McDermott et al: Annual fecundity, batch fecundity, and oocyte atresia of Pleurogrammus monopterygius 



25 



Location of tissue samples in ovaries 



The effect of ovary lobe location or position 

 within the ovary was not significant for either 

 average egg size (P=0.07, 0.43, respectively) 

 or mean number of eggs per gram of ovary 

 tissue (P=0.07, 0.53, respectively). Maturity 

 stage of ovary had a significant effect on both 

 eggs per gram (P<0.0001) and average egg size 

 (P<0.0001). 



Separation of oil-droplet— stage oocytes 

 (stage 4) from more advanced oocytes 



There was no significant difference in mean egg 

 size for stage-4 oocytes in pre- and postspawn- 

 ing fish (P=0.59)(Fig. 3B). We determined that 

 all oocytes smaller than 525 f(m that were 

 measured were stage-4 oocytes (Fig. 3A). The 

 proportion of stage-4 oocytes less than 525 f(m 

 ( p ) was 0.755 based on samples of postspawn- 

 ing fish in 1993, with g, = 1592 (the number 

 of stage-4 oocytes that measured smaller than 

 525 f^m), and m, = 2110 (the total number of 

 stage-4 oocytes measured). This proportion 

 was treated as a constant when estimating the 

 number of oocytes in stage 5 and larger. 



Determinate versus indeterminate spawning 



With the exception of hydrated eggs, Atka 

 mackerel are characterized by a continuous oocyte-size 

 distribution much like that of an indeterminate spawner 

 (Fig. 4). Atka mackerel ovaries contain several stages of 

 developing oocytes throughout the spawning season. 



However, the length-fecundity relationship for the 

 total number of oocytes decreased after the fish had 

 spawned their first batch (Fig. 5A) and there was a 

 significant difference in intercept and slope (P=0.01 and 

 0.001, respectively). When the length-fecundity relation- 

 ship for oocytes in stage 5+ was examined, the potential 

 fecundity also decreased after spawning (Fig, 5B), with 

 a significant difference in intercept and slope (P= 0.003 

 and P= 0.009, respectively). The numbers of stage-4 

 oocytes did not appear to differ substantially before 

 and after spawning for fish of comparable size (Fig. 5C) 

 although the slope of the length-fecundity relationship 

 differed significantly (P=0.012). Furthermore, there was 

 no reduction in the number of stage-4 oocytes at compa- 

 rable fish lengths for the nine spent fish collected late 

 in the spawning season (October) versus prespawning 

 and spawning fish collected in June-August. 



Oocyte atresia 



Incidence of oocyte atresia in prespawning and spawn- 

 ing fish was found to be low. Only 15 of the 150 ova- 

 ries examined showed any signs of atresia. Of thesel5 

 ovaries, high occurrence of atresia (more than 10% of 

 oocytes affected) was found in 3, and low occurrence of 



8/2/1 994 

 7/31/1994 

 7/30/1 994 

 7/27/1 994 

 7/24/1 999 

 9/1994 O 

 8/1994 m 

 994 



7/6/1994 

 6/20/1 994 

 1994 

 994 



Q. 



c-ometers 



Figure 4 



Atka mackerel [Pleurogrammus monopterygius) oocyte-size fre- 

 quency distribution summarized by date for 1994. Oocyte-size 

 frequencies were averaged per day. 



atresia (less than 10% of oocytes affected) was found in 

 12. Therefore, atresia in ovaries of pre- and postspawn- 

 ing fish was not quantified further. 



Atresia was found to be present in all of the nine 

 ovaries of postspawning fish examined in the study 

 and the number of atretic oocytes corresponded roughly 

 with the number of ooccytes spawned in a batch (batch 

 size). This finding indicates that Atka mackerel may 

 use atresia to regulate reproductive output at the end 

 of the spawning season. 



Estimates of annual fecundity and batch fecundity 



Length-fecundity parameters (a and b) for predicted 

 fecundities for scenario 1 (oocytes astage 4) and scenario 

 2 (oocytes sstage 5), as well as estimates of batch fecun- 

 dity and atretic eggs are shown in Table 2 with their 

 respective standard errors. Predicted potential fecundity 

 for both scenarios is shown in Figure 6A. 



The length-fecundity relationship for oocytes at stage 

 5+ is the most realistic because it appears that most 

 stage-4 oocytes will not be spawned in the current sea- 

 son. Predicted potential and realized fecundity for stage 

 5-1- oocytes are illustrated in Figure 6A with their re- 

 spective confidence intervals. Because the estimates of 

 potential fecundity were treated as data in the length- 

 fecundity regressions, the variance estimates and confi- 

 dence intervals are conservative. Data and model fit for 

 the relationship of batch fecundity to length is shown in 

 Figure 6B, and the data and model fit for the number of 



