328 
Fishery Bulletin 95(2), 1997 
In order to define maturity stages, the most ad- 
vanced oocyte stage in each specimen was used (Table 
3). In most cases, oocytes in all stages up to the most 
advanced stage observed were present. For some of 
the spawning fish, however, stage 5 oocytes (vitel- 
logenic) were absent. Since Atka mackerel are batch 
spawners (Zolotov, 1993), the number of advanced 
oocytes (egg stage 5 and larger) decreased with the 
number of batches spawned. 
For the Aleutian Islands region, the ovaries of the 
mature females were far enough advanced to distin- 
guish mature from immature fish by the presence of 
advanced oocyte stages (stages 5-9). Because of the 
timing of the collection of samples for the Gulf of 
Alaska, some of the maturity classification was done 
by comparing GSI values. Certain Gulf of Alaska 
samples showed a GSI that was almost an order of 
magnitude smaller than the GSI of the mature fish 
even though the oocytes appeared to be in a similar 
oocyte stage (stage 4, cortical alveoli and oil globules 
present). Because the GSI value was not continuous 
but showed a distinctive gap and because there was 
no evidence of yolk in the presumably immature ova- 
ries, fish that belonged in the group with the lower 
GSI value were classified as immature. This GSI 
value coincided with the GSI value of the immature 
fish in the Aleutian Island region, and the age at 
maturity calculated also coincided with the age at 
maturity determined for the samples in the Aleutian 
Island region. However, until year-round samples for 
the Gulf of Alaska can be obtained, the possibility of 
the presumably immature fish spawning later in the 
year cannot be excluded. 
Reproductive cycle 
Since data were not available throughout the year 
in all of the areas, the data were pooled and com- 
pared by month only. Mean oocyte stage did not in- 
crease substantially from January until June, when 
most females possessed ovaries with stage 5 oocytes 
(vitellogenesis) (Fig. 3). Mean oocyte stage started 
to increase rapidly in June, peaked in August, and 
declined slightly in September. The mean GSI value 
Table 3 
Definition of maturity stages of Atka mackerel. 
Maturity stage 
Description 
Most advanced oocyte stages 
Stage 1: Immature 
Ovary small with small oocytes. Oogonial nests, early and 
late perinucleus stages and cortical alveoli stage present. 
In some ovaries early oil droplet stage present. 
Oocyte stages 1-3; 
early oocyte stage 4 
Stage 2: Developing 
Ovary increasing in size. Oocytes show oil droplets in 
advanced stage. Ovary wall thickens. Vascularization 
increases. 
Oocyte stage 4 
Stage 3: Vitellogenesis 
Large visible eggs undergoing yolk development. Yolk 
globules present in oocytes. Wide range in oocyte diameter 
since oocytes from stage 1 through stage 5 are present. 
Oocyte stage 5 
Stage 4: Early hydration 
Most advanced yolked oocytes are in migratory-nucleus 
and early hydration stage. Yolk is not completely coalesced 
in hydrated oocytes. Oocytes are present in stages 1-7 
Oocyte stages 6 and 7 
Stage 5: Spawning 
Large oocytes visible at 2 mm. In advanced oocytes, yolk 
is completely coalesced. Number of yolked oocytes 
decreases as multiple batches are spawned. After first 
spawning, postovulatory follicles (POF) are present. In 
ovaries of fish that have spawned more than one batch, 
different stages of POF are distinguishable. In some cases 
proportion of vitellogenic oocytes decreases with the 
increase of hydrated oocytes. Ovaries are highly 
vascularized. Ovulated oocytes are found free-flowing in 
the center of the ovary. 
Oocyte stages 8 and 9 
Stage 6: Spent 
Ovary appears flaccid and highly vascularized. Ovary 
shows abundance of late POF, and atretic hydrated 
oocytes. Healthy oocytes are all in early developing stage. 
Oocyte stage 3 and 4, presence of post- 
ovulatory follicles and atretic hydrated 
oocytes. Atresia of oocyte stages 5-8. 
