Alaska plaice. 



Distribution and abundance. — During the entire survey period, 

 April-June 1976, Alaska plaice were taken over a broad, central 

 region of the study area, from inner Bristol Bay to the outer conti- 

 nental shelf (Fig. 58, 59). Although Alaska plaice were moderately 

 abundant over their geographical range compared to most fish 

 taxa, very high densities ( > 200 kg/km trawled) were never ob- 

 served. Overall, Alaska plaice occurred at 261 (60.0%) of the 435 

 grid stations at a mean abundance of 12.19 kg/km (Table 47). 



However, like yellowfin sole, the Alaska plaice population was 

 apparently migrating inshore during the survey period. In April, a 

 large high-density concentration was encountered between 

 Unimak Island and the pack ice edge. In May, after the ice edge 

 had receded north and allowed fishing on the inner shelf, two ma- 

 jor concentrations were encountered east of the Pribilof Islands. 

 Two large high-density regions were also evident in June, west and 

 southwest of Cape Newenham, in addition to smaller concentra- 

 tions directly north of the Alaska Peninsula and in midshelf. By 

 July and August, Alaska plaice showed only restricted low-density 

 distributions in outer shelf subareas 2 and 3S. The overall ob- 

 served pattern of spring migration followed the general descrip- 

 tion of Fadeev (1965). 



Because survey sampling and the migrating Alaska plaice pop- 

 ulation apparently progressed together, following spring recession 

 of the pack ice, coverage of the population may have been fairly 

 complete each month. As with yellowfin sole, population and bio- 

 mass estimates were computed overall and for individual months 

 of survey coverage (Table 47). 



The overall apparent population biomass for Alaska plaice was 

 243,700 1 (95% confidence limits 190,000-297,200 1); the estimated 

 population was 856.4 million individuals. Estimates of population 

 biomass obtained from sampling coverage during individual months 

 were April, 83,700 1; May, 221,300 1; and June, 169,900 1 (Table 47). 



Because of the sampling problems caused by inshore migration, 

 all of the 1976 survey biomass estimates for Alaska plaice were of 

 dubious accuracy. Compared to the 1975 survey estimate of 

 127,000 t (Pereyra et al. see footnote 2), the 1976 biomass esti- 

 mates obtained from the overall survey period, May, and June 

 appeared to be high. For these cases, biasing may have resulted 



from oversampling of high-density concentrations, particularly in 

 subarea 4S. 



Although the precision obtained in the 1975 and 1976 survey 

 overall biomass estimates was approximately the same, the 1976 

 monthly estimates were relatively imprecise. As a measure of rel- 

 ative variance, the width of the 95% confidence limits for each 

 1976 biomass estimate (expressed as a percentage of the estimated 

 total biomass) was overall, ±22%; April, ±73%; May, ±43%; 

 and June, ±36%. The relative variation observed in the 1975 

 survey estimate for Alaska plaice was ±20%. 



During May and June 1976, approximately 82% of the ap- 

 parent population biomass was distributed in survey subareas 4S 

 and 1. 



Size composition. — Alaska plaice taken during the 1976 spring 

 survey ranged from 11 to 50 cm TL, with an overall mean total 

 length of 28.3 cm (based upon 9,788 field measurements; Fig. 60). 

 Similar size-frequency distributions were observed from all geo- 

 graphical areas. Overall (sexes combined), individuals in the size 

 range 25-31 cm TL accounted for 50.2% of the total apparent 

 population. Small individuals (< 20 cm) occurred only in inner 

 shelf subareas 1, 4S, and 4N. Largest size ranges were shown by 

 the populations in subareas 4S (11-50 cm) and 1 (13^17 cm). 



In most subareas, male populations (overall size range 11-41 

 cm) were represented by smaller sized individuals than female 

 populations (overall, 17-50 cm), with less variance in size about 

 their mean total length. 



Age composition. — Estimates of the age-frequency distribution 

 of Alaska plaice were limited by two problem areas: 1) Few age- 

 length observations, with inadequate coverage of small indi- 

 viduals, and 2) poor population estimates caused by sampling 

 problems. 



Totals of 69 male and 88 female saccular otoliths were collected 

 from Alaska plaice in the size range 22-48 cm. The observed 

 ranges in ages were males, 6-14 yr, and females, 6-16 yr. 



Although estimates of Alaska plaice population abundance 

 were apparently biased by effects of migration, the overall 

 estimated numbers of individuals at each length were used as best 

 estimators of relative age-frequency distribution. 



Table 47. — F-stimalcd biomass and population number of Alaska plaice in the Bering Sea by subarea and tor all subareas combined during April, May, and June 1976. ' 



Percentage 



frequency of Mean CPUE 



occurrence April-June 

 Subarea 7 April-June (kg/km) 



Eslimatcd biomass it) 



April 



May 



Proportion of total 

 estimated biomass 



April May June 



Estimated population 

 (millions) 



Proportion of 



total estimated 



population 



April May 



June 



April 



May 



June 



Mean size 

 April-June 

 Weight TL 

 (kg) (cm) 



Inner shelf 







4N 



100.0 



10.40 



4S 



100.0 



29.07 



1 



80.0 



10.71 



Outer shell 







and slope 







3 



50.4 



1.92 



3 Slope 



- 



— 



2 



49.4 



10.02 



— 19,214 — — 0.113 



(134,055) (90,947) — 0.606 0.535 



40,453 47,211 49,440 0.483 0.213 0.291 



— — 73.4 



— (434.0) (297.7) 

 173.2 149.9 162.9 



0.551 



— 0.129 

 0.600 0.522 

 0.207 0.286 



2.306 8,888 10,318 0.028 0.040 0.061 4.3 28.6 36.1 0.015 0.040 0.063 



40,940 31,154 _ 0.489 0.141 — 136.6 110.7 0.434 0.153 — 



2 Slope 



0.262 27.5 

 0.270 27.7 

 0.323 29.6 



0.302 29.0 

 0.298 29.0 



All subareas 

 combined 



60.0 



12.19 



'83,698 (221.307)' (169,919)' 



314.6 (723.2) (570.0) 



28.3 



Parentheses indicate estimates that may be badly biased due to sampling problems. 

 : See Figure 3. 

 '95% confidence limits: April: 22,632-144,764 t. 



May: 126,070-316,545 t 



June: 108,464-231,3751. 



82 



