HUNTER ET AL.: SPAWNING FREQUENCY OF SKIPJACK TUNA 



meaningful because the oocyte resorption seems to 

 follow a similar sequence of stages in most teleosts 

 (Bretschneider and Duyvene de Wit 1947; Lambert 

 1970). The rate a skipjack tuna ovary passes from 

 one atretic state to another is not specified and 

 would require an additional study of captive fish. 

 The characteristics of the two classification systems 

 are outlined below. 



Spawning Frequency 



Hydrated and Migratory Nucleus Stages 



Ovaries with many translucent hydrated oocytes 

 (oocytes enlarged by fluid uptake just prior to ovula- 

 tion) are classified in the hydrated stage. Spawn- 

 ing is considered to be imminent. In northern an- 

 chovy, spawning takes place in <12 h after the onset 

 of hydration. No skipjack tuna with hydrated 

 oocytes were taken in our field collections. Female 

 skipjack tuna were taken with ovaries in the migra- 

 tory nucleus stage. This stage occurs just before the 

 onset of hydration and is characterized by the migra- 

 tion of the nucleus to the animal pole of the oocyte 



and the beginning of the fusion of its yolk globules 

 (Fig. 1). 



Age 0-H Postovulatory Follicles 



Ovaries with new postovulatory follicles with no 

 signs of follicle degeneration are classed as age 0-h 

 postovulatory follicles. Hydrated oocytes may occa- 

 sionally be present. Estimated elapsed time from 

 spawning is 0-2 h. No skipjack tuna taken at sea 

 were in this stage, but from the laboratory speci- 

 men (Fig. 2a, b) we can discern the following histo- 

 logical characteristics: The new postovulatory 

 follicle has an irregular, convoluted shape. The 

 granulosa epithelial cell layer of the follicle appears 

 as an irregularly looped cord of slightly hypertro- 

 phied cuboidal cells with prominent healthy nuclei 

 linearly arranged. The granulosa appears only loose- 

 ly attached to the thecal connective tissue layer. 

 Although the theca is less convoluted than the gran- 

 ulosa layer, it is distinct, contains blood capillaries 

 and appears thicker than the thecal layer seen in 

 northern anchovy. 



Figure 1.— Skipjack tuna oocyte with migratory nucleus (n) and 

 large oil droplet (o); bar = 0.1 mm. 



Age 12-H Postovulatory Follicles 



Twelve-hour-old postovulatory follicles (Fig. 2c, 

 d) show signs of degeneration similar to that ob- 

 served in northern anchovy after about 24 h. Histo- 

 logical characteristics include the follicle which is 

 smaller with fewer convolutions; a lumen which is 

 evident; the degenerating granulosa which is no 

 longer a recognizable unbroken cord of cells, but 

 rather the cells are scattered in clumps in the lumen 

 or may be irregularly attached to the theca; and 

 some pycnotic or irregular nuclei which are evident. 

 The theca has begun to disintegrate although it still 

 remains thick and distinct. Deterioration of the 

 theca is indicated by its overall smaller size, a more 

 filamentous rather than cohesize cellular arrange- 

 ment, and some irregular nuclei. 



Age 24-H Postovulatory Follicles 



Ovaries containing 24-h-old postovulatory follicles 

 showed pronounced signs of degeneration similar 

 to that observed in northern anchovy 48 h after 

 spawning. At this stage the follicle is much smaller 

 than that at 12 h but a lumen is still evident (Fig. 

 2e, f). Only few granulosa cells remain; they usual- 

 ly have pycnotic nuclei and generally are loosely at- 

 tached to the thecal layer. The thecal layer is still 

 fairly thick although it contains some pycnotic 



897 



