concentrating along the outer shelf where food abundances — 

 euphausiids and juvenile walleye pollock — (Takahashi and Yama- 

 guchi 1972) are high; 2) during winter, shelf populations shift off- 

 shore to deep and warmer water; and 3) during early spring, 

 spawning concentrations are formed along the shelf edge. 



Like most Bering Sea demersal fish species, the spawning of 

 walleye pollock is apparently synchronized with spring plankton 

 production (Waldron and Vinter 1978 12 ). During both the 1975 

 and 1976 surveys, juvenile (age 1 yr) walleye pollock were primar- 

 ily found in central and inner shelf regions (subareas 1, 4S, 4N, 

 and 3S). 



If a substantial proportion of adult walleye pollock had mi- 

 grated out of the 1976 survey area by moving off -shelf during 

 winter, these movements could account for the large differences in 

 size and age structures, sex ratios, and length-weight relationships 

 measured for the shelf populations by the two surveys. 



The hypothesis that genetically distinct north and south walleye 

 pollock populations may exist was not well supported by data col- 

 lected during the 1975 and 1976 surveys, although rigorous testing 

 was not possible. Two principal centers of abundance, north and 

 south, were observed during August-October 1975 in subareas 3N 

 and 2 (Fig. 82). However, comparisons of the population 

 characteristics measured by the two surveys were not adequate to 

 evaluate the integrity or extent of behavioral and reproductive 

 isolation of the two apparent populations. 



Yellowfin sole. — Yellowfin sole showed strongly contrasting 

 distributional behavior between the 1975 and 1976 surveys, ap- 

 parently representing seasonal extremes of adult migration pat- 

 terns (Fig. 83, 84). During August -October 1975, adults (ages 6-13 

 yr) were broadly distributed over the central and inner continen- 

 tal shelf, and there were no indications of large-scale migratory 

 movements. During April-June 1976, however, dense frontal con- 

 centrations of adults followed the receding pack ice from the 

 outer shelf into Bristol Bay where spawning reportedly occurs 

 during summer (Musienko 1963, 1970). 



Although sampling problems encountered during the spring 

 1976 survey confounded rigorous comparisons of abundance 

 estimates, the strong similarities in size composition, age struc- 

 ture, and overall sex ratios suggest that a relatively homogeneous 

 population was sampled during both years. Because nearly the en- 

 tire range of the eastern Bering Sea population was included 

 within the 1975 and 1976 survey areas, effects due to in-migration 

 and out-migration were apparently small. 



During both the 1975 and 1976 surveys, juveniles (ages 2-4 yr) 

 were found primarily along the inner shelf, and there seemed to be 

 a positive relationship between the abundance of small individuals 

 and proximity to shore. Whereas adults appear to regularly 

 undergo extensive geographical migrations between seasons — 

 perhaps to maximize food supply, or as an adaptive response to 

 cold temperatures, juveniles apparently remain in shallow inshore 

 nursery areas during their first 1 -2 yr and then develop migratory 

 behavior that extends progressively offshore. 



The hypothesis that eastern Bering Sea yellowfin sole might 

 also have genetically distinct north and south populations was not 

 well supported by the survey data. Although three principal 

 centers of abundance were recognized during the April-June 1976 



survey— the large Unimak Island concentration that moved into 

 central Bristol Bay and the two small Pribilof concentrations that 

 appeared to migrate towards Nunivak Island, the population data 

 collected were not adequate to enable identifying potential stock 

 isolation from the overall background of geographical gradients 

 and variability. Recent tissue collections and analyses of protein 

 variation using starch gel electrophoresis have subsequently sug- 

 gested free genetic exchange between the Bristol Bay and Pribilof 

 Island groups (Grant et al. 1978"). 



Rock sole. — The density distribution of rock sole during the 

 April-June 1976 survey had also shifted (relative to August- 

 October 1975) southwest and towards the outer continental shelf 

 (Fig. 85). If the two surveys are interpreted as representing 

 seasonal extremes, then the distribution of the population ob- 

 served in 1975 — widespread, with several large centers of abun- 

 dance — apparently represents the results of extensive summer 

 migration onto the inner continental shelf, and the distribution 

 observed in 1976 — primarily restricted along the outer continental 

 shelf, with scattered locations of high abundance — represents a 

 retreat to deep water during winter. Because no large-scale move- 

 ments were observed during the 1976 survey, the timing of rock 

 sole summer migration to the inner shelf was apparently later than 

 that of yellowfin sole, and frontal concentrations were not as 

 pronounced. 



Like yellowfin sole, nearly the entire geographical range of the 

 eastern Bering Sea rock sole population was included within the 

 1975 and 1976 survey areas, and effects due to in-migration and 

 out-migration appeared to be small. In general, characteristics of 

 the population measured by the two surveys — absolute size, 

 length and age structure, and sex ratio — were similar. During 

 both surveys, juvenile rock sole (ages 2 and 3 yr) were taken 

 primarily along the Alaska Peninsula between Unimak Island and 

 Port Moller in subareas 1 and 2. 



Flathead sole. — During both the 1975 and 1976 surveys, 

 flathead sole were distributed mainly along the outer continental 

 shelf, and one large center of abundance was observed between 

 St. George and Unimak Islands (Fig. 86). During August-October 

 1975, the distribution of the population included relatively low 

 density extensions onto the central and inner continental shelf. 

 During April- June 1976, although scattered occurrences were 

 recorded on the central shelf, the population range was primarily 

 restricted to deep water and moderate levels of abundance were 

 also observed at slope depths. 



Although a large southeastern Bering Sea population was iden- 

 tified, the species range clearly extended beyond the northern 

 boundaries of the survey areas; so questions remain regarding re- 

 lationships and exchange between north and south populations, 

 and potential effects of in-migration and out-migration between 

 regions. Relationships between the eastern Bering Sea population 

 of flathead sole, Hippoglossoides elassodon, and that of the con- 

 generic Arctic species, H. robustus (Bering flounder), are also 

 unclear (Forrester et al. 1977). 



Population characteristics that showed similarity between the 

 1975 and 1976 surveys included overall mean density, estimated 



"Waldron, K. D.. and B. M. Vinter. 1978. Ichthyoplankton of the eastern 

 Bering Sea. Unpubl. manuscr., 88 p. Northwest and Alaska Fisheries Center, 

 National Marine Fisheries Service, NOAA, 2725 Montlake Boulevard East, Seattle, 

 WA 98112. 



"Grant, S., R. Bakkala, and F. Utter. 1978. Examination of biochemical 

 genetic variation in yellowfin sole (Limanda aspera) of the eastern Bering 

 Sea. Unpubl. manuscr., 21 p. Northwest and Alaska Fisheries Center, National 

 Marine Fisheries Service, NOAA, 2725 Montlake Boulevard East. Seattle, WA 

 98112. 



116 



