Evidence of spawning among Pacific bluefin tuna, Thunnus orientalis, in the Kuroshio and Kuroshio–Oyashio transition area

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Table 2

Histological classification criteria for Pacific bluefin tuna.

Class	Maturity status	Development class	MAGO and POFs	α and β atresia of yolked oocytes
1	Immature	Immature-regenerating (resting)	PNS or YVS, no POFs	Absent
2	Immature	Developing	EYS, no POFs	<50% α atresia, β atresia may be present
3	Mature	Spawning-capable	AYS, no POFs	<50% α atresia, β atresia may be present
4	Mature	Spawning	MNS or HS, and/or POFs	<50% α atresia, β atresia may be present
5	Mature	Regressing-potentially reproductive	AYS, no POFs	≥50% α atresia, β atresia present
6	Mature	Regressed	PNS or YVS, no POFs	100% α atresia, β atresia may be present

AYS, advanced yolked stage; EYS, early yolked stage; HS, hydrated stage; MAGO, most advanced group of oocytes; MNS, migratory nucleus stage; PNS, perinucleolus stage; POF, postovulatory follicles; YVS, yolk vesicle stage.

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