| Early growth and development stage as an important part of fish life history, whichrelated to all kinds of aspects like growth, development, morphological characteristics,ecology, is the mainly factor that influenced the population and age structure, and hasbecome a focus of fishery resource research correlated with exploitation, utilize,protectionism constitute. However, it is hard to utilize traditional methods to studydeeply on early growth and development stage of fish. Now, using morphologicaldevelopment measurement and microstructure in fish otolith to study early growth anddevelopment has been a hotspot in the world, which can solve many problemseffectively in formerly study. Therefore we plan to select larvae and juveniles of bigyellow croaker(Larimichthys crocea) and small yellow croaker(Larimichthys polyactis)and measure their morphological development and microstructure of otolith(sagitta) asobject, with the aim to establish foundation with several research about daily age,growth, development, death, population identification, and accumulate data for otolithresearch. Furthermore, similar method maybe applied to other fish studies.Our experiment data consisted of two parts: one is the larvae and juveniles of bigyellow croaker and small yellow croaker via artificial breeding at Jiangsu marinefisheries research institute during April2011to June2011; the other one is sampled fromtwo locations during April2011to June2011: Sanmen bay and inshore spawning site ofLüsi fishing ground. After measuring biological character, morphological developmentstage and microstructure in otolith(sagitta), we combined graphics analyses softwareand used mathematical statistics and methods from each aspects to systemic analysis ofthe character of morphological development and microstructure of otolith(sagitta), andthe relationship with early growth and development of each individual, and thenanalyzed the difference on microstructure in otolith(sagitta) between Sanmen bay and inshore spawning site of Lüsi fishing ground.In the first place, the larvae and juveniles of large yellow croaker wereobserved continually in rearing condition, and individual growth and development andmorphological development and microstructure of otolith(sagitta) have been researched.Results show as following:(1)New born larvae metamorphosed into juvenile after28d,including yolk-sac larvae(3d), pre-flexion larvae(12d), flexion larvae(3d), post–flexion(10d). The days after hatching(t) for large yellow croaker was significant exponentialrelated to standard length(LS) of all individuals, and the equation is LS=4.07e0.025t(N=211, R2=0.97, P<0.001).(2)The otolith(sagitta) has appeared since38h embryodevelopment. The otolith(sagitta) during yolk-sac larvae and pre-flexion larvae stagewere round in shape, subsequently the long axis elongated rapidly during flexion larvaestage, and then changed to ellipse in shape during post-flexion larvae stage. During theearly juvenile stage, the first secondary primordium and secondary growth zone(SGZ)began to form at29~39(32±0.39)day, the fringe of otolith(sagitta) hunched up,corresponding to the metamorphism from larvae to juveniles, subsequently the numberof secondary primordium increased gradually and the shape of otolith(sagitta) has asignificant change. For47~78-day-old juveniles, the number of secondary primordiumreaches to5~7stably, and secondary growth zone surrounding the first growth zone(FGZ) formed an “enclosed structure”. In this stage, the otolith(sagitta) were nearlypeltate in shape and close to the adult. The radius(R) of otolith(sagitta) were significantlinearly related to standard length(LS) of all individuals, and the equation is LS=-145.51+37.52R(N=211, R2=0.98, P＜0.001).(3)The increment count(I) in otolith(sagitta) wassignificant linearly related to the days after hatching(t), and the equation is I=-0.99+1.01t(N=169, R2=0.99, P＜0.001), which indicated the daily periodicity of growthincrement formed in otolith(sagitta). The first daily increment was formed on day2afterhatch, corresponding to the initial feeding. The Growth rates during the larvae stage(b)that calculated by daily increment width in otolith(sagitta) of juveniles were linearlyrelated to the time that the first secondary primordium formed(tsp), and the equation isb=38.60-161.91tsp(N=42, R2=0.72, P＜0.01), which indicated the formation ofsecondary primordium was much earlier and the metamorphosed time from larvae tojuveniles was much shorter for faster growing larvae. To sum up, the growth anddevelopment in early life history of large yellow croaker could be back-calculated fromotolith(sagitta) size, morphology and microstructure. These results would provide theoretical basis for the early growth and development as well as resource recruitmentresearch of large yellow croaker in field．In the next place, we sampled the larvaes and juveniles of small yellow croakerfrom two locations: Sanmen bay and inshore spawning site of Lüsi fishing ground, andthe morphological development and microstructure in otolith(sagitta) have beenresearched. Results show as following:(1)The radius(R) of otolith(sagitta) wassignificant linearly related to standard length(LS) at two sampling locations, and theequations were LS=122.24+38.12R(N=78, R~2=0.92, P＜0.001) and LS=-115.43+31.84R(N=84, R~2=0.94, P＜0.001) respectively, which indicated the radius growing of otolith(sagitta) was synchronous with body length growing of wildlife small yellow croaker.(2)The mean value of growth rates during the larvae stage(b) that calculated by dailyincrement width in otolith(sagitta) of juveniles at Sanmen bay and inshore spawning siteof Lüsi fishing ground were0.039and0.024respectively. There was significantdifference between them(P＜0.001, df=230.58), indicated that the growth rate anddevelopment rate during the larvae stage of inshore spawning site of Lüsi fishingground juveniles were faster than Sanmen bay juveniles.(3)The mean value of time thatthe first secondary primordium formed at Sanmen bay and inshore spawning site of Lüsifishing ground were37d and46d respectively, which had the significance difference(P＜0.001, df=151.03), and indicated larvaes metamorphosis to juveniles at inshorespawning site of Lüsi fishing ground occurred earlier than Sanmen bay.(4)The growthrates during the larvae stage(b) that calculated by daily increment width inotolith(sagitta) of two sampling locations juveniles were linearly related to the time thatthe first secondary primordium formed(tsp), and the equation is tsp=0.07-0.001b(N=162,R2=0.51, P＜0.001), which indicated the formation of secondary primordium was muchearlier and the metamorphosis time from larvaes to juveniles was much shorter thanfaster growing larvae.(5)At two sampling locations, the more the number of secondaryprimordium were, the faster the growth rates during the larvae stage kept. To sum up,morphological development and microstructure in otolith(sagitta) could mirrored thegrowth and development in early life history of small yellow croaker. For small yellowcroaker in different waters, the microstructure in otolith(sagitta) varied. These resultswill provide useful information for many researches about small yellow croaker, such asthe estimate of spawning ground and spawning time, population identification, earlyselective mortality, and so on. |
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