hectare environ. Jusqu'à présent, on a fait très peu d'expériences dans les lacs et rien ne permet de dire qu'un stock de 10 000 individus à l'hectare dans un lac faiblement stratifié et très eutrophe entraîne une altération massive de la biomasse du phytoplancton. De même, l'accumulation de H2S dans l'hypolimnion ne change guère. Les conditions sont restées assez favorables aux brèmes (Abramis brama L.) et aux anguilles (Anguilla anguilla L.), comme elles l'étaient déjà auparavant. En revanche, on a observé une nette diminution du zooplancton en cas de repeuplement au taux de 10 000 individus à l'hectare; par ailleurs, les essais de repeuplement avec du frai de sandre (Stizostedion lucioperca (L.)) ont échoué et le frai de perche (Perca fluviatilis L.) n'a pas survécu. Etant donné que la production de zooplancton était importante dans le littoral, la croissance du frai des cyprinidés a été bonne et il en a probablement été de même de leur taux de survie. Comme le repeuplement intensif en carpes argentées rend impossible la pêche à la senne, on risque d'aboutir à un surpeuplement de cyprinidés indigènes, d'où un ralentissement considérable de la croissance des carpes argentées. On essaie d'optimiser le rendement total des petits lacs très eutrophes de la République démocratique allemande en utilisant 1 000 individus à l'hectare et en procédant à une pêche complète tous les quatre ans. Une autre solution serait de déverser de petites quantités de carpes argentées chaque année et de capturer ce poisson au filet maillant lorsqu'il a atteint un stade de développement plus avancé. On connait encore mal les effets à long terme de ces deux stratégies d'aménagement.
When rearing of silver carp spread over European countries in the fifties the main objectives were to increase fishery yields from eutrophic waters and to improve water quality in the stocked ecosystems. The practicality of these objectives have been tested mainly in carp ponds and in similar waters. The results from the experiments in ponds are taken as the starting point for the experiments in lakes described below. In spite of the limitations of one-year class of fish only and well mixed water column, carp pond experiments with silver carp have yielded valuable basic information on how the fish interact with their environment.
Silver carp induce alterations in the environment at a threshold value of feeding pressure somewhat below 1 000 3–4 year old individuals/ha for zooplankton and between 1 000 and 2 000 3–4 year old individuals/ha for phytoplankton (Barthelmes and Kleibs, 1978). Zooplankton declines, the smaller sized elements being most vulnerable, while phytoplankton increases, as does primary production. Related alterations take place in phytoplankton-dependent variables such as nutrient and oxygen concentrations or pH (Opuszynski, 1979, 1979a). When these thresholds are converted to the more informative figures of metabolic silver carp biomass (mean individual fish weight for the growing season multiplied by the exponent 0.8 according to Vinberg (1956) and then further multiplied by the number of fish present) zooplankton decline begins at about 0.5 t/ha and phytoplankton increase begins at about 1 t/ha. There is some evidence that temperature (Barthelmes and Kleibs, 1978) and trophic state (Adamek, pers.comm.) influence these thresholds which seems reasonable since temperature accelerates nutrient recycling by the fish. Furthermore, a given amount of nutrients will produce more pronounced effects on primary production if nutrients are in shorter supply.
Metabolic silver carp biomasses in excess of these thresholds due to heavy stocking have only slow, predictable effects on phytoplankton as opposed to effects on fish yield and the rapid decrease of zooplankton. Indeed, phytoplankton may increase (e.g., Tujutjunik et al., 1976, Henderson, 1977, Januszko, 1978) decrease (e.g., Milanovski, 1974; Kajak et al., 1975, Barthelmes, 1975; Schroeder, 1978; Bednadz et al., 1978) and/or change in community structure (e.g., Grygierek, 1973; Januszko, 1974 and the papers cited above). These effects do not always correlate well with observations on the composition of the food of the fish implying indirect and complex relationships (Kajak, 1977). Fish yield increases continuously with stocking density above the threshold values, but the increment per unit stocking reaches a maximum and then decreases. The maximum may possibly reach 1 to 1.5 t of fish flesh increment per hectare under mid-European conditions in carp ponds (Opuszynski, 1976; Barthelmes and Kleibs, 1978). In warmer regions of the world yields are much higher. For instance, Liang et al. (1981) report maximum yields in China (31°N, 114°E) of up to 14 t/ha, though the stocked waters seems to have trophic conditions similar to carp ponds in Europe. It is not clear from Liang et al. (1981) what changes in phytoplankton and zooplankton underlie these figures and, therefore, what would be the possible effect on other indigenous fish species. In any case, the apparent influence of temperature on fish and on the thresholds strongly suggests that comparisons of results can only be made between waters with identical or similar climatic conditions. For this reason, in the following experiment 10 000 two-year old individuals/ha were stocked into a small lake of 5 ha (L. Grunz) approximately 100 km NNE of Berlin. In the same area and at the same time additional experiments were performed in other lakes, stocking densities were only 1 000 2-year old individuals/ha. However, the results are only quoted exceptionally, since the experiment with the greatest stocking density may be expected to give the strongest effects on the environment and the resident fish fauna.
The experiment in Lake Grunz was only one of several experiments involving lesser stocking rates. Thus, the results demonstrating effects on the indigenous fish stocks should be considered somewhat more than preliminary. A detailed description of the experiment in Lake Grunz is given in Barthelmes et al. (1982) and only the main background information and the results are presented here. The lake area is 5 ha with a maximum depth of 6 m. The water mass is weakly stratified in summer, and a real hypolimnion is missing. Each year, soon after formation of the thermocline, H2S appears at depths greater than 3 m. The lake is hypertrophic with primary production greater than 500 g C/m2/year; total phosphorus - 1 493 mg/m3; chlorophyll a - 127 mg/m3 and Secchi depth of 0.44 m (mean values for the growing seasons from 1978 to 1981). Silver carp were stocked in the autumn of 1977 and harvested in early spring 1981. Results are shown in Fig. 1. The return was 79.3 percent of the number stocked. One year later the lake was fished again and the number of returns rose to over 90 percent (not included in Fig. 1).
Fig. 1 shows that individual gain in weight decreases with time which suggests that the food of silver carp becomes poorer in quality. The quantity of food available did not change, however, if the first sample with its predominance of Microcystis is excluded (Fig. 2). This interpretation is justified since there was no trend in phytoplankton abundance, parallelling the decline in growth of the fish. Zooplankton reacts more to the growing silver carp biomass as was expected from the carp-pond experiments (Fig. 3). This implies that zooplankton represents an important component of high quality food. Apparently feeding pressure by silver carp and indigenous cyprinids together is sufficient to reduce zooplankton population to low levels, as may be seen to some degree in the experiments with the lower stocking density of 1 000 fish/ha. Since the indigenous cyprinid stocks are only thinned out by common fishing gear and the small, zooplankton-eating individuals especially are likely to increase in response to strong fishing pressure on the adults these compete with silver carp. Because the trophic state reached nearly maximum values and primary production was highly light-limited, the silver carp yields of 377 kg/ha as in Lake Grunz, might represent the maximum possible yield from cyprinid lakes in mid-Europe, however, this figure should be kept under continuous review. The decline of zooplankton is of importance especially to the indigenous fish stocks but as in Lake Grunz littoral zooplankton compensates to some degree for losses in the open waters of the lake and provides a very good food base for fish fry spawned in the littoral. This was confirmed by special investigations on littoral zooplankton production as opposed to zooplankton production in the limnion (Wellner, 1980) and by investigations on the growth of fish fry in the littoral. Zoobenthos was present in only moderate numbers and biomass and its area of occurrence was not reduced by an assumed increase of the part of the bottom influenced by H2S. The most important bottom-feeding fish species (by number) of the lake developed as well as before. These include white bream (Abramis bjorkna (L.)), roach (Rutilus rutilus (L.)) and eel (Anguilla anguilla L.), the latter being stocked in 1977 with approximately 400 specimens/ha and 19 g individual weight. It was also found by mark and recapture that the nearly closed lake still contained practically all the stocked eel in 1979, the individual weight of which had increased to 110 g. In 1979 the stock of white bream amounted to 2 425 ± 80 percent individuals/ha of greater than 15 cm total length. Growth of this species was near the mean growth in northern Germany (Bauch, 1953) both before and after stocking with silver carp (Fig. 4). No signs of massive growth alteration between years were found by scale analysis. In 1979 the number of roach of more than 15 cm long was 1 652± 43 percent/ha. Its growth was also near the mean growth in the region investigated (Fig. 5). Bream (Abramis brama (L.)) was nearly eradicated by very strong fishing pressure 5–10 years prior to our stocking experiment in connexion with carp management of Lake Grunz but was re-established by the time of our investigations. When the silver carp were harvested in 1980/81 the yield of roach, bream and white bream was 192 kg/ha with bream amounting to 35 percent in weight. The growth of bream was very good and scale analysis revealed a growth increase during the years when silver carp were present, but this was probably connected with the shift in feeding from zooplankton to zoobenthos (Fig. 6).
In contrast to the littoral spawning cyprinids which eat zoobenthos as adults, these species that spawn in the sub-littoral and have pelagic zooplankton-eating fry seem to be adversely affected. In 1979 Lake Grunz was stocked with 1 000 pike perch (Stizostedion lucioperca (L.)) fingerlings approximately 2 cm long. Only a few of these fish were found in 1980/81 and their growth was only moderate. The very small return contrasts sharply with the spawning success of these few survivors in 1981 when the silver carp stock was strongly reduced by harvesting and zooplankton began to recover. Today there is a multitude of young, well-grown pike-perch in Lake Grunz. While silver carp were present in the lake survival of perch fry was also very low. There were two other species which were at least partly planktonophage and which were initially abundant in the lake, Leucaspius delineatus (Heckel) and Alburnus alburnus (L.) which were not investigated. However, from the results of several electrofishings for eel it appeared that the number of these fish did not change much in response to silver carp stocking, possibly because they fed more in the littoral.
It seems advisable to manage suitable small lakes by stocking with 1 000 two-year old fish/ha for periods of four years. Under these conditions some competition for zooplankton apparently exists, which in severe cases may lead to stunting of both indigenous fish and silver carp, but this effect appears to be within tolerable limits. If the full growth potential of individual silver carp is desired however, it seems necessary to further reduce the numbers stocked within the temperature limits prevailing in central Europe to only 100 fish/ha or so. In any case, the niche occupied by silver carp seems to overlap with that of indigenous fish more than was thought initially. Moreover, the influence of silver carp on water quality is less than was assumed earlier. In Lake Grunz much of the phytoplankton (Oscillatoria) was not eaten or digested so this genus became predominant in the phytoplankton. Thus, the only effect of silver carp was to shift the phytoplankton structure from Microcystis-dominated to Oscillatoria-dominated associations. This slight effect of silver carp on water quality was expected because of the rapid recycling of nutrients in non- or weakly-stratified water bodies and because of heavy loading with nutrients from the surrounding land. This should not be taken as an argument against further heavy silver carp stocking investigations in strongly stratified lakes. Indeed such work is badly needed.
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Fig. 1 Increase in mean weight and biomass of one batch of silver carp stocked into Lake Grünz
Fig. 2 Abundance of phytoplankton in Lake Grünz from 1977 to 1981
Fig. 3 Abundance of zooplankton in Lake Grünz from 1977 to 1981
Fig. 4 Growth of white bream (Abramis bjoerkna (L.)) in Lake Grünz (
) in relation to mean growth
in the area according to Bauch, 1953 (x). Density of white bream > age 6 in 1979,
2 425 sp/ha ± 80 percent (mark and recapture, P = 95 percent)
Fig. 5 Growth of roach (Rutilus rutilus (L.)) in Lake Grünz () in relation to mean growth in the area
according to Bauch, 1953 ( x ). Density of roach > age 4 in 1979, 1 652 sp/ha ± 43 percent (mark
and recapture, P = 95 percent)
Fig. 6 Growth of bream (Abramis brama (L.)) in Lake Grünz () in relation to mean growth in the
area according to Bauch, 1953 ( x ). Density of bream of age 4 and 5 ca. 300–500 sp/ha
beach seining)
