Coulson et al.: Biological features of Achoerodus gou/dii 
63 
rising progressively in the ensuing months (Fig. 2). 
Although the numbers of otoliths with one opaque zone 
in each month were far less, their mean values could 
still be seen to follow a similar annual trend. Although 
the mean monthly marginal increments on otoliths with 
11-20, 21-30, and >30 zones followed trends that were 
very similar to those described for otoliths with 2-10 
zones, the minima of the last two groups were reached 
later. Consequently, as the number of zones in otoliths 
increases, the new opaque zone in otoliths becomes 
visually detectable later, i.e., in late summer to early 
autumn, rather than in late spring to mid-summer. 
The similar single decline and subsequent progressive 
rise in mean monthly marginal increments, irrespec- 
tive of the number of opaque zones, demonstrate that 
a single opaque zone is formed annually in the otoliths 
of A. gouldii. The numbers of opaque zones in otoliths 
could therefore be used for aging this species. 
From the trends exhibited by the mean monthly GSIs 
and prevalence in each month of females with stages 
V and VI ovaries, the approximate mid-point of the 
spawning period was estimated to be August, i.e., the 
end of the Austral winter. The small fish caught in 
November were ~40 mm in length, and those captured 
in February and March were ~60 and 90 mm, respec- 
tively. The otoliths of the latter (Feb. and Mar.) two 
collections of fish contained no opaque zones, which 
is consistent with these fish, on average, having been 
spawned in late winter and therefore not having had 
the opportunity to lay down the opaque zone that is 
deposited annually during that season in older fish. The 
first of these zones becomes delineated in the spring of 
the second year of life, i.e., when fish are -140 mm in 
length and -18 months old. 
The individuals in samples of A. gouldii ranged in 
total length from 40 to 1162 mm and in age from a 
few months to 70 years (Fig. 3). The largest and oldest 
A. gouldii , from which the gonads had not been removed 
and could therefore be sexed, were 880 mm and 49 
years, for females, and 1134 mm and 57 years, for males. 
Although the 822 A. gouldii collected by spear fish- 
ing ranged from 40 to 1050 mm TL, the majority of 
those individuals measured between 100 and 600 mm, 
a range of TLs that corresponds to ages 1 to 11 years 
(Fig 4). The 1107 A. gouldii obtained from the commer- 
cial gillnet fishery ranged from 428 to 1162 mm (TL) 
and from 6 to 70 years, but most were between 500 and 
800 mm (TL) and 10 and 34 years (Fig. 3). 
A von Bertalanffy growth curve provided a good fit 
to the lengths at age of A. gouldii (r 2 =0.84; Table 1, 
Fig. 4A). On the basis of the von Bertalanffy growth 
equation, A. gouldii attain lengths of 335, 509, 678, 
741, 764, and 773 mm by ages 5, 10, 20, 30, 40, and 50 
years, respectively. The marked similarity in the esti- 
mated lengths at 30, 40, and 50 years of age reflects the 
markedly asymptotic pattern of growth of A. gouldii, 
with relatively little overall growth occurring after 15 
years with females and 30 years with males. 
The von Bertalanffy growth curves fitted separately 
to the lengths at age of sexed fish demonstrated that, 
I zone 
J ASONDJFMA M .1 
Month 
Figure 2 
Mean monthly marginal increments ±1 standard 
error for sectioned sagittal otoliths of the western 
blue groper {Achoerodus gouldii) with different 
numbers of opaque zones. The marginal incre- 
ment is expressed as a proportion of the distance 
between the primordium and the outer edge of 
the opaque zone, when one such zone was pres- 
ent, or as a proportion of the distance between 
the outer edges of the two outermost opaque 
zones when two or more such zones were pres- 
ent. Sample sizes are shown above each mean. 
Closed rectangles on the x-axis refer to winter 
and summer months and the open rectangles 
to spring and autumn months. 
after the age at which some females had changed to 
males, the growth curve for males increasingly diverged 
upwards from that for females (Fig. 4B). Thus, for ex- 
ample, at 20, 35, and 50 years, the estimated lengths 
at age for males were 805, 923, and 965 mm, respec- 
tively; whereas those for females were 679, 737, and 
746 mm, respectively. The above differences in growth 
