McDermott and Lowe: Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 
329 
reflected a similar pattern (Fig. 3), although a slight 
increase was noted from February through June, re- 
flecting slow growth of oocytes during that time. The 
rapid increase from June through August suggests a 
rapid period of oocyte growth during vitellogenesis and 
hydration. The decrease in GSI after August reflects 
the loss of ovary weight due to spawning single batches, 
but it should be noted that the GSI in October is still 
higher than the GSI value in June, which suggests, 
that the fish might be still spawning their last batches. 
High variance in GSI and mean egg stage during the 
spawning period (July though October) could be attrib- 
uted to batch spawning since most females were not 
spawning synchronously and the ovary weight and oo- 
cyte stages differed accordingly. 
Examination of maturity stages over time indi- 
cated the same cycle with a long period of initial oo- 
cyte development, the appearance of vitellogenesis 
in June, and peak spawning in August (Fig. 4). Vi- 
tellogenesis progressed rather rapidly and was ob- 
served almost exclusively in June. However, 
vitellogenic eggs were found throughout most of the 
early hydration stage and during the spawning in 
some ovaries. It should be mentioned that in one 
cruise a few spawning fish were found in the central 
Aleutians in March and April 1992. While this was 
an oddity not observed in other years, it suggests 
that under certain circumstances fish can spawn as 
early (or late) as March. 
In general, Atka mackerel develop their oocytes 
slowly in the oil droplet phase from at least January 
until May, with a gradual increase in oocyte size and 
ovary weight. Vitellogenesis starts in June and early 
migratory nucleus and early hydration is observed 
in July. Spawning individuals were observed from 
July until October with the peak in August, when 
the highest mean egg stage and GSI values were 
observed. Fish with ovaries having hydrated eggs in 
October were clearly spawning their last batch as 
they had many atretic hydrated oocytes, no vitel- 
logenic, and few early hydrated oocytes present. Spent 
ovaries were found in September and October. Since 
no samples were collected in November and December, 
it is not clear how long the spawning season could last. 
However, by January all fish collected were in the early 
developing phase for the next year’s cycle. 
Size and age at 50% maturity 
Size at 50% maturity ranged from 33 
cm to 38 cm (Table 4). However, subarea 
was a highly significant factor 
(P<0.001), exhibiting a cline in the size 
at 50% maturity from east to west (Table 
4, Fig. 5). Samples from the eastern ar- 
eas reached 50% maturity at larger 
sizes. Length at 50% maturity in the 
Gulf of Alaska was 38.24 cm, while in 
the eastern Aleutian subarea the fish 
matured at 35.91 cm. Samples from the 
central and western Aleutian subareas 
matured at essentially the same size, 
33.55 cm and 33.64 cm, respectively . 
Age at maturity was not significantly 
different among the different Aleutian 
subareas (P=0.66) or between the Gulf 
of Alaska versus the Aleutian subareas 
combined (P=0.69), therefore the data 
were pooled. The age at 50% maturity 
(all areas combined) was 3.6 years for 
Atka mackerel (Table 4, Fig. 6). 
Discussion 
Figure 3 
Mean oocyte stage and mean gonad somatic index (GSI) by month for ma- 
ture Atka mackerel. 
In order to make inferences about popu- 
lation parameters and biology, it is nec- 
essary to take representative samples 
of the population’s true age and size 
