THE REPRODUCTIVE BIOLOGY OF WALLEYE POLLOCK, 

 THERAGRA CHALCOGRAMMA, IN THE BERING SEA, WITH REFERENCE 



TO SPAWNING STOCK STRUCTURE 



Sarah Hinckley' 



ABSTRACT 



The reproductive biology of walleye pollock, Theragra chalcogramma, in the Bering Sea was studied 

 from collections of ovaries and observations of spawning in 1984. Spawning occurred in the Aleutian 

 Basin from January through March, in the southeastern Bering Sea from March through June, and 

 northwest of the Pribilof Islands from June through August. Spawning concentrations found in these 

 areas showed significant differences in length at age and fecundity. Histological evidence indicated 

 that the spawning period of an individual female probably lasts less than 1 month. Results indicate 

 at least three separate spawning stocks of walleye pollock within the Bering Sea. These are located 

 in the Aleutian Basin, over the southeastern continental shelf and slope and northwest shelf areas, 

 and over the continental slope northwest of the Pribilof Islands. Mixing of stocks between widely 

 separated spawning grounds due to extended spawning and migration is not likely. 



Walleye pollock, Theragra chalcogramma, a 

 member of the gadid family found in the North 

 Pacific Ocean, currently supports the largest 

 single-species fishery in the world. In the eastern 

 portion of its range, catches of pollock average 

 100,000 to 300,000 metric tons (t) per year in the 

 Gulf of Alaska (Alton et al. in press) and 

 1 ,000,000 1 per year in the Bering Sea ( Bakkala et 

 al. in press). The walleye pollock resource in the 

 Bering Sea is presently managed as a single stock 

 but there is increasing evidence that substocks 

 may exist (Lynde et al. 1986'^). If substocks exist, 

 this information should be considered in manage- 

 ment strategy. 



If a stock is defined as a production unit or a 

 group of fish showing similar responses to envi- 

 ronmental conditions within a certain geographic 

 area, then population characteristics such as 

 growth rates, fecundity, and size or age at matu- 

 rity may provide the most practical means of dif- 

 ferentiating these units. As these parameters de- 

 termine the yield of a stock to a fishery, 

 identification of production units based on some 



^Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, 7600 Sand Point Way N.E., Seattle, 

 WA 98115. 



2Lynde, C. M., M. Van Houghton Lynde, and R. C. Fran- 

 cis. 1986. Regional and temporal differences in growth of 

 walleye pollock (Theragra chalcogramma) in the eastern 

 Bering Sea and Aleutian Basin with implications for manage- 

 ment. NWAFC Proc. Rep. 86-10. Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 

 7600 Sand Point Way N.E., Seattle, WA 98115. 



Manuscript accepted April 1987. 



FISHERY BULLETIN; VOL. 85. NO. 3, 1987. 



or all of them may improve the effectiveness of 

 fisheries management. 



A clearer understanding of the reproductive 

 process in walleye pollock may also aid in differ- 

 entiating stocks or production units. Nishiyama 

 and Haryu (1981) proposed that walleye pollock 

 in the Bering Sea may migrate and spawn over 

 extensive distances during the long spawning 

 season, implying that spawning groups mix over 

 large areas. Sakurai (1982) has shown in labora- 

 tory experiments that walleye pollock spawn a 

 single group of matured eggs in successive 

 batches, possibly over a period of 1 month. It is 

 not known, however, whether an individual fe- 

 male is capable of repeating vitellogenesis with 

 more than one separate group of eggs in 1 year 

 (i.e., of rematuring the ovary). Evidence of batch 

 spawning or rematuration in Bering Sea walleye 

 pollock may indicate the duration of individual 

 spawning and the potential for mixing between 

 spawning concentrations found in widely sepa- 

 rated areas. 



In this study, the spatial and temporal distribu- 

 tion of spawning for 1984 has been documented 

 and the length at age and fecundity of walleye 

 pollock from spawning concentrations in different 

 areas has been determined. Walleye pollock 

 ovaries were examined histologically to clarify 

 the process of oocyte development, to learn 

 whether annual fecundity is determinate or inde- 

 terminate and to learn the optimal stage for esti- 

 mating fecundity, and to determine whether 



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