Comparison of food selection and growth performance of koi carp, Cyprinus carpio L., and goldfish, Carassius auratus (L.) in mono- and polyculture rearing in tropical ponds

To compare the effect of polyculture against conventional monoculture on ornamental carp production, investigations on food selection and growth performance of koi carp (K), Cyprinus carpio L. and goldfish (G), Carassius auratus (L.) were conducted in a 11‐week rearing experiment in two monoculture (100% K and 100% G) and five polyculture (90% K–10% G, 70% K–30% G, 50% K–50% G, 30% K–70% G and 10% K–90% G) conditions in tropical ponds. There were three replicates for each treatment. Environmental conditions and food availability were similar in all the treatments. Ivlev's electivity index showed that both fish species avoided phytoplankton and preferred cladocerans to other zooplankton groups (copepods and rotifers) in monotypic conditions. However, in the polyculture treatments, the positive electivity of goldfish towards cladocerans reduced significantly (P<0.05), while the percentage of copepods, rotifers and phytoplankton in the gut content increased. No significant differences in weight gain, specific growth rate and deformities were recorded at harvest for koi carp between the different treatments (P>0.05). Even the survival rate of koi carp recorded above 90% in all the treatments. However, the goldfish recorded significantly better weight gain, specific growth rate and survival in monoculture (100% G), compared with the polyculture treatments (P<0.05). Goldfish deformities were lowest (P<0.05) in the monoculture treatment (2.42%). The number of marketable fish above a set size limit of 4 g total weight was significantly higher in the two monoculture treatments, compared with the five polyculture treatments (P<0.05). Keeping in view of the dietary similarities of koi carp and goldfish, and the aggressive nature of koi carp in polyculture, it is suggested to refrain from polyculture of goldfish and koi carp until further documentations relating to optimum stocking density and management of polyculture of ornamental carps are available.     

锦鲤墨蝶呤还原酶基因的克隆、表达和定位分析

为探究SPR在锦鲤体色形成中的作用,实验利用RACE技术获得spr cDNA全长序列,并分析其时空表达模式,同时利用Western blot和免疫组织化学方法检测SPR蛋白在皮肤、鳍条和鳞片中的分布和表达情况。结果显示,spr cDNA全长879 bp,包含132 bp和134 bp的5′和3′非编码区,开放阅读框510 bp,编码170个氨基酸残基。氨基酸序列比对和系统进化树分析显示,锦鲤SPR具有保守的adh_short_C2结构域,与金鱼相似性高达97.7%。spr在各组织中均有表达,其中皮肤的表达量最高。spr在锦鲤个体发育的4个阶段表现为先降后升。纯红、纯白及红白3种体色锦鲤皮肤、鳞片和鳍条中spr mRNA和蛋白表达水平基本一致,在纯红锦鲤皮肤中表达量最高,红白锦鲤白色皮肤、鳞片和鳍条的表达量最低。SPR组织定位分析显示,红色锦鲤和白色锦鲤皮肤中均检测到阳性信号,其中红色皮肤阳性信号强度高于白色皮肤。研究表明,spr可能与锦鲤红/黄色素细胞的分化和形成具有一定相关性,参与了锦鲤体色的形成。     

Fish-management relationships in Israeli commercial fish farming

To analyse the relationships among fish species performance and management procedures, a database was built up with data from 31 fish farms during the period 1976–1987 (1673 observations) and analysed through multivariate statistics (factor analysis). The data include nurseries, grow-out and operational ponds with mono- and polycultures of common carp, Cyprinus carpio, tilapia hybrid, Oreochromis niloticus × O. aureus, silver carp, Hypophthalmichthys molitrix, and mullet, Mugil cephalus. The main conclusions include the following. (1) The highest total yields and best tilapia performances were obtained in polyculture ponds where tilapia was the main species. (2) The best carp performances occurred in grow-out polyculture ponds where carp was the main species. Carp performance was improved in polycultures with mullet and silver carp, irrespective of whether tilapia were present or not. (3) Carp and tilapia yields increased as the nutritional inputs (feed pellets, sorghum pellets, manure), pond size and culture duration increased. The effect of the nutritional input was not linear, but logarithmic. (4) Growth rate of common carp was more affected by total density and stocking size than that of tilapia. Better carp and tilapia growth occurred in grow-out ponds when stocked at large sizes and cultured during short periods, mainly when both species were present. (5) Carp growth varied with the geographical region and size of fish pond, being better in smaller than in larger ponds due to reduced access to natural benthic food in deep ponds.     

Effect of density on survival and growth of cyprinid fish fry

One of the bottlenecks in Israel's cold water ornamental fish industry is the large loss of fish during the post-larval stage. As a first step towards increasing survival rates, the optimal stocking density in earthen ponds for fry of each species should be determined. The results of five consecutive experiments with fry of goldfish (Carassius auratus), common carp and koi (respectively, edible and ornamental morphotypes of Cyprinus carpio) are presented here. The experiments were run in 18 cages of 1 m³ and 0.8 mm mesh size placed in a 0.02 ha earthen pond, stocked at 50,100 and 200 fry per cage. The variables analyzed were growth rate, harvesting weight, biomass and survival. Equations to estimate growth rates under different conditions are given. The three fish types present different growth rates and survival patterns in relation to stocking density and season, which lead to different management implications. Koi can be stocked up to 2 million fry per ha without showing negative density effects. Goldfish should be stocked at low density (500,000–1 million fry per ha) in spring and at higher rate (2 million fry per ha) in summer. Common carp stocking density should be adjusted to obtain the required fish size at harvest.     

Grass carp (Ctenopharyngodon idella) grazing on duckweed (Spirodela polyrhiza)

The aims of this experiment were (1) toquantify the ability of grass carp to processduckweed and (2) to assess indirect changes inwater chemistry and phytoplankton community,caused by grass carp feeding. Yearling grass carp sized 126 ± 7.7 mm (TL) and19.6 g in weight were kept in 9 laminate tanksof 1 m³ for 14 days. Two stockingdensities (2 and 6 fish per m³) anda control without fish were used. Standard growthrate (SGR) of grass carp fed exclusively onduckweed was 0.70% body weight (BW) d^–1and food conversion ratio (FCR) reached 2.0(average water temperature =21.1 ± 3.8 °C). Daily food intakewas 0.2 g of duckweed dry weight (DW), i.e.,1% of average BW of grass carp. SGR ofduckweed growing in 20 × 20 cm floatingenclosures, differed significantly[F(6,2) = 417.9; p = 0.002] between the twostocking densities of grass carp and thecontrol tanks (without fish). Mean SGR ofduckweed was 0.02 g g^–1 day^–1 and thehighest SGR was recorded in the control tanks.Both decrease in NH₄-N and increase inNO₂-N concentrations differedsignificantly between the treatments[F(2,2) = 45.3; p = 0.02 and F(2,2) = 19.2; p = 0.04 respectively]. Changes in other nitrogenand phosphorus components (NO₃-N, TN, TPand PO₄-P) caused by stocking of grasscarp were not significant. Biomass ofphytoplankton, dominated by filamentous algaeand blue-greens, increased proportionately tostocking density of grass carp. Althoughduckweed has a large potential for nutrientremoval, the most common pathway for thenutrients released through grass carp grazingif duckweed cover is loose is theirincorporation into phytoplankton biomass.     

Ammonia autointoxication of common carp: case studies

Three case studies of ammonia autointoxication of the common carp (Cyprinus carpio L.) are described. In the first case, carp yearlings with a full digestive tract were transferred during the growing period from pond water (22°C) to tap water (17°C). In the second case, marketable carp were transferred from the fishing ground of a pond (18°C) to a storage pond (10–12°C). Harvest was performed in late September when the fish were still ingesting natural feed. In the third case, marketable carp after storage for 1 month were transferred to storage ponds with markedly lower water temperature. Stress because of harvest and handling also occurred in this case. In all cases, highly increased concentrations of ammonia were found in the blood plasma of the fish (mean ± SD 1760 ± 350 μmol L⁻¹ in the first case, 870 ± 540 μmol L⁻¹ in the second case, and 880 ± 150 μmol L⁻¹ in the third case). Highly congested, dark-red coloured, oedematous gills were observed for all specimens. We can avoid similar cases of ammonia autointoxication by protection of fish from sudden changes of temperature during rearing, harvesting, and handling.     

Influence of supplemental cereal feeding on the content and structure of fatty acids during long-lasting storage of common carp (Cyprinus carpio L.)

Supplemental cereal feeding (maize, wheat and triticale compared with a control group with natural food only) and its effect on fatty acid (FA) expression in the flesh during long-lasting storage of common carp (Cyprinus carpio) was investigated. The fish were cultured in earthen ponds in the Trebon region (Czech Republic). The content of fatty acid was investigated in the flesh of carp during 8 months of long-lasting storage without additional feeding. Sixty common carp in their third year of life were used for the analyses. The weight of the fish (marketable fish) ranged from 1,358 g to 2,221 g. Polyunsaturated fatty acid (PUFA, n–3) content and composition in fish flesh were determined by gas chromatography (VARIAN 3300). Supplemental cereals caused lower levels of PUFAs and n–3 PUFAs in fish fat. The content of these fatty acids did not decrease, even during 8 months of fish storage. The average percentage of PUFAs in total fat from edible parts was: for maize 13.7% ± 1.58%, for wheat 11.6% ± 1.17% and for triticale 10.7% ± 1.00%. The percentage of n–3 PUFA for maize was 2.5% ± 0.36%, for wheat 3.38% ± 0.44% and for triticale 3.1% ± 0.39%.     

Genetic variations in survival of rohu carp (Labeo rohita, Hamilton) after Aeromonas hydrophila infection in challenge tests

A challenge test against Aeromonas hydrophila was conducted using 2062 rohu carp (Labeo rohita) fingerlings obtained from 52 dams and 87 sires (87 full-sib families) of two year-classes (2003 and 2004). Attempts to establish a cohabitant challenge model were not successful. Therefore, fish were challenged by intraperitoneal injection with A. hydrophila in two replicate tanks per year-class, and dead fish were collected hourly. The mortality reached its peak at 16–22 h after challenge and had almost completely stopped after 58 h. The test was terminated after 382 h at which the average survival was 43.9 and 48.9% in the two 2003 year-class tanks, and 77.1 and 35.8% in the two 2004 year-class tanks. Heritability estimates for survival were obtained from sire and dam threshold models (THR) and sire and dam linear repeatability models (LINR). For both types of models the heritability estimates based on data from single tanks were not consistent. For both year-classes, data from one of the two challenged tanks demonstrated significant additive genetic variation in survival during the A. hydrophila infection, whereas the heritability estimates were not significantly different from zero for the other tank. Further, genetic correlation between survivals in the two replicate tanks in each year-class was not significantly different from zero. The differential results from the replicate tanks demonstrate that additional challenge test experiments are needed before firm conclusions can be drawn about the magnitude of additive genetic variation for survival to aeromonasis in rohu carp. A cohabitant challenge model that allows the testing of important defence mechanisms in the skin and mucous membranes of the fish might have been more appropriate. To establish a valid cohabitant challenge model for rohu carp should be given high priority.     

Brackishwater carp culture in potentially waterlogged areas using animal wastes as pond fertilizers

In order to assess the possibilities of utilizing drainage effluents (salinity range 5.0–12.5), fish culture experiments were carried out. Experiments on polyculture using cow dung (24 000 kg ha^–1 y^–1) as pond fertilizer were conducted at five different salinity levels (0.3–8.5). Studies have revealed that carp perform well in salinities up to 7.5 and reasonably high fish production has been obtained. Even though the ponds had a high trophic status, higher salinities ( > 7.5) appear to repress fish growth probably due to low dissolved oxygen (DO), high BOD and high NH₄-N. Experiments on monoculture of common carp (Cyprinus carpio) conducted at two different salinity levels (0.3–0.9 and 6.0–7.0) using four different organic fertilizers (cow dung at 24 000 kg and 20 000 kg, poultry at 1500 kg, duck at 6000 kg and sheep/goat at 1500 kg ha^–1 y^–1) have revealed the highest fish growth to be in poultry-treated ponds, followed in decreasing order by duck and sheep/goat wastes. Similar trends in fish production were observed both in fresh- and salt-water ponds. However, fish production was lower in ponds having higher salinities ( > 7.5). Nevertheless, these studies indicated that inland saline waters can be utilized for fish culture. With minor modifications in the existing technology of fish culture in stagnant freshwater fish ponds, animal wastes could be used to fertilize brackishwater fish ponds.     

Influence of nutrition on the lipid quality of pond fish: common carp (Cyprinus carpio) and tench (Tinca tinca)

Like marine fish freshwater fish are an important source of essential fatty acids for human nutrition. However, the fatty acid composition of pond fish can vary considerably and strongly depends on that of the ingested food. Investigations on the fatty acid composition of common carp (Cyprinus carpio) and tench (Tinca tinca) have shown that different methods of rearing and feeding cause substantial variations in the proportions of the n-6 and n-3 polyunsaturated fatty acids of these fish species. Carp reared on the basis of natural food in ponds exhibit high contents of n-6 as well as n-3 fatty acids in their muscle triacylglycerols. On the other hand carp fed supplementary wheat in ponds resulted in somewhat lower levels of these essential fatty acids. High amounts of n-3 fatty acids can be found in carp fed high-energy diets containing high levels of fish oil. Analogous results were obtained in experiments with tench reared under different nutritional conditions. While rearing on the basis of only natural food in ponds as well as feeding supplementary wheat yielded in similar levels of n-3 and n-6 polyunsaturated fatty acids, higher contents of n-3 fatty acids were recorded in tench fed pellets. High levels of n-3 polyunsaturated fatty acids in foodstuffs have positive effects on human health. Experiments with different cultured fish species proved that the fatty acid composition of the edible parts can be influenced by the diet. Therefore, a finishing diet with a suitable fatty acid profile can be used to improve the nutritional quality of fish products of farmed origin.     

Effect of using different types of organic animal manure on plankton abundance, and on growth and survival of Tilapia rendalli (Boulenger) in ponds

An experiment was conducted to evaluate the effect of using different types of organic manure on the plankton abundance, and growth and survival of Tilapia rendalli juveniles in ponds. Fish weighing 18.15±0.44 g were stocked into 12, 20 m² ponds at 2 fish m^?2 (40 fish pond^?1). There were three replicate ponds per treatment (chicken manure, cattle manure, pig manure, and no‐manure as a control). After 84 days the T. rendalli in the chicken manure treatment were significantly larger and had higher net annual yields than those in the cattle manure, pig manure and no‐manure treatments. The survival rates were not significantly different across the treatments. Significantly higher amounts of chlorophyll a and higher numbers of zooplankton were found in ponds fertilized with the chicken manure treatment. The overall results obtained in this study suggest that the use of chicken manure produces better results than cattle and pig manure treatments on unfertilized ponds.     

Effects of stocking density on the survival, growth, size variation and condition index of juvenile rabbitfish Siganus rivulatus

Spinefoot rabbitfish, Siganus rivulatus, is an economically important species of herbivorous fish that is relatively easy to rear and thus considered to be suitable for aquaculture. Juveniles are generally reared in nursery systems before being stocked into growout cages or ponds. We report here our evaluation of the effects of stocking density on the survival, growth, feed efficiency and condition index of S. rivulatus juveniles in nursery tanks. The experiment was conducted in a recirculating system of twelve 52-l aquaria connected to a biological filter and a sand filter. Juvenile fish (average weight 6.5 g) were stocked into aquaria at four stocking densities (10, 20, 30, and 40 fish/aquarium) with three replicate aquaria per treatment. Diet was provided at 3% body weight daily divided into two feedings. Fish were weighed weekly for 8 weeks and the diet increased accordingly. Survival was greater than 95% in all treatments, with no significant differences observed among treatments. There were also no differences in specific growth rate (SGR 2.12–2.27) of the fish among treatments. Growth rate was linear during the 8 weeks in all treatments, and harvested biomass increased proportionally to stocking density (198, 401, 600 and 785 g per increasing stocking density, respectively). Feed efficiency (FE 0.67–0.71) of the fish did not vary significantly among treatments. The coefficient of variation was high (35–41%) among the harvested fish, but it also did not differ significantly among treatments. The final condition indices of the fish in all treatments were similar to each other but significantly greater than the initial values (P < 0.05). The results suggest that there is no apparent effect of stocking density at the levels tested on the survival and growth of juvenile rabbitfish.     

POLYCULTURE OF THE FRESHWATER PRAWN (Macrobrachium rosenbergii) WITH TWO COMBINATIONS OF CARPS IN MANURED PONDS

A study was conducted to 1) evaluate the compatibility of prawns (Macrobrachiurn rosenbergii) with carps, especially the effect of the common carp (Cyprinus carpio); and 2) measure the ability of prawns to utilize the foods available in manured systems. Carps used were the silver (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis), grass (Ctenopharyngodon idella) and common carp (Cyprinus carpio). Four ponds (0.09 to 0.17 ha) were stocked with silver, bighead and grass carps while common carp were stocked in only two ponds. Total fish densities were 8,600/ha where four carps were stocked, but only 6,200/ha when common carp were omitted. Post-larval prawns (wt 0.116 g) were stocked at densities of 17/m². Swine (approximately 60/ha of pond surface area) were housed on pond dikes so that fresh manure continuously entered each pond. Two lots of swine were fattened during the experimental period and loadings of swine manure were calculated based on hog size and feed consumption. Growth periods were 163 days for fish and 107 and 121 days for separate stockings of prawns. Gains in biomass by prawns averaged 714 kg/ha in ponds without common carp and 364 kg/ha in ponds containing common carp. Gains in biomass for both fish and prawns were 3,619 kg/ha (19.8 kg/ha/day) in ponds with common carp and 2,924 kg/ha (18.0 kg/ha/day) in those without common carp. Prawn survival was higher (X = 84.5% vs X = 72.5%), but final average weight of prawns was lower (2.64 vs 5.86 g) in the presence of common carp due to competition for food between common carp and prawns. Competition from carp is believed to have exerted less influence on prawn survival than predation on small prawns by larger, faster-growing prawns.     

Effects of soy sauce <Emphasis Type="Italic">koji</Emphasis> and lactic acid bacteria on the fermentation of fish sauce from freshwater silver carp <Emphasis Type="Italic">Hypophthalmichthys molitrix</Emphasis>

The effects of soy sauce koji and the lactic acid bacterium, Tetragenococcus halophilus, were studied on the fermentation of fish sauce prepared from Chinese silver carp. The fish sauce prepared without koji and the lactic acid bacterium contained low levels of organic materials, total nitrogen, and organic acids. The use of koji was effective in increasing these qualitative parameters and further improved the amino acid score of the fish sauce. Addition of T. halophilus had an effect on lowering the pH value during the initial period of fermentation when the soy sauce koji was also supplemented. In contrast, T. halophilus-like bacteria were found to be predominant for all tanks fermented under the different starting conditions. Although it was not examined whether the T. halophilus-like bacteria observed after fermentation were the same as the starter-bacterium or not, it was suggested that T. halophilus plays an important role in the successful fermentation of silver carp fish sauce. Sensory evaluation conducted with Japanese and Chinese panelists also suggested the superiority of the use of koji for fermentation of silver carp fish sauce.     

Intensive polyculture of fish in freshwater ponds. I. Substitution of expensive feeds by liquid cow manure

In 1974 and 1975 nine experimental treatments of fish polyculture in stagnant water ponds without aeration were conducted at Dor. The polyculture was composed of common carp, silver carp, white amur (grass carp) and Tilapia. The treatments differed in stocking densities, feeding and manuring levels. The most productive treatment of the experiment, in which the fish were fed with protein-rich pellets, produced 50 kg/ha per day, probably a record for unaerated ponds of stagnant water. Two treatments (low and high stocking densities) fed exclusively with liquid cow manure produced an average yield of around 32 kg/ha per day. The yields of the treatments receiving high-protein pellets exceeded those of the treatments receiving grain pellets by 20 and 9.6 kg/day per ha, at high and low stocking densities, respectively, and in both cases the yield increments justified the extra cost of high-protein feed. The responses of the four fish species to the different levels of feeding and stocking densities were widely different. The common carp and white amur showed the greatest responses to increased feeding inputs while the silver carp and Tilapia, even at high densities, have done equally well at low feeding levels. Total body fat contents of the common carp were 20%, 15% and 6.2% when fed with high-protein pellets, grains pellets and liquid cow manure, respectively. Intermittent harvesting did not result in increased yields.     

Microbial risk assessment of livestock-integrated aquaculture and fish handling in Vietnam

Animal manure has been traditionally used to fertilize fish ponds in Vietnam. While this practice effectively reuses agricultural wastes, high concentrations of pathogenic microorganisms in animal manure raise public health concerns. Working in fish ponds and handling of contaminated fish in unhygienic manner were identified as potential factors of occupational risk. Escherichia coli occurred in numbers <10³ colony forming units (CFU)/100 mL in irrigation water and <10⁴ CFU/100 mL in fish pond water that uses animal manure. Escherichia coli on tilapia skin in numbers were <10³ CFU/100 cm in excreta-based systems and <10¹ CFU/100 cm in feed-based systems, respectively. The study identified direct use of animal manure as major contributors of the fecal contamination of pond water as well as skin of cultured fish. Estimated risks of enteric infection were 100–1000 times higher than the US Environmental Protection Agency acceptable risk. While these risk values are not likely to accurately predict infection rates in Vietnam, they indicate a potential occupational risk in the long term. Therefore, a need for risk mitigation measures was realized for health protection of future generation in agricultural communities.     

草鱼养殖水体中参与氮转化途径的异养菌分析

为分析草鱼池塘中参与氮代谢的异养细菌比例及其代谢途径,从杭州郊区取得4个草鱼池塘的水样,每个水样通过涂布随即挑选100株菌株进行定性显色试验,并据此选取11株异养菌进行16S rRNA序列分析。结果表明,4个草鱼养殖池塘中NH4+-N和NO2--N的平均水平分别为5.597 mg/L和0.135 mg/L。池塘中可培养的异养菌平均为3.26×105cfu/mL,其中的89.75%参与了氮的不同代谢途径,其中31.25%的氨化菌和33.50%NO3--N(NO2--N)还原菌参与了NH4+-N的生成,32.45%的氨氧化菌参与了NH4+-N的降低;NO2--N生成途径主要包括蛋白质直接转化(11.26%)、氨氧化(4.25%)和硝酸盐氮还原(10.75%),而NO2--N降低主要通过15.50%的亚硝酸氧化菌、8.75%的NO2--N还原菌和10.75%的反硝化菌实现。结果提示,草鱼养殖水体中存在大量的异养硝化菌参与不同的氮代谢途径,且产生氨氮的异养菌比例远高于去除氨氮的菌,这是草鱼养殖水体中氨氮含量易偏高的原因。同时,11株不同功能的异养菌16SrRNA鉴定结果为寡养食单胞菌(Stenotrophomonas)6株、假单胞菌(Pseudomonas)3株、克雷伯氏菌(Klebsiella)和肠杆菌(Enterobacter)各1株,而且细菌对氮源的利用具有菌株特异性。     

Effects of introduction of common carp, Cyprinus carpio (L.), on the pond ecology and growth of fish in polyculture

The effects of introduction of common carp, Cyprinus carpio (L.). in polyculture with major Indian carps and silver carp were studied in fertilized and fed earthen ponds in Mymensingh, Bangladesh. Bottom-dwelling common carp were found to exert an influence on the quality and quantity of planktonic food organisms. The presence of common carp significantly increased (P < 0.05) the growth of other carps including the bottom-feeder mrigal. Cirrhinus mrigala (Hamilton). A significantly higher fish yield (P < 0.05) was observed in the fish ponds containing common carp when the combined yields of all species in three treatments were compared.     

Chromosome set manipulations in the black carp

The mollusc-eating black carp (Mylopharyngodon piceus) has economic and health-care potential for biological control of nuisance aquatic molluscs. The present study investigates the production of gynogenetic-monosex and triploid-sterile populations of black carp. The goal was to provide a method which would eliminate unwanted biological and environmental impacts of introducing this exotic species into areas with nuisance mollusc infestation. Meiotic gynogenesis was induced by inseminating black carp eggs with UV-irradiated (800 Jm⁻²) sperm of common carp (Cyprinus carpio) or Japanese ornamental (koi) carp. Diploidy was restored through retention of the second polar body (2PB), by shocking activated eggs at 1–8 min post-fertilization (embryological age of 0.07–0.57τ₀, a parameter defined by the cell cycle duration) at 1 min intervals, with heat-shocks (41.0±1.0 °C for 1 min) or pressure-shocks (7500–7600 psi for 1.5 min). Highest survival was found when embryos were heat-shocked 1.5–4.5 min post-fertilization (0.10–0.25τ₀). The highest survival of free-swimming larvae from pressure-shocked eggs, was achieved at 7500 psi at 1–2 min post-fertilization (0.08–0.16τ₀). Triploidy was induced by retention of 2PB following normal fertilization. Batches of 30 000 eggs were fertilized with intact sperm and pressure-shocked (6000–8500 psi for 1.5 min) 2 min post-fertilization (0.15–0.16τ₀). The highest survival of triploid swim-up larvae was 5.1% in eggs shocked with 7500 psi. In random samples of individual larvae taken from each treatment, triploidy was analysed by cytofluorometry of the cellular DNA content. In DNA analysis performed in fingerlings (N≥15), 50% of the fish were triploids.