FISHERY BULLETIN: VOL. 85, NO. 3 



pletely separated from the remaining reserve 

 fund (Fig. 9E). This appears to be the best stage to 

 estimate fecundity, as no more eggs will be re- 

 cruited into the developing mode, and spawning 

 has not yet begun. 



The final part of the maturation process ap- 

 pears to occur in a synchronized manner, with 

 successive groups of fully yolked oocytes proceed- 

 ing through the last stages (homogenization of 

 yolk, hydration, ovulation, and release) in dis- 

 crete batches. During the spawning period, the 

 proportion of nonhydrated (stage 8) oocytes grad- 

 ually decreases, and that of hydrated oocytes 

 (stage 11) increases, until only hydrated oocytes 

 remain. This progression is visible in Figure 9H 

 through L. The spawning of matured oocytes in 

 discrete groups appears to represent the "batch 

 spawning" process in walleye pollock. 



Spent ovaries contain oocytes in the early and 

 late perinucleus stages (the reserve fund), post- 

 ovulatory follicles, and, in some cases, yolked but 

 unspawned oocytes undergoing resorption. Rede- 

 veloping ovaries, i.e., those containing signs of 

 prior spawning and vitellogenic oocytes, were 

 found in small numbers (9 out of 122 ovaries ex- 

 amined). Of the 18 ovaries examined for March, 

 only one appeared to be developing new oocytes, 

 possibly early enough for a second spawning that 

 year. The rest of the redeveloping ovaries were 

 collected from June to September from walleye 

 pollock in the southeast shelf area. Redeveloped 

 ovaries found in the summer with oocytes at the 

 yolk vessicle stage may spawn in the autumn or 

 during the next year (small numbers of walleye 

 pollock have been observed in spawning condition 

 throughout the year). Rematuring ovaries con- 

 taining oocytes at the primary and secondary 

 yolk stages were found only in August and Sep- 

 tember. More than one-half of these ovaries (5 out 

 of 8) showed signs of resorption of the developing 

 oocytes, and would probably not spawn again that 

 year. 



DISCUSSION 



The results of this study appear to indicate that 

 at least three separate spawning stocks of walleye 

 pollock exist in the Bering Sea. One is located in 

 the Aleutian Basin, a second over the northwest 

 continental slope, and a third in the southeast 

 shelf, southeast slope, and northwest shelf areas. 



As noted by Ogawa (1956), geographical isola- 

 tion or ecological separation of spawning concen- 



trations may indicate population separation. 

 Overall, the spawning season in the Bering Sea 

 lasts about 8 months, and spawning within the 

 different areas is separated by 500 to 1,000 km. 

 Within the different areas, spawning lasts 2 to 

 3 months. 



Dissimilarities in several population character- 

 istics were observed between groups spawning in 

 the different areas, supporting the concept of mul- 

 tiple stocks. Length at age differed by area, with 

 larger length at age seen in walleye pollock 

 spawning over the southeast shelf, southeast 

 slope, and northwest shelf, and smaller length at 

 age seen in walleye pollock spawning in the Aleu- 

 tian Basin and over the northwest slope. These 

 results were also found by Lynde et al. (fn. 2). 



Fecundity relationships in all shelf and slope 

 areas were similar, and differed from that seen in 

 the Aleutian Basin. Aleutian Basin walleye pol- 

 lock showed the lowest fecundity. Walleye pollock 

 from the northwest and the southeast slope areas 

 showed the highest fecundity. Fecundity esti- 

 mates for walleye pollock in Shelikof Strait in 

 1982 (Miller et al. 1986^; F = 1.2604L32169) and 

 in British Columbia waters in 1979 (Thompson 

 1981; F = 6.771L2^«^) are higher than the Bering 

 Sea estimates from this study. A general trend of 

 declining fecundity exists towards the northern 

 range of walleye pollock. Due to possible interan- 

 nual variability in fecundity, caution should be 

 taken in comparing studies done in different re- 

 gions and years. 



Further research is needed for walleye pollock, 

 in the Bering Sea and elsewhere, to determine the 

 proportion of annual fecundity actually realized, 

 i.e., whether resorption of yolked oocytes during 

 maturation and after spawning is significant. 

 Preliminary histological analysis of walleye pol- 

 lock ovaries from Shelikof Strait (Hinckley un- 

 publ. data) suggests that resorption of yolked 

 oocytes may not be significant. 



Based on similarities in growth, Lynde et al. 

 (fn. 2) proposed that mixing of walleye pollock 

 stocks occurs between the Aleutian Basin and the 

 northwest slope; however, the findings of this 

 study indicate the mixing does not occur during 

 the spawning season. Spawning over the basin 

 and the northwest slope is separated by about 

 5 months and 500 km, yet there was no sign of 



9Miller, B. S., D. R. Gunderson, D. Glass, D. B. Powell, and B. 

 A. Megrey. 1986. Fecundity of walleye pollock (Theragra 

 chalcogramma ) from Shelikof Strait, Gulf of Alaska. Fish. 

 Res. Inst. Rep. FRI-UW-8608. Univ. Washington, Seattle, WA 

 98195. 



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