HINCKLEY: REPRODUCTIVE BIOLOGY OF WALLEYE POLLOCK 



rematuration in walleye pollock ovaries collected 

 from the northwest slope late in the season, which 

 could have indicated that these fish spawned first 

 in the basin and then later on the northwest 

 slope. The difference in fecundity between these 

 areas also supports the theory that spawners 

 found in the northwest slope area form a separate 

 group from those '"ound in the Aleutian Basin. 

 The similarities in gi'owth found by Lynde et al. 

 (fn. 2) may be a result of mixing occurring at 

 other times of the year. 



A reduced food supply may produce the smaller 

 length at age and lower fecundity of Aleutian 

 Basin walleye pollock. The reduced growth of 

 walleye pollock in the basin is probably due to the 

 lack offish, particularly juvenile walleye pollock, 

 in the diet (Okada fn. 7; Traynor and Nelson 

 1985; Dwyer et al. in press). Dwyer (1984) also 

 found that the mean weight of stomach contents 

 of basin-caught fish was low compared with that 

 of fish caught over the shelf and slope. Reduced 

 food supplies have been shown to lower fecundity 

 in several species (Scott 1962; Hester 1964; Bage- 

 nal 1969; Leggett and Power 1969; Wootton 1973, 

 1977; Hislop et al. 1978). 



Histological examination of walleye pollock 

 ovaries further supports the theory that spawn- 

 ing concentrations found in widely separated 

 areas do not mix extensively. Walleye pollock 

 spawning in the Bering Sea can be classified as 

 partially synchronous, with one discrete group of 

 oocytes brought to maturation and then spawned 

 in successive batches. This is similar to the pro- 

 cess described for walleye pollock from Japan 

 (Sakurai 1977). The maturation of a second group 

 of oocytes from vitellogenesis to spawning within 

 1 year does not appear to be common. Maximum 

 walleye pollock fecundity is therefore annually 

 determinate, and the duration of an individual 

 female's spawning period is limited by the dura- 

 tion of the batch spawning process. If an individ- 

 ual does batch spawn a group of matured oocytes 

 over 1 month, as Sakurai's (1982) laboratory 

 studies suggest, it seems unlikely that it would 

 migrate any great distance over this time while 

 actively spawning. 



To infer stock separation from the results of 

 this study (i.e., that fish return to the same dis- 

 crete areas each year to spawn) requires assum- 

 ing that the timing and distribution of spawning, 

 the dynamics of ovarian maturation, and the dif- 

 ferences in growth and fecundity remain rela- 

 tively constant from year to year. The timing and 

 location of spawning have been similar from 1982 



to 1986 (Hinckley unpubl. data; R. Nelson fn. 5). 

 The process of maturation is basically the same 

 for walleye pollock found in the Bering Sea, in the 

 Gulf of Alaska (Miller et al. fn. 9), and in 

 Japanese waters (Sakurai 1977, 1982). Differ- 

 ences in mean length at age represent cumulative 

 differences in growth over the life of a fish, and 

 systematic variation by area such as that seen in 

 this study probably reflects separation over a pe- 

 riod of years. The study by Lynde et al. (fn. 2) 

 documented the same differences in growth by 

 area over a period of 8 years as was seen in this 

 study for 1984. It is not known at what point 

 annual fecundity is determined in walleye pol- 

 lock, but as egg production is influenced by food 

 supply in many species, and as walleye pollock 

 feed mostly during the spring and summer and 

 less during the winter (Dwyer et al. 1986) and the 

 spawning season, yearly fecundity may be deter- 

 mined about 1 year before spawning. The results 

 of this study suggest that the assumption of stock 

 separation over a period of years is reasonable. 



This study has outlined the timing and distri- 

 bution of walleye pollock spawning in the Bering 

 Sea for 1984, and postulates the existence of at 

 least three separate spawning stocks. Further re- 

 search is needed on the biological and oceano- 

 graphic conditions occurring in the different 

 spawning areas in order to understand the rea- 

 sons for the apparent separation of stocks, and to 

 clarify differences in recruitment and production 

 by these stocks. 



ACKNOWLEDGMENTS 



I would like to express my thanks to Kevin 

 Bailey and Robert Francis for their guidance and 

 support. Russell Nelson and the U.S. foreign fish- 

 eries observers made possible the collection of 

 data and samples. Anne Hollowed assisted in the 

 length at age analysis, Linda Rhodes and Beverly 

 Macewicz gave advice on histology, and Berne 

 Megrey assisted with the analysis of fecundity. 

 Arthur Kendall, William Karp, and Gary Stauf- 

 fer made useful comments on the manuscript. 



LITERATURE CITED 



Alton, M. S.. M. O. Nelson, and B. A. Megrey. 



In press. Changes in the abundance, composition, and dis- 

 tribution of pollock {Theragra chalcogramma) in the 

 western Gulf of Alaska (1961-1984). Fish. Res. (Amst.) 

 Bagenal. T. B. 



1969. The relationship between food supply and fecundity 

 in brown trout Salmo trutta L. J. Fish. Biol. 1:167-182. 



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