Carp Stocking Research Articles

The effects of partially substituting Indian carps or adding silver carp on polycultures including small indigenous fish species (SIS)

A sustainable semi-intensive pond aquaculture technology including major carp species (Indian, Chinese and common carp) as cash-crop and small indigenous fish species (SIS) as food for the farmers' families is being optimized in Bangladesh. Silver carp inclusion in the polyculture is now being considered, because this very efficient filter feeder has a strong impact on pond ecology and also on the farmers' family nutrition because it is a cheap fish that the family can afford to eat instead of selling. The present paper is centered on the reduction of silver carp negative effects on other species while keeping the advantages of increased total yield and income due to silver carp stocking. It presents the results of two experiments, one on-station and one on-farm, in which 3–5 silver carp/100 m 2 were added or partially substituted major carp filter feeders. The basic stocking density was 100 carps (rohu, catla and a bottom feeder, either mrigal or common carp, at a 1:1:1 ratio) and 250 SIS (punti and mola) per 100 m 2. In the on-station experiment silver carp density was 3 and 5 fish/100 m 2 and the large carp bottom feeder was common carp. In the on-farm experiment silver carp density was 5 fish/100 m 2 and the bottom feeder was either common carp or mrigal. Most of the water quality and fish performance parameters tested were not affected by the polyculture composition. Adding 3–5% silver carp or substituting 3–5% of the herbivorous fish species by this highly efficient filter feeder increased grazing pressure on the phytoplankton, which led to a 25–40% reduction of the chlorophyll concentration in the water column. The increased grazing pressure was not enough to affect other water quality parameters and fewer effects on the availability of food for the other fish species occurred than when the silver carp addition was 10% of the polyculture, as reported in a previous work. The strong negative effects of silver carp on the other species of the polyculture and the higher total yields and income recorded in previous experiments with the addition of 10 silver carp/100 m 2 were much weaker and their expression depended on other pond conditions when 3 or 5 silver carp/100 m 2 were added or substituted the same number of rohu or catla, either when the bottom feeder was mrigal or common carp. It was concluded that stocking 3 silver carp/100 m 2 over the usual 100 large carp and 250 SIS /100 m 2 can be considered a ‘no effect’ stocking density in relation to the control without silver carp, while stocking 10 silver carp/100 m 2 should be preferred by farmers to keep the option of selling or consuming the silver carp.

Fluctuating asymmetry, abnormalities and parasitism as indicators of environmental stress in cultured stocks of goldfish and carp

Knowledge about the effects of captivity and environmental stress on fish populations is basic to culture them for ornamental, conservation or exploitation purposes. The aim of this study was to test how captivity affect morphological development of bilateral characters and whether levels of fluctuating asymmetry, morphological abnormalities and ectoparasites reflect developmental stress caused by adverse environmental conditions in two species of cyprinids, goldfish Carassius auratus and two varieties of carp Cyprinus carpio, scale and mirror carp. Samples from captivity were caught in an artificial pond (Madrid City, Spain), which presents poor environmental conditions; while samples from nature were captured in the Ebro River basin (NE Spain). Six meristic and four morphometric bilateral characters were measured and compared between the two samples. Additionally, the incidence of morphological abnormalities and ectoparasites was also recorded. Captive goldfish presented significant lower number of scales of the lateral line, scale rows above lateral line and length of pectoral and ventral fins. Captive carps showed significant shorter first barbels and longer ventral fins. Ornamental goldfish showed significant higher levels of fluctuating asymmetry than wild samples for number of branched rays of pectoral fin, lateral line, scale rows below lateral line and length of pectoral and ventral fins. Ornamental carps presented a similar trend for the same characters, except for branched rays and length of pectoral fins, together with the length of second barbels. In the artificial pond, the incidence of abnormalities was higher in carp, whereas goldfish showed a higher occurrence of ectoparasites. These facts support the hypothesis of a different species response to environmental stress. The overall results indicate that fish from the artificial pond are subjected to environmental stress caused by adverse conditions such as overcrowding and lower water quality. Moreover, estimated parameters in this study mean a useful tool to detect developmental instability in confined fish.

Genetic diversity in common carp stocks assayed by random-amplified polymorphic DNA markers

The common carp (Cyprinus carpio L.) is one of the major aquaculture species, contributing nearly 35% to the inland fish production in Karnataka, India. Stocks collected from Hungary (2), Indonesia and Vietnam were assessed alongside two local stocks in a series of culture performance trials with the objective of setting up a base population for developing a breeding programme. The present study deals with the genetic divergence and polymorphism in these six stocks using random-amplified polymorphic DNA (RAPD) markers. A total of 180 decamer random primers were screened for polymerase chain reaction (PCR) amplification (OPA 1-20, OPB1-20, OPC1-20, OPD1-20, OPE1-20, OPF1-20, OPG1-20, OPP1-20 and OPM1-20). Eight primers were selected for analysis of common carp genotypes (OPA-7, OPA-20, OPB-17, OPF-10, OP F-9, OPG-4, OPG-9 and OPP-16). Out of 492 bands recorded, 57.1% were polymorphic. Stepwise regression analysis was carried out to find best combination markers affecting body weight (P<0.001). The results demonstrate major differences in the genetic structures between different stocks. Dendrogram data showed grouping of individuals according to stocks and corresponding data variables revealed the per cent homology within the stock and also found markers correlating to the body weight.

Impact of carp stocking on the yield and biodiversity of nonstocked indigenous fish species in selected oxbow lakes of Bangladesh

Impact of carp stocking on the yield and biodiversity of nonstocked indigenous fish species in selected oxbow lakes of Bangladesh

Effects of the phytoplanktivorous silver carp (Hypophthalmichthys molitrixon) on plankton and the hepatotoxic microcystins in an enclosure experiment in a eutrophic lake, Lake Shichahai in Beijing

The responses of nutrients, water transparency, zooplankton, phytoplankton and microcystins to a gradient of silver carp biomass (0, 18, 55, 110 g/m3) were assessed using enclosures in Lake Shichahai (Beijing). Picophytoplankton biomass increased with increasing fish stocking density (r=0.64, p=0.09). Silver carp significantly depressed zooplankton biomass, and thus, zooplankton grazing was too low to control phytoplankton. Intracellular microcystin (MC) content in the enclosures was correlated only to Microcystis biomass in the present study. Microcystis spp. biomass and intracellular microcystins content were much higher in lake water than those of enclosures with and without stocking fish. Stocking of silver carp could be an appropriate in highly productive Lake Shichahai, which naturally lacks of large cladoceran zooplankton. A fish stocking density of 55 g/m3 was most efficient at controlling Microcystis blooms and increasing water clarity. Mean extracellular MC concentration in the lake water was almost the same with that of the enclosures with fish.

The Public Sector Role in the Establishment of Grass Carp in the United States

On 16 November 1963, the U. S. Fish and Wildlife Service Fish Farming Experimental Station at Stuttgart, Arkansas, became the first institution to import grass carp (Ctenopharyngodon idella) into the United States. This introduction was the result of at least seven years of effort to find an effective biological control for problematic aquatic weeds. The introduction was in keeping with a strong environmental and political mandate of that day to replace the broad use of chemicals with biological controls. For about 10 years, federal and state agencies and university systems strongly promoted introductions, spawning, and nationwide stocking of the grass carp. In 1966, the USFWS laboratory at Stuttgart, Arkansas, was apparently responsible for the first accidental release of grass carp to the environment. By 1972, grass carp were stocked in open water systems, documented in 16 states, and established in the Mississippi River system. All this occurred before the first private-sector commercial producers received and spawned the fish in 1972 and 1973, respectively. From 1972 to 1985, public agencies were actively involved in research to produce non-reproductive grass carp for stocking purposes and in 1983, in conjunction with a private producer, these efforts led to the development of triploid grass carp. In 1985, an official U.S. Fish and Wildlife Service biological opinion was rendered that grass carp triploids were an environmentally safe, weed-controlling fish. In the same year, the U.S. Fish and Wildlife Service established a triploid grass carp ploidy inspection program that opened the way to ship certified triploid grass carp around the country. From 1985 to 2005, more than 7 million triploid grass carp were shipped throughout the United States. With grass carp now officially recorded from 45 states it is important to be aware of (or remember) the facts surrounding its introduction, early spread, present usage, and need. Understanding these will in turn will allow for more rational decisions to be made concerning the future of these fish.

Changes in the Phytoplankton Community of Lake Donghu Since the 1980s

ABSTRACT The phytoplankton of Lake Donghu was compared between two periods (i.e., 1982–83 and 1999–2001). During 1982–1983, Lake Donghu was characterized by cyanobacterial blooms, very high summer phytoplankton biomass, and a spring clear-water phase. However, during 1999–2001, the lake was dominated by dinophytes and diatoms and had a relatively low phytoplankton biomass and a very turbid state. Increased stocking of silver carp and bighead carp during the interim years eliminated cyanobacterial blooms and large-bodied daphnids from the lake, creating favorable conditions for growth of the small-sized algae.

Microsatellite-based genetic variability and differentiation of domesticated, wild and feral common carp ( Cyprinus carpio L.) populations

In the past, the genetics of common carp populations has mainly been studied on regional levels using traditional protein markers. We analysed four microsatellite loci of 577 individuals from 22 domesticated, wild and feral populations spanning a geographical area as wide as from Europe through Central Asia to East and South-East Asia. The variability of these loci was much higher than that of the allozyme loci examined in our previous studies. A total number of 143 alleles were recorded across loci ranging from 27 at MFW28 to 47 at MFW7. However, the mean number of alleles per locus was remarkably lower: from 2.50 in a captive stock of River Amur wild carp to 14.25 in a wild population from Lake Arnasaiskie, Uzbekistan, and domesticated Chinese carp from Wuhan. The variability of populations did not show a clear geographical pattern but a highly significant difference was found in allelic richness between the 13 domesticated/captive stocks (average A r = 4.436) and the nine wild−caught populations (average A r = 8.221). The prevalence of different alleles in different populations resulted in a high degree of population differentiation: the F ST values for all but four pairwise comparisons of Central Asian wild populations were significant. Within geographical regions the D A distances between populations were smallest in Central Asia (average D A = 0.139), intermediate in Europe (average D A = 0.434) and largest in East/South-East Asia (average D A = 0.833). Between geographical regions the European and Central Asian populations showed the smallest distances (average D A = 0.484) whereas the distances of these two regions and East/South-East Asia were almost identical but substantially larger (Europe vs. East/South-East Asia: average D A = 0.801, and Central Asia vs. East/South-East Asia: average D A = 0.806). All populations clustered into only two highly divergent major groups with 91% bootstrap support: Europe/Central Asia and East/South-East Asia. Thus, the microsatellite data also suggest an ancient separation of European/Central Asian from East/South-East Asian carp and a single origin of European carp in Central Asia as already inferred from our previous allozyme and mtDNA RFLP studies. The taxonomic status of subspecies assigned to European ( C. c. carpio) and East Asian carp ( C. c. haematopterus) is supported. However, because of their close relationship to European carp the Central Asian carp do not deserve a separate subspecies status ( C. c. aralensis).

Population biology of carp (Cyprinus carpio L.) in the mid-Murray River and Barmah Forest Wetlands, Australia

The present study quantitatively describes a significant stock of carp (Cyprinus carpio L.), an exotic pest species, in a temperate riverine floodplain wetland. Intensity and duration of flooding influenced relative abundance, distribution and recruitment. Average growth (mm) in length was described with the von Bertalanffy growth model for males (L∞ = 489, k = 0.249, t0 = –0.519), and females (L∞ = 594, k = 0.177, t0 = –0.609) to age 28. Variation in growth was described with a lognormal distribution of k. Total mortality (Z year–1) was 0.268–0.407 for males, 0.311–0.422 for females, 3.24 for age-0 juveniles and 1.80 for age-1 juveniles. Natural mortality (M year–1) was 0.199 for males and 0.262 for females. Fishing mortality (F year–1) was &lt;0.05 for males and 0.11–0.30 for females. Gonadal changes indicated extended spawning seasons peaking in September 1999 and October 2000. Median sizes and ages at initial maturation were 307 mm, 584 g and 1.1 years for males and 328 mm, 688 g and 2.7 years for females. Sex ratio varied significantly with age from equal as juveniles to a significant male-bias as adults. This description will enable better stock assessment and development of simulations that evaluate potential pest management strategies.

Effects of adding different proportions of the small fish punti (Puntius sophore) and mola (Amblypharyngodon mola) to a polyculture of large carp

This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for the farmers' family consumption and of large carp species as a cash crop. The objectives of this experiment were to assess the effects of adding punti and mola in different proportions on the large carp and on the environment, and to assess the effects of punti on mola and mola on punti. The polyculture included the large carp rohu, catla and common carp (as cash crop fish), and the small indigenous fish punti and mola (as food for the small-scale farmer family). The total large carp stocking density was 10 000 fish ha−1, at a species ratio of 1:1:1. The total small fish stocking densities were 0 in the control and 30 000 punti and mola ha−1 in the treatments, these at rates 2:1, 1:1 and 1:2. Stocking punti and mola at the density and all ratios tested were viable solutions to obtain simultaneously large carp cash crops and small fish to feed the farmer's family. Statistically marginal differences in large carp production were obtained in stocking small fish in the different proportions. These marginal differences could be explained by food competition between punti and common carp and between mola and rohu, which had different outcomes depending on the proportions of the small fish stocked. Stocking punti and mola at a 1:1 ratio would result in more small fish for the farmer's family, while the individual size of rohu, the most expensive large carp, would be somewhat smaller, but not necessarily small enough to decrease its selling price. Stocking one of the small fish in higher proportion than the other (2:1 or 1:2) would result in less mola for the family consumption, while harvesting of common carp would be somewhat lower and of smaller fish. Since common carp is the cheapest of the large carps, this small reduction would not necessarily affect the family income in an important way. With these results, farmers would now be able to reorganize their stocking practices with large carps and small fish and decide the appropriate small fish stocking ratios to meet their needs.

Maturation and Reproductive Biology of Female Wild Carp, Cyprinus carpio, in Victoria, Australia

The reproduction of carp Cyprinus carpio L. (Family: Cyprinidae), from Victorian waters in Australia, is studied with detailed analysis of gonad maturation, spawning season, fecundity and oocyte diameter. Results show that carp has a high annual fecundity (AF) (0.12–1.54 million oocytes per fish), which is positively correlated with caudal fork-length (L, mm) and total weight (W, kg) but not age. The relationships between length or weight and AF were statistically significant and best described with the simple linear or quadratic regressions. Mean relative fecundity was 0.163 million eggs kg−1 whole weight. Egg size was estimated from oocyte diameter in carp from eight stocks. Egg size was proportional to maternal size but not age. Seasonal trends in gonadosomatic indices, together with the changes in the macroscopic and microscopic condition of ovaries, demonstrated that spawning generally peaks during spring–early summer, but also occurs through until autumn and can even start in late winter at some sites. In Victoria, this species is a multiple spawner with asynchronous oocyte development and a protracted spawning season. Stocks generally contain both females that spawn once, and females that spawn repeatedly, within a spawning season. Implications for management for the control of feral carp stocks are discussed.

Measurement of total body composition changes of common carp by computer tomography

The crude fat and protein content of the total body was estimated by X-ray computer tomography (CT) and determined by chemical analyses during a feeding experiment with sexually matured common carp. Between 21 and 35 serial scans were taken of altogether 41 fish and samples from the homogenized body were prepared for chemical analyses. Experimental fish with an average body weight of 1453 g originated from a commercial stock of mirror carp. Two feeding regimes (carp feed and carp feed+ad libitum maize) were applied for 57 days and followed by a fasting period of 27 days. Both feeding regimes significantly increased the crude fat content of the whole body that did not decrease during fasting in spite of high water temperature (22°C). No significant changes were observed in crude protein content. The variables used for producing the prediction equations were taken from the density values of the Hounsfield scale, on a range between −90 and +160, by summing the frequencies within each interval of 10 values. Whole body fat content could be estimated with R2=0.89−0.91 accuracy with the principal component analysis using data of all (seven) body regions and only dorsal fin region respectively. Adequate linear regression model could not be calculated by the same procedure for crude protein. Three-dimensional ‘volumetric’ estimation of fat tissue was also carried out on the basis of fat index showing high correlation with measured fat content. The changes of body composition of individual fish can be followed by sequential CT scanning.

Variability of a cyprinid lake ecosystem with special emphasis on the native fish fauna under intensive fisheries management including common carp (Cyprinus carpio) and silver carp (Hypophthalmichthys molitrix)

The lake investigated (Lake Heiliger See near the town of Angermünde, North-East Germany) is small (10.2 ha) but deep (z max = 12 m, ź = 6.5 m). During summer the thermocline is at a depth of 4–5 m, and the hypolimnion is filled with H 2S already by mid-June, making 60–75% of the bottom area inaccessible to fish. The native fish fauna is dominated by planktivorous roach ( Rutilus rutilus). A test stocking of silver carp ( Hypophthalmichthys molitrix) together with an unplanned and erroneous stocking of common carp ( Cyprinus carpio), coupled with an incorrectly calculated strong stocking of eel ( Anguilla anguilla), which assumed that the total lake area yielded fish food organisms, brought about a strong depression of the food biomass available to native fish. Both silver carp and common carp showed a pattern of behaviour which only partly agreed with results obtained in experimental ponds prior to lake stocking. At least for 10 years there was a very low zooplankton biomass (mostly below 1 mg/l fresh weight) caused by silver carp and a strongly reduced benthos biomass (frequently below 1 g/m 2 fresh weight) caused by common carp and eel on H 2S-free bottom areas. Despite this extreme situation, which lasted for 20 years and is rarely found in lakes elsewhere, it was not possible to demonstrate negative effects on the native fish fauna comprising 16 species. Also the dominance pattern seems uninfluenced in the long run. A strong decline of the catchable roach stock was observed for some years, but the stock reappeared soon in comparable quantity. During the time of roach stock depression the individual growth of roach (and common carp) was particularly low. Nevertheless, roach succeeded in reproduction during this time in spite of the reduced zooplankton and detritus concentrations. Fry numbers per year ranged from 500–15 000 individuals/ha. The pike perch ( Stizostedion lucioperca) was perhaps the most endangered species of all those living at small population numbers in the lake, since its fry is confined to the limnion where competition from silver carp and adult roach and perhaps predation by roach is highest. However the pike perch stock remained unchanged at the low level of 1950, the time prior of carp and eel stocking. All other rare species of Lake Heiliger See spawn in the littoral zone where fry survival conditions were less impacted. In summary, we present more evidence that the fish fauna of cyprinid lakes is more robust than in salmonid lakes. The regulation capacity of fish communities of eutrophic lakes seems to be able to offset even severe direct and indirect influences of fisheries management.

Effects of adding the small fish punti (Puntius sophore Hamilton) and/or mola (Amblypharyngodon mola Hamilton) to a polyculture of large carp

Abstract This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for farm families' consumption and of large carp species as a cash crop. The polyculture included the large carp rohu, catla and mirror carp as cash crop fish, and the small indigenous fish punti and mola as food for the small-scale household. Total large carp stocking density was 10 000 fish ha−1, at a species ratio 1:1:1. Total small fish stocking densities were 0, 25 000 or 50 000 fish ha−1. The objectives were to assess the effects of adding 25 000 punti and/or mola ha−1 on the large carp and environment, and to assess the effects of punti on mola and mola on punti. It was found that catla was not affected by the addition of small fish in any of the combinations tested; rohu was not affected by punti, and mola reduced rohu performance by 30–40% only when punti was not present; mirror carp was not affected by punti, and mola increased mirror carp growth rate and harvesting weight by 25–30% whether punti was present or not; small fish did not significantly affect total yield and food conversion ratio; punti performance was not affected by mola; mola harvesting weight was not affected by punti, while mola harvesting numbers and biomass were reduced by 55–65% by punti. Factor analysis of water quality data identified photosynthesis–respiration and algal biomass– temperature as the main processes governing water quality. Effects of treatment on those water quality factors are analysed, and the fish–water quality relationships discussed. In a parallel polyculture experiment in 25 farm ponds, the performance of large carp species was found to be unaffected by the addition of punti and/or mola. The results indicate that, at the densities tested, punti and mola addition to the large carp polyculture is viable as they do not reduce cash crop production and might be a good food source for the farmer's family.

Socio-economic characterisation of specialised common carp ( Cyprinus carpio L.) anglers in Germany, and implications for inland fisheries management and eutrophication control

Many freshwater ecosystems suffer from dense common carp ( Cyprinus carpio L.) stocks. From the ecological point of view, high carp densities are not desirable because of several negative impacts (e.g. bioturbation, competition with other benthivores). A mail and internet survey among specialised carp anglers (SCAs) ( n=710) suggests that, in Germany, carp anglers’ catch exceeds commercial carp harvest by up to 2500%. This indicates that by following at least five steps (marketing, education, specific regulations, risk communication and monitoring), carp angling may reduce carp stocks efficiently. Moreover, demographics, participation patterns, economic impact and value of SCAs suggest a great potential for marketing (tourism, commercial fishermen). Thus, specialised carp angling provides a means for inland (carp) fisheries management world-wide. However, input–output balances for total phosphorus revealed that, under certain conditions, carp angling may contribute substantially to anthropogenic eutrophication if ground- and pre-baiting is used in excess and harvest rates are low. A simple equation was developed to provide managers with a tool for a quick appraisal of the likelihood of a negative ecological impact of phosphorus inputs by angling. Further research on this topic is recommended to develop management guidelines on maximum tolerable amounts of bait because ground-baiting is intensively practised not only by carp anglers but also by the general coarse fish angler.

Effects of a filter‐feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto‐ and zooplankton in a mesotrophic reservoir: results from an enclosure experiment

SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto‐ and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m−3) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir.2. Phytoplankton biomass &lt;30 μm and particulate organic carbon (POC) &lt;30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish‐free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton.3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.

자연산 3배체 붕어 (Carassius auratus) 클론 집단에 대한 유전학적 동정

Natural clonal stock of triploid crucian carp, Carassius aurahs was identified and its cytogenetic, molecular genetic and morphological traits were studied. Cytogenetic analysis of the clonal crucian carp revealed that they were natural triploidy, evidenced by 1.5-fold increases of cell size, DNA content, and chromosome number. Multi-locus DNA fingerprinting using probe showed that they had an identical fingerprint profile, indicating the clonal propagation of the population. External morphology and morphometric characteristics of triploid individuals were much uniform compared to those of diploids. Natural triploid crucian carp was proven to be all-female in this study.

Enclosure experiments on and lacustrine practice for eliminatingMicrocystis bloom

Microcystis bloom, one of the most objectionable characteristics of eutrophication in tropical and subtropical waters, occurred in Donghu Lake (East Lake) of Wuhan every summer from the 1970s up to 1984, but from 1985 up to now failed to occur there. The cause of its disappearance remained in obscurity until recently.In situ enclosure experiments in the lake for three years showed that the stocking of the filter-feeding silver carp (Hypophthalmichthys molitrix) and big-head carp (Aristichthys nobilis) played a decisive role in eliminatingMicrocystis bloom from the lake; but that recurrence of the bloom is possible under certain conditions. This paper presents the details and the results of enclosure experiments. The authors' analysis of fish biomass data obtained by echo-sounding and the fishery production of the lake over the years, revealed that the recurrence ofMicrocystis bloom can be prevented so long as the combined biomass of silve carp and big-head carp remains at or exceeds 50 g per cubic meter of lakewater, as was the case in the lake's 1985 fish yield of 1015 t.

Initial impact of low stocking density of grass carp on aquatic macrophytes

Grass carp ( Ctenopharyngodon idella Val.) stocked at a low density (29 kg ha −1) to one of two small ponds significantly reduced the biomass of aquatic macrophytes (especially of the filamentous alga Cladophora globulina Kütz.) and changed the species composition of the higher aquatic plant community. After grass carp stocking (in 1999), Myriophyllum spicatum L., Ceratophyllum demersum L. and Lemnaceae species replaced the species present originally: Eleocharis acicularis (L.) Roem. et Schult., Potamogeton pusillus L. and P. pectinatus L., which were dominant in 1998.

Growth and survival in two complete diallele crosses with five stocks of Rohu carp ( Labeo rohita)

The experiment of two 3×3 complete diallele crosses was carried out with four wild river stocks (Ganga, Yamuna, Brahmaputra and Sutlej) and one farmed stock (Local) of Rohu carp ( Labeo rohita). The Local stock was included in both diallele crosses. In total, 864 fish were individually tagged at 6 months of age and reared for 14 months in three monoculture and two polyculture earthen ponds in Orissa, India. For harvest weight, total heterosis for each of the six stock crosses was low or negative and average heterosis was also low and in most cases not significantly different from zero. For survival, total and average heterosis was negligible and not significantly different from zero. It is concluded that genetic improvement through crossbreeding of Indian stocks of Rohu carp has little practical significance.