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Fishery Bulletin 96(3), 1998 
group (10-19 m, 20-29 m, etc.), weighted by the sta- 
tion CPUE (no. fish/hectare). For nearshore beam- 
trawl samples, variance surrounding mean male-pro- 
portions was estimated following Cochran ( 1977) for 
estimation of proportions in cluster sampling. For 
standard and exploratory surveys, the error struc- 
ture of proportion means was approximated by the 
boot-strap method (Efron and Tibshirani, 1993). The 
station-specific data were randomly resampled and 
the mean estimated 1000 times. Twenty-fifth and 
975th quantiles were chosen for the 95% confidence 
bounds. 
Results 
Between-sex variation 
Male yellowfin sole within the standard survey area 
were distributed closer to shore than were females 
in all years, 1982-96 (Fig. 3; Table 1). In addition to 
having an overall distribution in deeper water, fe- 
males also appeared to have a more extended north- 
westerly distribution than males (Fig. 3). Males out- 
numbered females near shore, whereas farther off 
shore ( >60 m bottom depth), females outnumbered 
males (Fig. 4). The proportion of males (no. males/ 
no. both sexes) and CPUE increased with decreas- 
ing depth (Fig. 4). Togiak and Kuskokwim Bay area 
samples from exploratory trawls were also charac- 
terized by a higher proportion of males as well as 
greater overall yellowfin sole concentrations. The 
proportion of males from Togiak and Kuskowim Bay 
areas averaged 0.60 and 0.67, respectively (Fig. 4). 
Sex proportions also varied between mature and 
immature yellowfin sole. Among beam-trawl samples 
from the Togiak Bay area (Fig. 2), the overall mean 
proportion of males was 0.54 (SE=0.0033). Among 
mature fish, however, the proportion of males aver- 
aged 0.65 (SE=0.0067) and among immature fish, 
mostly small <20 cm fish, the proportion of males 
averaged 0.50 (SE=0.0024). Within the spawning 
area (<30 m), the proportion of males among mature 
fish averaged 0.68 (SE=0.0056) (Fig. 5). 
Amomg-year variation 
Yellowfin sole spring-summer distributions varied 
from year to year, although the most notable differ- 
ence was for years 1982-84 when distributions of 
males in particular within the standard AFSC sur- 
vey area were shifted off shore (deeper waters) rela- 
tive to subsequent years ( 1985-96) (Fig. 3). A greater 
mean CPUE-weighted bottom depth in conjunction 
with a decreased percentage of nearshore ( <30 m) 
CPUE (kg/hectare) during 1982-84 confirmed a 
bathymetric shift to deeper waters for both male and 
female distributions in relation to subsequent years 
(Table 1). Interestingly, yellowfin sole biomass lev- 
els, as well as the overall proportion of males within 
the standard AFSC survey, were considerably higher 
during 1982-83 than in subsequent years (Table 1). 
Discussion 
Between-sex variation 
Distributional differences between male and female 
yellowfin sole exist primarily because males mature 
earlier than females. Standard AFSC surveys, ex- 
ploratory nearshore samples, and Togiak area beam- 
trawl samples have shown conclusively that male 
yellowfin sole outnumber females in the nearshore 
areas ( <30 m bottom depth) of the eastern Bering 
Sea during spring-summer. Given that males ma- 
ture approximately 4 years earlier than females 
(Wilderbuer et al., 1992), spawning males should 
considerably outnumber spawning females. Because 
yellowfin sole spawn primarily in nearshore areas 
( <30 m) during spring-summer (Nichol, 1995), ma- 
ture males outnumber mature females by nearly 2:1 
in this region. In deeper waters, females outnumber 
males because of the abundance of larger (25-32 cm 
TL) immature females that generally do not move 
into the spawning area (Nichol, 1997). 
Factors such as differential life-spans and differ- 
ential catchability between sexes have been shown 
to cause similar sex-ratio patterns in other species 
(Beverton, 1964). Such factors were assumed negli- 
gible in this case. Since 1982, maximum ages for yel- 
lowfin sole males and females averaged 25.7 and 26.1 
years, 3 respectively, and have not differed signifi- 
cantly (paired t-test; P-value=0.7758, df=28), suggest- 
ing similar life-spans for males and females. Beverton 
(1964) discussed the potential effects of differential 
natural mortality, fishing mortality, and catchability 
on sex-ratios of plaice (Pleuronectes platessa L.) in 
the North Sea. The possibility that male yellowfin 
sole may be more readily captured by trawl gear, at 
least among immature fish, is unlikely given that 
the sex proportion among immature fish in the 
spawning area was near 0.5 (Fig. 5). The possibility 
that spawning males are more catchable than fe- 
males owing to differential spawning behavior, as 
Beverton ( 1964) reported for plaice, is not known for 
3 Ages were determined by the Age and Growth Unit of the Alaska 
Fisheries Science Center (AFSC) from annual otolith collections 
made during the standard AFSC groundfish trawl surveys in 
the eastern Bering Sea. 
