inhabit the coasts of south korea  the yellowtail kingfish ( s. lalandi ) , yellowtail ( s. quinqueradiata ) , and greater amberjack ( s. dumerili ) ( kim et al . , 2001 ) . yellowtail kingfish do not only occur in south korea , however , but also in temperate waters worldwide , including japan and china ( nfrdi , 2004 ) . in japan , 3 species of s. lalandi , s. quinqueradiata and s. dumerili are cultivated ( nakada , 2000 ) . yellowtail kingfish are also cultivated in australia and new zealand ( moran et al . , 2007 ; kolkovski & sakakura , 2007 ) , as well as in california , usa ( stuart & drawbridge , 2012 ) . aquaculture of seriola spp . in south korea has been attempted , beginning with pond culture of yellowtail in 1963 . since then , there was an experimental culture with juvenile yellowtail captured off the south coast . after the export of captured seeds for cultivating was banned in south korea in 1976 , pond culture of yellowtail began in earnest . however , domestic aquaculture of seriola spp . has so far been restricted to a method of pond culture of captured juveniles . in the future , culture production systems , such as seed production and intermediate rearing , clause 2 of the  enforcement decree of the fishery resources management act ( prohibition of capturing and gathering of fishery resources  weight and length , presidential decree no . 25276 ) , implemented from early 2014 , it restricts the capture of yellowtail to individuals of 30 cm or shorter . in japan , the aquaculture of yellowtail kingfish used hormones for the induction of maturation and production of fertilized eggs ( fugita & yogata , 1984 ) . however , there has been no research into reproductive biology for the cultivation of yellowtail kingfish in south korea . this study therefore aimed to develop a method for the production of stable , healthy fertilized eggs from yellowtail kingfish . since yellowtail kingfish are spring - summer spawning fish , that typically spawn april to june , broodstock management was conducted by controlling the light cycle in an indoor tank using the characteristics of spring - summer spawning , during which the spawning behavior , spawning induction , and spawning frequency were examined . in addition , the spawning amount , buoyancy rate and fertilization rate among spawned eggs , the size change of egg and oil droplet for buoyant eggs were investigated . fifty - nine adult yellowtail kingfish were caught off the coast of jeju , south korea , and raised in a concrete indoor tank ( 8.0  8.0  2.0 m , usable capacity 100 m ) . in all , 20 fish ( 4.5  0.7 kg ) were raised in march 2008 , while the remaining 39 fish ( 2.6  0.7 kg ) were raised in october 2012 ( fig . the tank was filled with natural seawater , with the flow rate set at 6 cycles per day . the water temperature during the breeding period ranged from 12.0 to 29.0c . during the spawning season , the flow rate was adjusted to 3.5 cycles per day to protect the eggs , thus enabling good quality fertilized eggs to be collected . to induce maturation , a light intensity of 130 lux was maintained , in accordance with conditions for inducing maturation in spring - summer spawning fish ( song , 2013 ) , a light cycle of 12 h light ( l ) : 12 h dark ( d ) was used from october 2013 to january 2014 , and a light cycle of 15 h l : 9 h d from february 2014 to the end of spawning . fish were fed a commercial ep feed ( 1618 mm ) , comprising  46.0% crude protein ,  9.0% crude lipid ,  16.0% crude ash ,  5.0% crude fiber ,  1.0% calcium , and  2.7% phosphorus . from april to the end of spawning , feeds were supplemented with pacific flying squid ( todarodes pacificus ) . rearing management for the induced sexual maturation of captive - reared yellowtail kingfish ( s. lalandi ) broodstock in indoor culture tank spawning began after sunset , between 19:00 and 24:00 , and ended between 02:00 and 03:00 . eggs were collected every morning between 08:00 and 09:00 , before separating the buoyant eggs and sunken eggs using a measuring cylinder . the volume of each group was measured to calculate the spawning amount , while the buoyancy rate was calculated from the ratio of buoyant eggs to total spawning amount . the number of eggs was calculated volumetrically using a conversion rate of 500 eggs per ml . to calculate the rate of fertilization , a profile projector ( pj300 , mitutoyo ) was used to count the ratio of buoyant eggs showing signs of development . the egg diameter and oil droplet diameter for 30 buoyant eggs were measured to a precision of 0.001 mm at 1-week intervals , using the profile projector . the growth of the broodstock in the concrete indoor tank during the breeding period is displayed in table 1 and fig . 2 . those individuals acquired in march 2008 , grew from 3.15.9 kg ( mean 4.5  0.7 kg ) to 9.414.9 kg ( 11.4  1.3 kg ) in 77 months . individuals acquired in october 2012 grew from 1.34.0 kg ( 2.6  0.9 kg ) to 6.110.5 kg ( 8.0  1.1 kg ) in 22 months . males and females could not be distinguished owing to the similarity of their external appearance . growth of captive - reared yellowtail kingfish ( s. lalandi ) broodstock used for the induction of sexual maturation and natural spawning in an indoor culture tank weight of captive - reared yellowtail kingfish ( s. lalandi ) broodstock used for the induction of sexual maturation and natural spawning in 2014 . 2008 , wild - caught broodstock in 2008 ;  2012 , wild - caught broodstock in 2012 . following 3 months of long - day conditions ( 15l:9d ) , the spawning of yellowtail kingfish began in early may 2014 , between the times of 19:00 and 24:00 . while the fish were swimming together in an orderly manner near the edges of the tank , a relatively large female was seen being followed by 2 males , twisting and intertwining . the males stuck close to the female s genital pore while swimming rapidly , and applied pressure to the female s abdomen . after approximately 1 hour of courtship behavior , the female began spawning , and the orderly swimming of males became started to spawn , in a frenzy of circling activity . after a while , the clear and still water in the tank became clouded with the spawned eggs and the sperm of the males , while small bubbles rose to the surface of the water . spawning first began on may 3 , when the water temperature was 17.0c , and it ended in mid - june , when the temperature was 20.5c . the yellowtail kingfish undergoing rearing management spawned a total of 26 times during the spawning period , with the total spawning amount being 4,449  10 individuals , making it an average spawning amount of 171  10 each time . among mature yellowtail kingfish eggs , the mean ratio of buoyant eggs was 76.1% , while the fertilization rate for buoyant eggs during the spawning period was 100% ( fig . 3 ) . right up until the third spawning event , the majority of eggs were cloudy or sunken , with a buoyancy rate of 0% . from may 23 to the end of spawning , the fish exhibited a pattern of spawning at 23 day intervals , with the buoyancy rate ranging from 33.396.4% . spawn frequency and egg production of captivereared yellowtail kingfish ( s. lalandi ) broodstock in an indoor culture tank from may to june , 2014 . the egg diameter of fertilized yellowtail kingfish eggs was 1.388  0.041 mm , during which those eggs were buoyant with a single oil globule ( fig . the egg diameter changed from 1.416  0.030 mm at the start of spawning on may 8 , to 1.351  0.037 mm at the end of spawning on june 9 . changes in mean egg diameter and oil globule diameter of natural spawned eggs from captivereared yellowtail kingfish ( s. lalandi ) broodstock in 2014 . the oil globule diameter at the start of spawning ( may 8) was 0.348  0.025 mm , which gradually decreased towards the end of the spawning period . the egg diameter and oil globule diameter were reduced at the end of spawning by 4.1% and 13.0% , respectively , from that at the start of spawning . the growth of the broodstock in the concrete indoor tank during the breeding period is displayed in table 1 and fig . 2 . those individuals acquired in march 2008 , grew from 3.15.9 kg ( mean 4.5  0.7 kg ) to 9.414.9 kg ( 11.4  1.3 kg ) in 77 months . individuals acquired in october 2012 grew from 1.34.0 kg ( 2.6  0.9 kg ) to 6.110.5 kg ( 8.0  1.1 kg ) in 22 months . males and females could not be distinguished owing to the similarity of their external appearance . growth of captive - reared yellowtail kingfish ( s. lalandi ) broodstock used for the induction of sexual maturation and natural spawning in an indoor culture tank weight of captive - reared yellowtail kingfish ( s. lalandi ) broodstock used for the induction of sexual maturation and natural spawning in 2014 . 2008 , wild - caught broodstock in 2008 ;  2012 , wild - caught broodstock in 2012 . following 3 months of long - day conditions ( 15l:9d ) , the spawning of yellowtail kingfish began in early may 2014 , between the times of 19:00 and 24:00 . while the fish were swimming together in an orderly manner near the edges of the tank , a relatively large female was seen being followed by 2 males , twisting and intertwining . the males stuck close to the female s genital pore while swimming rapidly , and applied pressure to the female s abdomen . after approximately 1 hour of courtship behavior , the female began spawning , and the orderly swimming of males became started to spawn , in a frenzy of circling activity . after a while , the clear and still water in the tank became clouded with the spawned eggs and the sperm of the males , while small bubbles rose to the surface of the water . spawning first began on may 3 , when the water temperature was 17.0c , and it ended in mid - june , when the temperature was 20.5c . the yellowtail kingfish undergoing rearing management spawned a total of 26 times during the spawning period , with the total spawning amount being 4,449  10 individuals , making it an average spawning amount of 171  10 each time . among mature yellowtail kingfish eggs , the mean ratio of buoyant eggs was 76.1% , while the fertilization rate for buoyant eggs during the spawning period was 100% ( fig . 3 ) . right up until the third spawning event , the majority of eggs were cloudy or sunken , with a buoyancy rate of 0% . from may 23 to the end of spawning , the fish exhibited a pattern of spawning at 23 day intervals , with the buoyancy rate ranging from 33.396.4% . spawn frequency and egg production of captivereared yellowtail kingfish ( s. lalandi ) broodstock in an indoor culture tank from may to june , 2014 . the egg diameter of fertilized yellowtail kingfish eggs was 1.388  0.041 mm , during which those eggs were buoyant with a single oil globule ( fig . the egg diameter changed from 1.416  0.030 mm at the start of spawning on may 8 , to 1.351  0.037 mm at the end of spawning on june 9 . changes in mean egg diameter and oil globule diameter of natural spawned eggs from captivereared yellowtail kingfish ( s. lalandi ) broodstock in 2014 . the oil globule diameter at the start of spawning ( may 8) was 0.348  0.025 mm , which gradually decreased towards the end of the spawning period . the egg diameter and oil globule diameter were reduced at the end of spawning by 4.1% and 13.0% , respectively , from that at the start of spawning . in broodstock management and rearing management for the induction of spawning in indoor tanks , it is important for the size and shape of the tank to be suited to the swimming characteristics and spawning behaviors of the broodstock . in previous studies , a 140-m circular tank was used for california yellowtail ( seriola lalandi ) ( stuart and drawbridge , 2012 ) , while spawning was induced in 14 yellowtail kingfish ( mean weight , 17 kg ) in a 70-m circular tank ( moran et . in this study , the size of the tank was 100 m. the fish acquired in march 2008 grew from in mass from a mean of 4.5  0.7 kg to 11.4  1.3 kg in 77 months , while the fish acquired in october 2012 grew from 2.6  0.9 kg to 8.0  1.1 kg in 22 months . by adjusting the light cycle , we were able to induce spawning and retrieve fertilized eggs . in determining the size of broodstock management tanks for seriola spp . , it was assumed that there is some relationship between broodstock density and spawning behavior characteristics . since this was the first time that fertilized eggs had been produced by yellowtail kingfish broodstock in an indoor tank , future studies will be required for the development of a breeding tank system that accounts for spawning behavior characteristics and the optimal male - to - female ratio and breeding density for stable healthy egg production . in new zealand yellowtail kingfish , spawning occurred 3090 minutes after courtship , but another male was observed to participate 1015 minutes prior to spawning ( moranet al , 2007 ) . in this study , we also observed 2 males entwining and twisting as they followed a single large female , sticking close to the genital pore , and swimming rapidly while pressing the abdomen of the female to induce spawning . having information about the pre - spawning courtship behavior makes it possible to predict when spawning will occur and prepare for egg collection , thereby reducing the risk of bacterial infection due to delayed collection and mechanical damage from the collecting net ( moran et al,2007 ) . during this study , in which light cycle regulation was used to induce maturation of yellowtail kingfish in an indoor tank , it is difficult to find instances of broodstock management with light cycle - induced maturation of spring - summer spawning yellowtail kingfish off the coast of jeju ; however , sevenband groupers ( epinephelus septemfasciatus ) , which are summer spawning fish inhabiting the jeju region , undergo induction of maturation and fertilized egg production under similar broodstock management conditions of long days with water temperature regulation ( song , 2013 ) . yellowtail kingfish fertilized egg production in goto in the south of japan was conducted with hormone treatment between april and may at a water temperature of 15.5 18.5c , followed by artificial insemination ( fugita & yogata,1984 ) . california yellowtail in california , san diego , and catalina island spawn between april and july at water temperatures of 16.522.0c , while yellowtail kingfish inhabiting east coast in northern new zealand spawn between november and february , when the water temperature is at least 17.0c ( stuart & drawbridge , 2012 ; moranet al . , 2007 ) . in terms of the size of individuals participating in spawning , one study of gonadal development in yellowtail kingfish reported that the minimum size fork length of females reaching maturity was 78 cm , 50% of females reached maturity at 94 cm , and 100% of females reached maturity at 128 cm . , 50% of males reached maturity at 81 cm , and 100% of males reached maturity at 93 cm ( poortenaar etal . , 2001 ) . another study in new south wales , australia , showed that females first matured at 3 years and a fork length of 70 cm , and that 50% reached maturity at 83 cm between 45 years . in all , 50% of males matured at 47 cm and 1 year ( gillanders etal . mckenzie et al . ( 2014 ) reported that the reason why yellowtail kingfish living in new south wales and new zealand reach maturity and different sizes and ages is the water temperature , the unique habits of individuals , and the differences in the habitat conditions . in this study , the yellowtail kingfish broodstock were caught in the wild , and bred to a length of 85.0103.0 cm ( mean 97.8  7.0 cm ) in an indoor tank , meaning that , in contrast to the study of gonadal development in yellowtail kingfish off the coast of northern new zealand , the females should have exceeded the length at which they first started reaching maturity , while the males would have reached 100% maturity . a previous study showed that california yellowtails spawn 3743 times in water temperatures ranging from 16.522.0c ( stuart & drawbridge , 2012 ) . in the present study , this is consistent with results from southern japan , california , and new zealand , which showed that spawning begins at temperatures of 15c and above , but the regional differences in the number of spawns and the end of spawning can be considered to result from adaptive physiological characteristics . fugita & yogata ( 1984 ) reported a fertilization rate of 81.6% for artificial insemination in yellowtail kingfish , while natural spawning by yellowtail kingfish in an indoor tank exhibited a fertilization rate of over 99% for buoyant eggs ( moran et al . , 2007 ) . in this study , the buoyancy rate was 76.1% , while the fertilization rate of buoyant eggs was 100% . fugita & yogata ( 1984 ) measured the egg diameter and oil globule diameter of yellowtail kingfish that had been artificially inseminated following hormone treatment , and reported diameters of 1.36  0.07 mm and 0.34  0.02 mm , respectively . in this study , the egg diameter and oil globule diameter of yellowtail kingfish after natural spawning were 1.388  0.041 mm and 0.0378  0.029 mm , showing slightly greater size than that observed following artificial insemination . the egg diameter and oil globule diameter of california yellowtail were reported at 1.36  0.03 mm and 0.29  0.02 mm in 2009 , and 1.40  0.05 mm and 0.32  0.05 mm in 2010 ( stuart & drawbridge , 2012 ) . another study found that the egg diameter and oil globule diameter in yellowtail kingfish were 1.40  0.04 mm and 0.31  0.01 mm , respectively ( moran et al . , 2007 ) . the egg diameters in california yellowtail and yellowtail kingfish were similar . in this study , the egg diameter and oil globule diameter changed over during the course of spawning season , showing reductions of 4% and 13% , respectively , at the end of spawning from those at the start of spawning . the decrease in oil globule diameter size over the course of the spawning season also tended to follow the change in egg diameter . these results show that the egg diameter in yellowtail kingfish decreases volumetrically by approximately 15% during the spawning season , and that the oil globule diameter is closely related to the egg diameter and tends to decrease in size more than the egg diameter , resulting in a reduction in oil globule volume of approximately 20% from that at the start of spawning ( moran et al . , 2007 ) . the reasons for this reduction in egg diameter include differences between females in egg production and spawning period ( fugita & yogata , 1984 ) , as well as the general reduction in egg quality as the spawning period continues ( brooks et al . , 1997 ) . however , although egg quality is assumed to be better for larger eggs ( kamler , 1992 ) , there was no noticeable trend during the spawning period . it was therefore difficult to draw any conclusions about whether or not egg diameter affects egg quality ( moran et al , 2007 ) . although this protocol produced fertilized eggs , future studies will be required investigating management to improve the health of broodstock for the stable acquisition of healthy eggs .
this study describes results on sexual maturation and characteristics of natural spawned eggs to develop a method for the production of stable , healthy fertilized eggs from captive - reared yellowtail kingfish , seriola lalandi . 
 a total of 59 yellowtail kingfish were captured off the coast of jeju island , after which the broodstock was cultured in indoor culture tank ( 100 m3 ) until they were 6.114.9 kg in body weight . as part of the rearing management for induced sex maturation , the intensity of illumination 
 was maintained at 130 lux . 
 the photoperiod ( light / dark ; l / d ) was set to a 12 l/12 d from october 2013 to january 2014 , and 15 l/9 d from february 2014 to june 2014 . 
 feeds comprised mainly ep ( extruded pellets ) , with squid cuttlefish added for improvement of egg quality , and was given from april to june 2014 . 
 the first spawning of yellowtail kingfish occurred in may 3 , 2014 , at a water temperature of 17.0c . 
 spawning continued until june 12 , 2014 , with the water temperature set at 20.5c . 
 time of spawning was 26 times at this period . 
 the total number of eggs that spawned during the spawning period was 4,449103 . 
 the buoyant rate of spawning eggs and fertilization rate of buoyant eggs during the spawned period were 76.1% and 100% , respectively . 
 the diameters of the egg and oil globule were 1.388  0.041 mm and 0.378  0.029 mm , respectively , which was higher in early eggs than in those from late during the spawned period .