164 
Fishery Bulletin 109(2) 
Developmental stages of eggs 
Stages of Alaska plaice eggs were determined according 
to developmental criteria standardized and described by 
Blood et al. (1994) for walleye pollock (Theragra chal- 
cogramma). Staging criteria for walleye pollock are the 
standard criteria used to determine northern marine 
teleost egg development and were easily adapted to 
egg development in Alaska plaice. Modifications were 
made to the protocol in order to accommodate additional 
embryonic growth in Alaska plaice. For the present 
study, the last stage (21) in the egg development schedule 
for walleye pollock is defined as that stage when the tail 
tip reaches the back of the embryo’s head and two stages 
are added: stage 22, when the tail tip extends to 14 of 
the circumference of the egg beyond the snout; and stage 
23, when the tail tip extends to V 2 of the circumference 
of the egg beyond the snout. 
Eggs were then binned into three broader categories 
that comprised 1) early-stage eggs (E: stages 1-12), 
which includes all stages before the closing of the blas- 
topore; 2) middle-stage eggs (M: stages 13-15), when 
the blastopore is closed, the margin of the tail is de- 
fined, and the tail bud is thick, but the margin remains 
attached to the yolk; and 3) late-stage eggs (L: 16-23), 
when the tail lifts away from yolk and lengthens and 
encircles the top half of the yolk to extend just beyond 
the head of the embryo. The late-stage period was fur- 
ther subdivided into three stages after determining that 
the majority of eggs were in one of the following stages 
of development; early-late stage (EL: stages 16-18); 
middle-late stage (ML: 19-21); and late-late stage (LL: 
22-23). 
Collections with the neuston net 
Eggs were collected from surface waters (<1 m depth) 
across the southern Bering Sea shelf in the vicinity of 
the Alaska Peninsula. Collections were made over the 
basin, outer, middle, and inner shelves. Eggs from each 
tow were staged and the hypothesis that there were 
differences in geographic (horizontal) distribution with 
stage of development was examined by using Cramer- 
von Mises tests. 
Vertical distributions of eggs determined 
with MOCNESS tows 
The hypothesis that vertical patterns in egg abundance 
vary with depth and developmental stage was evaluated 
by using data collected from MOCNESS tows. A fourth 
root transformation was used to improve the normality 
of the data. In 2003, a series of MOCNESS tows were 
conducted at a single station, whereas in 2005, MOC- 
NESS tows were conducted at multiple stations over 
the Bering Sea shelf. A general linear model ANOVA 
was used for each cruise by using haul as a blocking 
factor in 2003, and station as a blocking factor in 2005. 
If significant differences with depth were found, the 
analysis was followed with Fisher’s least significant dif- 
ference comparisons (Milliken and Johnson, 1992). We 
also checked for autocorrelation in 2003 (Durbin Watson 
statistic) because in that year samples were taken over 
time at a single station. 
Results 
Stages of developing eggs 
More than 2100 eggs were examined for this work (Table 
1), of which more than 950 eggs were collected from 
neuston nets and the remainder from MOCNESS tows. 
The earliest developmental stage observed was stage 
5 (32 cells) (Table 2); we estimated these eggs would 
be about 1 day old at in situ temperatures (4°C). Egg 
development studies of walleye pollock and arrowtooth 
flounder ( Atheresthes stomias) have established that the 
time required to reach stage 5 is 18-28 hours at about 
3°C (Blood, 2002; Blood et ah, 2007), and we assumed 
similar rates for Alaska plaice eggs. The presence of 
stage-5 eggs indicates that residual spawning occurred 
in mid-May; however, the vast majority of the eggs were 
late-stage eggs (Table 2), indicating that most spawn- 
ing occurred a few weeks before sampling (Pertseva- 
Ostroumova, 1961). Most eggs collected were at stage 
22. Hatching occurs at stage 23; the eyes of embryos 
are fully pigmented and numbers of eggs are greatly 
reduced in contrast to stage 22. 
Collections with the neuston net 
Eggs collected from neustonic surface collections repre- 
sented all stages of development (Fig. 2). Results of all 
pairwise Cramer-von Mises tests for differences in spa- 
tial distributions showed that there were no significant 
differences between the geographic distributions of any 
stages of larvae, except between the geographic distri- 
butions of EL and ML (P= 0.002). However, there were 
some trends that could be discerned. Earliest stage eggs 
collected in the neuston layer appeared to concentrate 
offshore of the 40-m isobath, over bottom depths ranging 
from 40 to 75 m. Eggs in middle stages of development 
appeared to spread shoreward toward shallower depths, 
but catches of eggs in both the early and middle stages 
of development were comparatively low. Late-stage eggs 
occurred over depths ranging from 40 to 100 m. 
Vertical distributions of eggs determined 
with MOCNESS tows 
Vertical distributions of Alaska plaice eggs showed dif- 
ferences in depth distribution with ontogenetic stage 
(Fig. 3), and differences between years. In 2003, there 
was no significant effect of haul and therefore no auto- 
correlation (Durbin Watson statistic =1.91; effect of haul), 
and eggs were generally distributed throughout the 
water column. There was only one egg collected in the 
early stage and it was located in the deepest depth 
stratum (40-50 m). There were no collections of eggs 
