22 



Fishery Bulletin 105(1) 



Determinate versus indeterminate spawning 



Direct estimation of annual fecundity is possible if 

 the number of eggs to be spawned that season is fixed 

 (determinate) and there is a well-defined gap in the 

 oocyte-size distribution between advanced oocytes that 

 will be spawned that year and the immature oocytes. 

 In contrast, indeterminate spawners have the ability to 

 develop unyolked oocytes continually and add them to 

 the standing stock of advanced-yolk oocytes even after 

 spawning begins. 



To examine whether Atka mackerel are determinate 

 spawners as previously described (Zolotov, 1993), we 

 measured their oocytes to obtain a size distribution for 

 oocytes over time. Additionally, we compared the po- 

 tential fecundity of prespawning fish with the potential 

 fecundity of fish that had spawned at least one batch. 

 When all oocytes stage 4 and greater were summed, 

 the tally exceeded the expected number of oocytes to be 

 spawned per year, given the number of eggs in the first 

 batch and the previous estimate of an average of three 

 batches per female (Zolotov, 1993). This total number of 

 oocytes led to the hypothesis that stage-4 oocytes may 

 be developed, but not spawned, in a given year and are 



1050 1300 1550 1800 2050 2300 2550 



140 

 120 

 100 - 



80 



60 



40 - 



20 



UlUlL 



-^-jH 



50 



1050 1300 1550 



Micrometers 



1800 2050 2300 2550 



Figure 2 



Atka mackerel iPleurogrammus monopterygius) oocyte-size frequency 

 distribution of (A) stage-4 (clear bars) and advanced atretic eggs (black 

 bars) typical of a recently spent ovary, measured from whole oocytes 

 (October 1993); (B) an ovary collected on 2 August 1994, with a hydrated 

 batch of oocytes ready to be spawned (oocytes larger than 1800 jim 

 are hydrated). 



part of the standing stock of oocytes to be spawned in 

 the following year. This hypothesis was supported when 

 we found both oil droplet oocytes and atretic advanced- 

 stage oocytes in the spent ovaries of nine specimens 

 collected in 1993 (Fig. 2A). 



The specimens from 1994 were consequently divided 

 into two ovary stages: 



1 Prespawning-stage ovaries: ovaries with the most 

 advanced oocyte stage of vitellogenic or hydrated 

 oocytes that showed no evidence of having been 

 spawned, i.e., did not have postovulatory follicles 

 (POFs). 



2 Spawning-stage ovaries: ovaries that had spawned 

 at least one batch of eggs and therefore contained 

 POFs. 



Potential annual fecundity was estimated with the 

 gravimetric method and the total number of oocytes 

 were further divided into stage-4 and stage-5-i- oocytes 

 by using the methods described below. The data were 

 log-transformed and linear regressions were fitted by 

 using S-plus. Ovaries of prespawning and spawning fish 

 were compared by testing for differences in intercepts 

 and slopes with S-Plus (Venables and Ri- 

 pley, 2002). 



Separation of stage-4 oocytes from more 

 advanced oocytes 



To test whether Atka mackerel retain 

 all of their stage-4 oocytes as a stand- 

 ing stock for the next year's fecundity, it 

 became necessary to separate the stage-4 

 oocytes from the rest of the more developed 

 oocyte stages and then subtract the stage- 

 4 oocytes from the total oocyte count. It 

 was not possible to distinguish the oil- 

 droplet-stage oocytes from oocytes in the 

 vitellogenic stage or migratory nucleus 

 stage when examining whole oocytes with 

 the dissecting microscope image and the 

 Optimas software. Therefore, we exam- 

 ined nine ovaries that were recently spent, 

 which contained only healthy stage-4 

 oocytes and atretic hydrated oocytes that 

 could be easily distinguished (Fig. 2A). 

 The average size distribution of stage- 

 4 oocytes was estimated by measuring 

 approximately 250 stage-4 oocytes per 

 ovary. This size distribution could then be 

 applied to the prespawning-stage ovaries, 

 assuming that stage-4 oocyte-size distri- 

 butions in spent ovaries were the same as 

 those in prespawning-stage ovaries. To 

 test this assumption, the size distribution 

 from prespawning-stage ovaries in histo- 

 logical preparations was compared to the 

 size distribution in postspawning fish by 

 using ANOVA. 



