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Abstract [Objectives] This study was conducted to study early reproduction techniques of yellow catfish （Pelteobagrus fulvidraco） in Jianghan Plain.
[Methods] Parents of P. fulvidraco were cultured by the methods of artificial temperature control and nutrition enhancement.
[Results] After culture， the body weight， gonad weight， maturity coefficient and egg diameter of P. fulvidraco increased gradually. Up to April 30， the gonad weight of the female parents of P. fulvidraco cultured in greenhouses was significantly higher than that of the parents under natural conditions （P<0.05）， 54% higher than that of the parents naturally cultured at the same period; the maturity coefficient of the parents cultured under strengthened conditions was also significantly higher than that of the parents under natural conditions at the same period （P<0.05）; and the ovaries of the strengthened parents were orange yellow and full of eggs， and the number of eggs per fish was relatively high. In early May， the parents cultured under strengthened conditions were induced to spawn artificially. The average induced spawning rate and fertilization rate reached 86.7% and 71.6%， respectively， reaching the respective normal level. The gonad maturity of P. fulvidraco cultured under strengthened conditions was 15 d earlier than that under natural conditions， resulting in mass production of earlypropagated larvae of P. fulvidraco.
[Conclusions] In Jianghan plain area， artificial propagation can be achieved in early May by means of artificial temperature control and strengthened culture， which is 20 d ahead of natural conditions.
Key words Pelteobagrus fulvidraco; Early propagation; Gonad development
Yellow catfish （Pelteobagrus fulvidraco） belongs to Pelteobagrus in Bagride of Silurformes， and is widely distributed in the middle and lower reaches of the Yangtze River and rivers and lakes in the Pearl River Basin[1-3]. It is a kind of small freshwater economic fish， which has tender meat， delicious taste， no intermuscular bones and rich nutrition， and is favored by consumers[4]. The domestic market demand has gradually increased， and P. fulvidraco has become one of the most popular aquaculture species[5]. In 2018， the total production of P. fulvidraco was 480 000 t， which increased by 15.03%compared with the previous year. At present， the artificial propagation of P. fulvidraco has been successful， but it is synchronized with the natural propagation time， and the fish cannot reach the commercial fish market specifications in the very year. In recent years， the high morbidity and mortality of twoyearold P. fulvidraco has become the bottleneck of the industrialization of P. fulvidraco. Therefore， at present， the research on the technology of P. fulvidraco propagation is mainly focused on artificial propagation[6]， efficient propagation[7-8]and disease control[9-10]. The basis of P. fulvidraco early breeding is the early maturity of parent fish， and the key to the early maturity of parent fish is the gonad development of parent fish. However， there have been few studies on the early propagation of P. fulvidraco， and no comprehensive scientific studies have been reported on the temperaturecontrolled breeding of parents， rational nutritional breeding， environmental stimulation breeding， temperaturecontrolled induced spawning， temperaturecontrolled fertilization， temperaturecontrolled hatching， and temperaturecontrolled larvae cultivation. The development of gonads is the basis of reproduction. The key to early propagation is the early development and maturity of the parental gonads， while the important factors affecting the development of the gonads of P. fulvidraco parents are nutrition and environmental conditions （mainly water temperature）[11]. In this study， the parent gonads of P. fulvidraco were matured earlier than natural conditions by strengthening cultivation in greenhouses and studying the effect of artificial strengthened culture on the development of gonads， which enabled the marketing of the fry ahead of time and extended the propagation cycle， thereby realizing marketing of P. fulvidraco in the very year， which meant that the breeding efficiency and market competitiveness were improved. This study provides theoretical basis and scientific basis for the early propagation of P. fulvidraco， and lays a foundation for industrialization development and structural adjustment. Materials and Methods
Culture of parents
This experiment was conducted at the breeding base of Shashi Bairong Aquatic Breeding Co.， Ltd. of Haida Group. The parents used in this experiment were selfbred in the breeding base. They were robust， bright in color， free of disease and injury， aged 3 winter years， and the average specification was over 60 g.
The strengthened culture was carried out in warm sheds. There were 2 ponds with an average area of 1 hm2， a water depth of 2 m， and a stocking density of 3 750 kg/hm2. In early November， the P. fulvidraco parents were moved to the ponds in greenhouses， and the culture was strengthened from January of the following year. The parents cultured under outdoor natural conditions served as a control， and there were 8 outdoor ponds with an average of 1 hm2 and a stocking density of 3 750 kg/hm2. Feed uptake began on March 10， at which the water temperature was around 14 ℃.
The P. fulvidraco parents were fed the material special for P. fulvidraco parent breeding （protein content： 46%）， with the addition of 0.5% Vc and 0.1% Ve. The feeding was performed twice a day， and the feeding amount was flexibly determined according to the remaining amount of bait and the weather conditions， generally 3% of body weight. The water was kept fresh and transparent at 30-40 cm. The aquaculture water was disinfected once a week. Aerators were turned on in time to keep the dissolved oxygen above 5 mg/L and the pH value between 6.8-8.5.
Detection of gonad development
During the culture period， 10 female parents were randomly selected from each culture pond every 15 d （i.e.， March 15， March 30， April 15 and April 30）， and the weight and ovarian weight were measured， respectively. The tail of the fish was cut off from the cloaca， and the blood was drained for 20-30 min. Then， the belly was opened with a clean paper towel to absorb the blood， and the ovary was taken out with forceps. The appearance， color and blood vessel distribution of the ovary were observed. The ovarian membrane was carefully removed with tweezers， and a mass of egg cells were taken out and placed on a glass slide， to which a few drops of normal saline were added， followed by mixing and dispersing the egg cells with a dissecting needle. The slide was moved under a microscope， to record the diameter of all the egg cells in the field of view and the shift of the nuclei， and about 200 eggs were counted each time. Artificial propagation
In this experiment， artificial propagation was performed through hybridization of P. fulvidraco， which used common P. fulvidraco as the female parent and P. vachelli as the male parent， with sufficient mature sperm provided. Parents subjected to strengthened culture in greenhouses were induced to spawn from May 5， and those cultivated under natural conditions were induced from May 20. The fish were stimulated with microflow water before spawning at a water temperature of about 24 ℃ under dissolved oxygen above 5 mg/L. The oxytocin drugs selected were luteinizing hormone A2 and chorionic gonadotropin （HCG）. Two injections were performed at the base of the pectoral fin， about 1 ml/tail， and the needle pitch was about 12 h. The induced spawning rate was calculated.
Invitro dry artificial insemination was adopted. The collected semen was mixed with the eggs， followed by debonding with yellow muddy water. After the fertilized eggs were rinsed clean， they were transferred to an incubation barrel for incubation. The incubation period lasted about 2-3 d， and the water flow was controlled to maintain good water temperature and dissolved oxygen conditions. Dead eggs were removed in time to prevent the occurrence of water mold， and the fertilization rate and hatching rate were calculated.
Indicator calculation and data processing
Related indicators were calculated as following： Maturity coefficient （GSI）=（Gonad weight/Body weight）×100%， Induced spawning rate=（Number of spawning fish/Number of induced fish）×100%， Fertilization rate=（Number of fertilized eggs/Total number of eggs）×100%， Hatching rate= （Number of live eggs in the rupture stage/Number of fertilized eggs）×100%. The data processing was performed using the software Excel 2007， and statistical analysis was performed using the software SPSS17.0 to describe the significance of the difference.
Results and Analysis
Changes in gonadal weight
As can be seen from Table 1， after culture， the weight and maturity coefficient of P. fulvidraco gradually increased. Although the parents subjected to strengthened culture gained weight， there were no significant differences between various time periods （P>0.05）， and the naturally cultured parents had no significant differences between various time period （P>0.05）. There were no significant differences in the body weight between the parents cultured under strengthened and natural conditions at various time periods （P>0.05）. It can be seen from Table 1 that the gonad weight of the parents subjected to strengthened culture on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from that on March 30 （P>0.05）; and the gonad weight of the parents cultured under the strengthened conditions on April 30 was significantly higher than those of other periods （P<0.05）. The gonad weight of the naturally cultured parents on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from March 30 （P>0.05）; and the gonad weight of the naturally cultured parents on April 30 was significantly higher than those on March 15 and March 30 （P<0.05）， but had no significant difference from that on April 15 （P>0.05）. At the beginning， there was a small difference in parental gonad weight between strengthened culture and natural culture of P. fulvidraco， which was not significant （P>0.05）. With the parental culture going on， the difference in gonad weight of the P. fulvidraco parents gradually increased and became significant （P<0.05）. Up to April 30， the gonad weight of the female P. fulvidraco subjected to strengthened culture in greenhouses was 54% higher than that of P. fulvidraco cultured under natural conditions during the same period.
As can be seen from Table 1， there were significant differences in maturity coefficient of the parents subjected to strengthened culture between various culture periods （P<0.05）. The maturity coefficient on March 15 was significantly lower than other time periods （P<0.05）， and from April 15 to April 30， the maturity coefficient of parents subjected to strengthened culture increased significantly. Up to April 30， the maturity coefficient averaged 16.2%， which was significantly higher than other time periods （P<0.05）. The maturity coefficient of the naturally cultured parents had significant differences between various time periods （P<0.05）. Specifically， The maturity coefficient on March 15 was significantly lower than other time periods （P<0.05）， and the maturity coefficient on April 30 was significantly higher than other time periods （P<0.05）. At the beginning， the maturity coefficients of the P. fulvidraco parents cultured under strengthened and natural conditions were similar， and the difference was not significant （P>0.05）. As the culture progressed， the maturity coefficient difference gradually increased and become significant （P<0.05）. Ovarian appearance and egg development
It can be seen from Table 2 that after culture， the P. fulvidraco egg diameter gradually increased with ovarian development. Significant differences in egg diameter were found between all periods for the parents cultured under strengthened conditions （P<0.05）. The egg diameter was significantly lower on March 15 than in other periods （P<0.05）， and significantly higher on April 30 than in other periods （P<0.05）. The egg diameter of the naturally cultivated parents on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from that on March 30 （P>0.05）. The egg diameter on April 30 was significantly higher than those in other periods （P<0.05）. The egg diameter of the P. fulvidraco parents subjected to strengthened culture was always larger than that of the parents cultured under natural conditions， and there were significant differences （P<0.05）. The anatomical results on April 30 showed that the female parents of P. fulvidraco cultured under strengthened conditions had an orangeyellow ovary full the ova， and the amount of ova was relatively large， reaching 6 657 ova/tail averagely. And there were some free eggs， and a small amount of eggs can be discharged by gently pressing the abdomen， so the ovaries had developed to stage V. It was observed that most of the nuclei （80%） had moved towards the animal pole. Under natural conditions， the female parents ovaries were pale yellow and contained eggs of different sizes， and the ovaries had developed to stage IV. It was observed that only a small portion （25%） of the nuclei had moved towards the animal pole.
Induced spawning condition
The P. fulvidraco parents cultured under strengthened conditions were induced for spawning from May 5， and naturally cultured P. fulvidraco parents were induced from May 20. As can be seen from Table 3， the average induced spawning rate of the P. fulvidraco parents cultured under strengthened conditions reached 86.7%， and the average fertilization rate reached 71.6%. The spawning of the P. fulvidraco parents cultured under strengthened conditions was 15 d earlier than that under the natural conditions. The induced spawning rate， fertilization rate and hatching rate of the parents cultured under strengthened conditions were lower than those cultured under natural conditions， but the differences were not significant （P>0.05）. Discussion
Effects of strengthened culture on the development of gonads of P. fulvidraco parents
Fishes distribute the nutrients and energy obtained from food to the gonads， which promotes gonad development[12]. Water temperature is the most important environmental factor affecting the sexual maturity， sex differentiation， ovulation and spawning of fish. The formation rate of sperm and eggs of warmwater fish is positively related to water temperature in a suitable water temperature range[13]. In northern China， warm water from power plants was used to culture freshwater white pomfret， which reached sexual maturity on May 16， thereby achieving early propagation and growing up in the very year. The Ogasawara Fisheries Center in Tokyo， Japan constantly infused to fish ponds at a water temperature of 20 ℃with fresher seawater at a higher temperature to promote spawning， by which the early rearing of juvenile pomfret was realized successfully， which was 1.5-2.0 months earlier than the general propagation time[14]. Similarly， in this study， the water temperature was manually adjusted to make fish gonads mature and make the fish spawn in advance in an artificial culture environment. Up to early May， the parents entered the breeding period， and had been cultured under strengthened conditions for 3 months， they were artificially induced on May 5， 15 d earlier than the parents cultivated under natural conditions， thus indicating that strengthened culture through appropriate artificial temperature control is feasible for the promotion of the early breeding of P. fulvidraco. In this study， there were no significant differences in gonad weight and maturity coefficient between the parents cultured under strengthened and natural conditions （P>0.05）; and as the cultivation progressed， the difference gradually became significant （P<0.05）. From midApril to early May， the maturiry coefficient of the P. fulvidraco cultured under strengthened conditions increased sharply， from 8.65% to 16.20%， and even female P. fulvidraco parents collected on April 30 showed egg spawning. The reason might be that since midMarch， compared with natural conditions， greenhouse water temperature was higher and suitable for parental development， and plus normal feeding and adequate nutrient intake， the gonad development was faster. Sun et al.[15]cultured parent fish at 25 ℃ for one month from April 30， and most of the parent fish developed to gonad maturity on May 28. This culture used coalfired heating to keep indoor water temperature suitable for parental development， which shortens the cultivation time compared with this study， but the cost is higher， so it is suitable for northern China. In addition， within the appropriate temperature range， the metabolic intensity of fish and temperature are directly proportional to each other， and fish consumption and feed rate are closely related to metabolic rate[16]. The parents need to reserve a large amount of protein nutrition before reproduction to provide material and energy bases for gonad development， while the parents cultivated under natural conditions suffer from poor water temperature and have poor food intake in the early stage， which results in insufficient nutrition and slowed gonad development. Effects of early propagation on the "three rates" in artificial propagation
From May 5 to May 10， a total of more than 2 500 kg of P. fulvidraco parents under strengthened conditions were induced to spawn， showing an average induced spawning rate of 86.7% and an average fertilization rate of 71.6%. The spawning induction effect reached a normal level， indicating that early propagation did not negatively affect spawning induction. Luo et al.[16]showed that whether the parent fish were sick significantly affected the fertilization rate and fertilized egg quality at the time of spawning induction. Lai et al.[17]performed spawning induction on mandarin fish and found that the maturity of mandarin fish gonads and the size and color of eggs were closely related to the occurrence and control of fish diseases. During the strengthened culture period， the P. fulvidraco parents in greenhouses suffered from abdominal dropsy and were not fully recovered until the period of spawning induction， and during artificial egg stripping， the situation of small yields， small eggs and poor color was found， leading to the induced spawning rate and fertilization rate slightly lower than those under natural conditions. The greenhouse strengthened culture technology in this study is not yet fully mature， and it is necessary to improve ventilation， water quality control， scientific and reasonable feeding and disease prevention and control in future research or actual production.
References
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[2] NI Y， WU HL. The Fauna of Fish from Jiangsu[M]. Beijing： China Agricultural Press， 2006.
[3] LI MF. Progress on Study on Biology of Pelteeobaagrus fulvidraco （Richardson）[J]. Modern fishery information， 2010， 25（9）： 16-21. （in Chinese）
[4] GAO C， YANG RB， HU WB， et al. Ontogeny of the stomach in yellow catfish （Pelteobagrus fulvidra： detection and quantifictation of pepsinogen and H+/K+ATPase gene expression[J]. Journal of Animal Nutrition， 2013， 97（1）： 20-26. （in Chinese）
[5] YANG YH， LI XS， MAI KS， et al. Effects of different protein sources on growth performance and metabolismrelated gene expression in juvenile darkbarbel catfish （Pelteobagrus vachelli）[J]. Acta Hydrobiologica Sinica， 2019， 43（4）： 693-700. （in Chinese） [6] ZHENG WY， LEI CS. Scale Production technology of artificial breeding of Pelteobagrus fulvidraco[J]. China Fisheries， 2000（10）： 32-33. （in Chinese）
[7] XU TB. Efficient ecological aquaculture technology of Pelteobagrus fulvidraco in ponds[J]. Henan Fisheries， 2016（3）： 7-9. （in Chinese）
[8] XIAO ZW. Study on high efficiency ecoculture technology of Pelteobagrus fulvidraco[J]. Aquaculture， 2010（11）： 15-16. （in Chinese）
[9] WANG WM， YAN AS， ZHOU LG， et al. Toxicity test of four drugs on Pelteobagrus fulvidraco[J]. Freshwater Fisheries， 2001（31）： 49-51. （in Chinese）
[10] GOU XL， WANG L. Pathogenicity and detection of virulence genes of bacterium Yersinia enterocolitica isolated from yellow catfish （Pelteobagrus fulvidraco）[J]. Fisheries Science， 2013， 32（5）： 293-296. （in Chinese）
[11] ZHAO WJ， QU YT. Study on new technology of early propagation of Siniperca chuatsi in Heilongjiang Province[J]. Fisheries of Heilongjiang， 2015（1）： 14-16. （in Chinese）
[12] DU CL. Studies on the changes of morphology and chemical components with ovary development and oogenesis in Megalobrama pellegrini[D]. Chongqing： Southwest University， 2011. （in Chinese）
[13] CAO J. Effect of water temperature on spawning activity of Acipenser sinensis[D]. Hubei： Hubei University of Technology， 2016.
[14] CHEN WJ. Early rearing of juvenile pomfret[J]. Fisheries Science & Technology Information， 1983（25）： 30. （in Chinese）
[15] SUN GX. Effects of flow rate and temperature on feed intake of turbot （Scophthatmus maximus L） and the establishment of dynamic feeding model[D]. Beijing： Graduate School of Chinese Academy of Sciences， 2011. （in Chinese）
[16] LUO XN， ZHAO XW， TIAN J， et al. Artificial breeding of yellow catfish Peltebagrus fulvidraco Richardson in a largescale[J]. Fisheries Science， 2010， 29（2）： 106-108. （in Chinese）
[17] LAI ZX. Experiments on improving fertilization rate and seedling survival rate of mandarin fish[J]. Journal of Guangdong Ocean University， 1994， 14（1）： 16-18. （in Chinese）
[Methods] Parents of P. fulvidraco were cultured by the methods of artificial temperature control and nutrition enhancement.
[Results] After culture， the body weight， gonad weight， maturity coefficient and egg diameter of P. fulvidraco increased gradually. Up to April 30， the gonad weight of the female parents of P. fulvidraco cultured in greenhouses was significantly higher than that of the parents under natural conditions （P<0.05）， 54% higher than that of the parents naturally cultured at the same period; the maturity coefficient of the parents cultured under strengthened conditions was also significantly higher than that of the parents under natural conditions at the same period （P<0.05）; and the ovaries of the strengthened parents were orange yellow and full of eggs， and the number of eggs per fish was relatively high. In early May， the parents cultured under strengthened conditions were induced to spawn artificially. The average induced spawning rate and fertilization rate reached 86.7% and 71.6%， respectively， reaching the respective normal level. The gonad maturity of P. fulvidraco cultured under strengthened conditions was 15 d earlier than that under natural conditions， resulting in mass production of earlypropagated larvae of P. fulvidraco.
[Conclusions] In Jianghan plain area， artificial propagation can be achieved in early May by means of artificial temperature control and strengthened culture， which is 20 d ahead of natural conditions.
Key words Pelteobagrus fulvidraco; Early propagation; Gonad development
Yellow catfish （Pelteobagrus fulvidraco） belongs to Pelteobagrus in Bagride of Silurformes， and is widely distributed in the middle and lower reaches of the Yangtze River and rivers and lakes in the Pearl River Basin[1-3]. It is a kind of small freshwater economic fish， which has tender meat， delicious taste， no intermuscular bones and rich nutrition， and is favored by consumers[4]. The domestic market demand has gradually increased， and P. fulvidraco has become one of the most popular aquaculture species[5]. In 2018， the total production of P. fulvidraco was 480 000 t， which increased by 15.03%compared with the previous year. At present， the artificial propagation of P. fulvidraco has been successful， but it is synchronized with the natural propagation time， and the fish cannot reach the commercial fish market specifications in the very year. In recent years， the high morbidity and mortality of twoyearold P. fulvidraco has become the bottleneck of the industrialization of P. fulvidraco. Therefore， at present， the research on the technology of P. fulvidraco propagation is mainly focused on artificial propagation[6]， efficient propagation[7-8]and disease control[9-10]. The basis of P. fulvidraco early breeding is the early maturity of parent fish， and the key to the early maturity of parent fish is the gonad development of parent fish. However， there have been few studies on the early propagation of P. fulvidraco， and no comprehensive scientific studies have been reported on the temperaturecontrolled breeding of parents， rational nutritional breeding， environmental stimulation breeding， temperaturecontrolled induced spawning， temperaturecontrolled fertilization， temperaturecontrolled hatching， and temperaturecontrolled larvae cultivation. The development of gonads is the basis of reproduction. The key to early propagation is the early development and maturity of the parental gonads， while the important factors affecting the development of the gonads of P. fulvidraco parents are nutrition and environmental conditions （mainly water temperature）[11]. In this study， the parent gonads of P. fulvidraco were matured earlier than natural conditions by strengthening cultivation in greenhouses and studying the effect of artificial strengthened culture on the development of gonads， which enabled the marketing of the fry ahead of time and extended the propagation cycle， thereby realizing marketing of P. fulvidraco in the very year， which meant that the breeding efficiency and market competitiveness were improved. This study provides theoretical basis and scientific basis for the early propagation of P. fulvidraco， and lays a foundation for industrialization development and structural adjustment. Materials and Methods
Culture of parents
This experiment was conducted at the breeding base of Shashi Bairong Aquatic Breeding Co.， Ltd. of Haida Group. The parents used in this experiment were selfbred in the breeding base. They were robust， bright in color， free of disease and injury， aged 3 winter years， and the average specification was over 60 g.
The strengthened culture was carried out in warm sheds. There were 2 ponds with an average area of 1 hm2， a water depth of 2 m， and a stocking density of 3 750 kg/hm2. In early November， the P. fulvidraco parents were moved to the ponds in greenhouses， and the culture was strengthened from January of the following year. The parents cultured under outdoor natural conditions served as a control， and there were 8 outdoor ponds with an average of 1 hm2 and a stocking density of 3 750 kg/hm2. Feed uptake began on March 10， at which the water temperature was around 14 ℃.
The P. fulvidraco parents were fed the material special for P. fulvidraco parent breeding （protein content： 46%）， with the addition of 0.5% Vc and 0.1% Ve. The feeding was performed twice a day， and the feeding amount was flexibly determined according to the remaining amount of bait and the weather conditions， generally 3% of body weight. The water was kept fresh and transparent at 30-40 cm. The aquaculture water was disinfected once a week. Aerators were turned on in time to keep the dissolved oxygen above 5 mg/L and the pH value between 6.8-8.5.
Detection of gonad development
During the culture period， 10 female parents were randomly selected from each culture pond every 15 d （i.e.， March 15， March 30， April 15 and April 30）， and the weight and ovarian weight were measured， respectively. The tail of the fish was cut off from the cloaca， and the blood was drained for 20-30 min. Then， the belly was opened with a clean paper towel to absorb the blood， and the ovary was taken out with forceps. The appearance， color and blood vessel distribution of the ovary were observed. The ovarian membrane was carefully removed with tweezers， and a mass of egg cells were taken out and placed on a glass slide， to which a few drops of normal saline were added， followed by mixing and dispersing the egg cells with a dissecting needle. The slide was moved under a microscope， to record the diameter of all the egg cells in the field of view and the shift of the nuclei， and about 200 eggs were counted each time. Artificial propagation
In this experiment， artificial propagation was performed through hybridization of P. fulvidraco， which used common P. fulvidraco as the female parent and P. vachelli as the male parent， with sufficient mature sperm provided. Parents subjected to strengthened culture in greenhouses were induced to spawn from May 5， and those cultivated under natural conditions were induced from May 20. The fish were stimulated with microflow water before spawning at a water temperature of about 24 ℃ under dissolved oxygen above 5 mg/L. The oxytocin drugs selected were luteinizing hormone A2 and chorionic gonadotropin （HCG）. Two injections were performed at the base of the pectoral fin， about 1 ml/tail， and the needle pitch was about 12 h. The induced spawning rate was calculated.
Invitro dry artificial insemination was adopted. The collected semen was mixed with the eggs， followed by debonding with yellow muddy water. After the fertilized eggs were rinsed clean， they were transferred to an incubation barrel for incubation. The incubation period lasted about 2-3 d， and the water flow was controlled to maintain good water temperature and dissolved oxygen conditions. Dead eggs were removed in time to prevent the occurrence of water mold， and the fertilization rate and hatching rate were calculated.
Indicator calculation and data processing
Related indicators were calculated as following： Maturity coefficient （GSI）=（Gonad weight/Body weight）×100%， Induced spawning rate=（Number of spawning fish/Number of induced fish）×100%， Fertilization rate=（Number of fertilized eggs/Total number of eggs）×100%， Hatching rate= （Number of live eggs in the rupture stage/Number of fertilized eggs）×100%. The data processing was performed using the software Excel 2007， and statistical analysis was performed using the software SPSS17.0 to describe the significance of the difference.
Results and Analysis
Changes in gonadal weight
As can be seen from Table 1， after culture， the weight and maturity coefficient of P. fulvidraco gradually increased. Although the parents subjected to strengthened culture gained weight， there were no significant differences between various time periods （P>0.05）， and the naturally cultured parents had no significant differences between various time period （P>0.05）. There were no significant differences in the body weight between the parents cultured under strengthened and natural conditions at various time periods （P>0.05）. It can be seen from Table 1 that the gonad weight of the parents subjected to strengthened culture on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from that on March 30 （P>0.05）; and the gonad weight of the parents cultured under the strengthened conditions on April 30 was significantly higher than those of other periods （P<0.05）. The gonad weight of the naturally cultured parents on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from March 30 （P>0.05）; and the gonad weight of the naturally cultured parents on April 30 was significantly higher than those on March 15 and March 30 （P<0.05）， but had no significant difference from that on April 15 （P>0.05）. At the beginning， there was a small difference in parental gonad weight between strengthened culture and natural culture of P. fulvidraco， which was not significant （P>0.05）. With the parental culture going on， the difference in gonad weight of the P. fulvidraco parents gradually increased and became significant （P<0.05）. Up to April 30， the gonad weight of the female P. fulvidraco subjected to strengthened culture in greenhouses was 54% higher than that of P. fulvidraco cultured under natural conditions during the same period.
As can be seen from Table 1， there were significant differences in maturity coefficient of the parents subjected to strengthened culture between various culture periods （P<0.05）. The maturity coefficient on March 15 was significantly lower than other time periods （P<0.05）， and from April 15 to April 30， the maturity coefficient of parents subjected to strengthened culture increased significantly. Up to April 30， the maturity coefficient averaged 16.2%， which was significantly higher than other time periods （P<0.05）. The maturity coefficient of the naturally cultured parents had significant differences between various time periods （P<0.05）. Specifically， The maturity coefficient on March 15 was significantly lower than other time periods （P<0.05）， and the maturity coefficient on April 30 was significantly higher than other time periods （P<0.05）. At the beginning， the maturity coefficients of the P. fulvidraco parents cultured under strengthened and natural conditions were similar， and the difference was not significant （P>0.05）. As the culture progressed， the maturity coefficient difference gradually increased and become significant （P<0.05）. Ovarian appearance and egg development
It can be seen from Table 2 that after culture， the P. fulvidraco egg diameter gradually increased with ovarian development. Significant differences in egg diameter were found between all periods for the parents cultured under strengthened conditions （P<0.05）. The egg diameter was significantly lower on March 15 than in other periods （P<0.05）， and significantly higher on April 30 than in other periods （P<0.05）. The egg diameter of the naturally cultivated parents on March 15 was significantly lower than those on April 15 and April 30 （P<0.05）， but had no significant difference from that on March 30 （P>0.05）. The egg diameter on April 30 was significantly higher than those in other periods （P<0.05）. The egg diameter of the P. fulvidraco parents subjected to strengthened culture was always larger than that of the parents cultured under natural conditions， and there were significant differences （P<0.05）. The anatomical results on April 30 showed that the female parents of P. fulvidraco cultured under strengthened conditions had an orangeyellow ovary full the ova， and the amount of ova was relatively large， reaching 6 657 ova/tail averagely. And there were some free eggs， and a small amount of eggs can be discharged by gently pressing the abdomen， so the ovaries had developed to stage V. It was observed that most of the nuclei （80%） had moved towards the animal pole. Under natural conditions， the female parents ovaries were pale yellow and contained eggs of different sizes， and the ovaries had developed to stage IV. It was observed that only a small portion （25%） of the nuclei had moved towards the animal pole.
Induced spawning condition
The P. fulvidraco parents cultured under strengthened conditions were induced for spawning from May 5， and naturally cultured P. fulvidraco parents were induced from May 20. As can be seen from Table 3， the average induced spawning rate of the P. fulvidraco parents cultured under strengthened conditions reached 86.7%， and the average fertilization rate reached 71.6%. The spawning of the P. fulvidraco parents cultured under strengthened conditions was 15 d earlier than that under the natural conditions. The induced spawning rate， fertilization rate and hatching rate of the parents cultured under strengthened conditions were lower than those cultured under natural conditions， but the differences were not significant （P>0.05）. Discussion
Effects of strengthened culture on the development of gonads of P. fulvidraco parents
Fishes distribute the nutrients and energy obtained from food to the gonads， which promotes gonad development[12]. Water temperature is the most important environmental factor affecting the sexual maturity， sex differentiation， ovulation and spawning of fish. The formation rate of sperm and eggs of warmwater fish is positively related to water temperature in a suitable water temperature range[13]. In northern China， warm water from power plants was used to culture freshwater white pomfret， which reached sexual maturity on May 16， thereby achieving early propagation and growing up in the very year. The Ogasawara Fisheries Center in Tokyo， Japan constantly infused to fish ponds at a water temperature of 20 ℃with fresher seawater at a higher temperature to promote spawning， by which the early rearing of juvenile pomfret was realized successfully， which was 1.5-2.0 months earlier than the general propagation time[14]. Similarly， in this study， the water temperature was manually adjusted to make fish gonads mature and make the fish spawn in advance in an artificial culture environment. Up to early May， the parents entered the breeding period， and had been cultured under strengthened conditions for 3 months， they were artificially induced on May 5， 15 d earlier than the parents cultivated under natural conditions， thus indicating that strengthened culture through appropriate artificial temperature control is feasible for the promotion of the early breeding of P. fulvidraco. In this study， there were no significant differences in gonad weight and maturity coefficient between the parents cultured under strengthened and natural conditions （P>0.05）; and as the cultivation progressed， the difference gradually became significant （P<0.05）. From midApril to early May， the maturiry coefficient of the P. fulvidraco cultured under strengthened conditions increased sharply， from 8.65% to 16.20%， and even female P. fulvidraco parents collected on April 30 showed egg spawning. The reason might be that since midMarch， compared with natural conditions， greenhouse water temperature was higher and suitable for parental development， and plus normal feeding and adequate nutrient intake， the gonad development was faster. Sun et al.[15]cultured parent fish at 25 ℃ for one month from April 30， and most of the parent fish developed to gonad maturity on May 28. This culture used coalfired heating to keep indoor water temperature suitable for parental development， which shortens the cultivation time compared with this study， but the cost is higher， so it is suitable for northern China. In addition， within the appropriate temperature range， the metabolic intensity of fish and temperature are directly proportional to each other， and fish consumption and feed rate are closely related to metabolic rate[16]. The parents need to reserve a large amount of protein nutrition before reproduction to provide material and energy bases for gonad development， while the parents cultivated under natural conditions suffer from poor water temperature and have poor food intake in the early stage， which results in insufficient nutrition and slowed gonad development. Effects of early propagation on the "three rates" in artificial propagation
From May 5 to May 10， a total of more than 2 500 kg of P. fulvidraco parents under strengthened conditions were induced to spawn， showing an average induced spawning rate of 86.7% and an average fertilization rate of 71.6%. The spawning induction effect reached a normal level， indicating that early propagation did not negatively affect spawning induction. Luo et al.[16]showed that whether the parent fish were sick significantly affected the fertilization rate and fertilized egg quality at the time of spawning induction. Lai et al.[17]performed spawning induction on mandarin fish and found that the maturity of mandarin fish gonads and the size and color of eggs were closely related to the occurrence and control of fish diseases. During the strengthened culture period， the P. fulvidraco parents in greenhouses suffered from abdominal dropsy and were not fully recovered until the period of spawning induction， and during artificial egg stripping， the situation of small yields， small eggs and poor color was found， leading to the induced spawning rate and fertilization rate slightly lower than those under natural conditions. The greenhouse strengthened culture technology in this study is not yet fully mature， and it is necessary to improve ventilation， water quality control， scientific and reasonable feeding and disease prevention and control in future research or actual production.
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