Rodgveller et al. : Age at maturity, skipped spawning, and fecundity of female Anoplopoma fimbria 
93 
_ro 
<D 
DC 
B 
0.2 
0.15 
0.1 
0.05 
0 
Age 
J 
30 34 38 42 46 50 54 58 62 66 70 74 78 82 86 
Length (cm) 
Figure 1 
The frequency (in numbers) of female sablefish (Anoplopoma fimbria) 
sampled in December 2011 on the continental shelf or slope in the 
central Gulf of Alaska, by (A) age and (B) length. 
and histological slides were examined for 
postovulatory follicles. Determinations of 
any oocyte loss due to atresia were done 
by examination of histological slides. The 
number of oocytes per gram was also 
compared for 2 categories of mature fish: 
younger (age <12 years) and older (age 
>12 years) fish. Age 12 is approximately 
the age at which female sablefish were 
100% mature in most maturity curves in 
our study. Differing values of the num- 
ber of eggs per gram of sample weight 
would indicate a larger egg size for one 
age group. Additionally, the relationship 
between relative fecundity (the number 
of mature oocytes per gram of ovary-free 
fish weight) and age was examined for in- 
dications that relative reproductive out- 
put changes with increasing age. 
Gonad weight in relation to body 
weight (gonadosomatic index [GSI], go- 
nad weight/body weightxlO) of fish that 
would skip spawning, as well as spawn- 
ing and immature fish was determined to 
see if it could be used to predict spawn- 
ing status during the winter. 
Results 
Distribution of fish on shelf and slope 
Because of weather, sampling in the win- 
ter of 2011 was restricted primarily to 
the shelf; there were 37 tows on the shelf 
and 4 tows on the slope. In total, 393 fe- 
male sablefish were sampled: 320 on the shelf and 73 
on the slope. The majority of fish from the shelf were 
smaller and younger than those sampled on the slope 
(Fig. 1). The average total length of fish was 628 mm 
on the shelf and 747 mm on the slope. Of the samples 
from the shelf, 90% (290 of 320) were age 1-7; 15% (11 
of 73) of the samples taken on the slope were age 2-7, 
and no age-1 fish were collected on the slope. 
Maturity classification for fish sampled during the winter 
Ovarian development was uniform throughout both 
ovaries for individual fish. Oocyte stages were consis- 
tent within the 33 specimens that were assessed at 3 
regions in both ovaries. Development of both ovaries 
was the same in the 188 specimens for which samples 
were taken from the posterior end of both ovaries. Im- 
mature gonads contained only primary growth oocytes 
and macroscopically looked thin and tubular and had a 
pinkish hue. Gonads were easily determined to be ma- 
ture on the basis of the presence of mature, advanced 
vitellogenic oocytes (Table 1). Without the aid of histo- 
logical examination, it could be seen that these ovaries 
were engorged with white, opaque oocytes (Table 1). 
The cortical alveoli stage was absent from all ovaries. 
The second category of mature fish was composed of 
fish that would skip spawning. These fish had a much 
thicker tunica than immature fish and sometimes con- 
tained atretic primary growth oocytes that had mean 
wall width of 35 pm (95% confidence intervals [Cl] 30- 
41) for immature fish and of 318 pm (95% Cl 267-370) 
for fish that would skip spawning (Fig. 2). They also 
had thick stroma and blood vessels in the lamellae be- 
tween oocytes, whereas immature ovaries had tightly 
packed oocytes with little tissue in between. These 
characteristics indicated that sablefish exhibit the rest- 
ing type of skipped spawning, in which vitellogenic oo- 
cytes are not produced. Macroscopically, fish that would 
skip spawning had small ovaries, more similar in size 
to an immature ovary than to an ovary with yolked 
oocytes, but they were more flaccid than immature ova- 
ries and had a red coloration. Without adequate experi- 
ence, or the aid of histological examination, fish that 
would skip spawning could be mistaken easily for im- 
mature fish. 
Of 110 mature fish, 23 fish (21%) were fish that 
would skip spawning (here, the number of mature fish 
