McDermott and Lowe: Reproductive cycle and sexual maturity of Pteurogrammus monopterygius 
325 
with geographical area as a factor. The variance for 
the estimated L 50 or Age bQ was calculated using the 
delta method (Seber, 1982): 
where e. 
'ij 
n j 
mean oocyte stage in month j\ 
the most advanced oocyte stage of speci- 
men i in month j; and 
number of specimens in month j. 
The estimated variance is 2 ) of the mean egg stage 
was determined using the formula: 
Sef 
Z (e »-^ )2 
n i in l - 1) 
S ( Age 50 , L 50 ) — 
S 2 ia) 2 aS{a)S((3)r a 2 S 2 (f3 ) 
P A 
where S 2 iL 50 ] Age 50 ) 
a 
P 
r 
Sid) 
Sip) 
variance of the length or 
age at 50% maturity; 
estimate of a; 
estimate of (3; 
correlation coefficient; 
standard error of a ; and 
standard error of /? . 
Calculating gonad somatic index 
Size measurement of oocytes 
Oocyte diameters were measured from histologically 
prepared ovary sections using a compound micro- 
scope with an ocular micrometer. Random measure- 
ments were taken by measuring oocytes along mul- 
tiple transect lines across the section. Only oocytes 
that touched the transect line and which had been sec- 
tioned through the nucleus were measured. For each 
oocyte stage a minimum of 15 oocytes per fish were 
measured from at least two individuals. 
Length and age at 50% maturity 
To minimize confusion between immature and rest- 
ing fish (mature females with oocytes smaller than 
oocyte stage 4; see Table 2), only samples in which 
the oocytes of mature fish were in advanced oocyte 
stages (oocyte stages 4-9, Table 2) were used for the 
calculation of length and age at maturity, except for 
some samples collected in the Gulf of Alaska as dis- 
cussed below. 
The proportion of fish mature at length or age was 
estimated by fitting a logistic model to the observed 
proportion mature. The logistic equation used was: 
y _ 1 
l + e- ( “ +/5r) 
where Y = proportion mature at length or age x; 
a, p = model parameters to be estimated; and 
x = fork length (cm). 
Length or age at 50% maturity (L 50 ; Age 50 ) was cal- 
culated as -a/p. The statistical program used was S- 
plus (Venables and Ripley, 1994). A general linear 
model with a binomial error distribution was applied 
Relative reproductive effort was expressed as a go- 
nad somatic index (GSI), defined as the ratio of go- 
nad weight to somatic body weight. In all cases the 
gonads were weighed after they had been preserved 
in formalin. For the samples that did not have 
weights for fresh gonads, fresh gonad weight was 
estimated with a linear regression using the samples 
for which both fresh weight and formalin-preserved 
weight of the gonads had been measured (ft =254). The 
regression line was forced through the origin using: 
y = cx, 
where y = fresh weight of ovary; 
x = formalin preserved weight of ovary; and 
c - constant. 
The GSI was calculated as: 
GSI = — x 100, 
B 
where GSI = gonad somatic index; 
G = fresh gonad weight; and 
B = somatic body weight (stomach empty, 
gonads removed). 
Results 
Definition of oocyte stages and maturity 
stages 
Oocyte development was classified into nine oocyte 
stages based on major histological characteristics 
(Fig. 2, Table 2). The oocyte stages were then used to 
determine maturity stages. 
