HUNTER and GOLDBERG: SPAWNING INCIDENCE OF NORTHERN ANCHOVY 



08 

 7 

 OG 

 05 

 4 



Spawning 



-Spawning 



ELAPSED TIME FROM SPAWNING (days) 



Figure 4. — Growth of oocytes in female northern anchovy dur- 

 ing an assumed 6-day spawning cycle. Open circles are the mean 

 diameter of eggs in the most advanced mode for: females with 

 hydrated eggs Ihydrated eggs excludedi combined with those 

 with recent postovulatory follicles (''spawning"!; females with 

 day-old postovulatory follicles (second open circle): and for 

 nonspawning females i third open circlet- Horizontal bar indi- 

 cates period in a 6-day cycle that females would be classified as 

 nonspawning; and the point is plotted at the midpoint of that 

 interval. Vertical bars are ±2 SE of mean and solid circle indi- 

 cates the mean diameter of eggs in the least advanced mode for 

 the nonspawning class. 



that in the second mode could produce a 6-day 

 spawning cycle, the second mode becoming the 

 most advanced mode at the time of spawning. The 

 mean diameter of the eggs in the most advanced 

 and the second mode for the nonspawning class, 

 when plotted at 3.5 days, seems in a reasonable 

 position relative to the other points, indicating 

 that the cycle may be about 6 days. A linear trajec- 

 tory of oocyte growth of 0.04 mm/day indicates 

 spawning at a diameter between 0.6 and 0.7 mm. 

 in keeping with laboratory findings. This analysis 

 indicates that the mean diameter of yolked oocytes 

 of females in various reproductive stages is consis- 

 tent with a 6-7 day spawning cycle. 



Batch Fecundity 



MacGregor ( 1968) estimated the number of eggs 

 in the most advanced mode in northern anchovy 

 ovaries to be 574 eggs/g wet weight, from an 

 analysis of frequency distribution of eggs in 19 

 females. Norberg (see footnote 5) concluded that 

 northern anchovy fecundity was 556 eggs/g, from 

 an examination of 119 females. The supposition 

 underlying both estimates was that the number of 

 eggs in the most advanced mode represents the 

 number of eggs spawned. Owing to the importance 

 of batch fecundity in any estimate of spawning 

 biomass from egg and larval production, we de- 



cided to reexamine spawning batch fecundity in 

 the northern anchovy. 



The assumptions underlying batch fecundity es- 

 timates are: all eggs in the most advanced mode 

 are spawned; fecundity is directly proportional to 

 weight; and no bias exists in the estimation of the 

 number of eggs within the most advanced mode 

 nor in the selection of mature females for analysis. 

 We consider these assumptions for females taken 

 in February 1978 using fecundity estimates for 

 each reproductive class. 



Histological examination of females with post- 

 ovulatory follicles indicated that very few hy- 

 drated eggs were retained after spawning. Thus, 

 the number of hydrated eggs within ovaries prior 

 to ovulation (females with no postovulatory folli- 

 cles) should give the most accurate estimate of the 

 number of eggs spawned. Another advantage of 

 using hydrated eggs was that they stand out as an 

 isolated class, distinct from all others; they differ 

 in appearance and are as much as 2-3 times larger 

 than yolked oocytes. Hence, they only need to be 

 counted; neither statistical techniques nor one's 

 judgment need be used to separate overlapping 

 modes. 



The mean number of hydrated eggs per gram of 

 female (ovary free) was 389 ±59 ( ±2 SE) eggs and 

 was only T^k less than that estimated for females 

 with the most mature ovaries (nonspawner class) 

 (Table 6). Thus, nearly all eggs in the most ad- 

 vanced mode were destined to be hydrated and 

 spawned. Fecundity estimates were substantially 

 higher and more variable in the other three repro- 

 ductive classes. Many of the females in these 

 classes had only one mode of yolked eggs whereas 

 about %W( of those classified as nonspawners had 

 two modes. Fecundity estimates for the less ma- 

 ture females tend to be higher because the eggs 

 destined to form a second mode have not grown 

 sufficiently to be separated from the rest of the 

 yolked oocytes. More variability exists because of 

 variation among females in the extent of the dif- 

 ferentiation of the second modal group of eggs. 



In summary, we believe our most accurate esti- 

 mate of batch fecundity is 389 hydrated eggs/g 

 ovary-free female weight. If an estimate based on 

 total female weight is needed, we recommend the 

 one for nonspawning fish (Table 6) reduced by the 

 fraction of eggs which may not be hydrated (7%). 

 The adjusted fecundity for nonspawners is 368 

 eggs/g female weight. 



Fecundity as estimated above is a function of 

 female weight, ovary weight, and the number of 



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