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Number of opaque zones in sectioned otoliths 
Figure 2 
Bias plot of opaque zone counts showing the level of agreement 
between the counts of the number of opaque zones in whole otoliths 
and their corresponding sections from Indian halibut (Psettodes eru- 
mei) caught as bycatch by commercial trawlers between February 
2014 and December 2015 and with an otter trawl during research 
surveys between August 2015 and September 2017 off the Pilbara 
coast in northwestern Australia. Values represent the number of 
individual otoliths. The diagonal dashed line represents perfect 
173 
by each reader, falling well below the reference 
level of 5% for correspondence recommended by 
Campana (2001). 
Length and age compositions and growth 
The numbers of female and male Indian halibut 
<250 mm TL, all of which were largely collected 
in trawl nets during research surveys, were very 
similar (Fig. 4). Although males were far more 
abundant in the length classes of 250-299 mm 
TL and 300-349 mm TL, caught by commer- 
cial trawlers, females were considerably more 
abundant than males in the length class of 350— 
399 mm TL. The 2 largest length classes were 
composed of females exclusively (Fig. 4). The 
ratios of females to males in the 3 length classes 
of the commercially caught fish that contained 
both sexes, 250-299 mm TL, 300-349 mm TL, 
and 350-399 mm TL, were 0.2:1.0, 0.8:1.0, and 
12.8:1.0, respectively. For all 3 of these length 
classes, the sex ratio was significantly different 
from parity (the x” statistic for each class was 
53.0, 4.1, and 56.8, respectively, with P>0.05 
for all of them). The TLs of the largest female 
and male were 469 and 380 mm TL, respec- 
tively, with these fish also having the greatest 
agreement between counts of the 2 readers. 
identify definitively the best model in terms of the Kull- 
back—Leibler distance for the 2 groups of otoliths (Burn- 
ham and Anderson, 2002). The number of opaque zones 
in sectioned otoliths can, therefore, be used for aging 
individual Indian halibut. 
The sectioned otoliths of 5 small Indian halibut (135- 
196 mm TL) caught in August (late winter) contained 
a large, central opaque region and a surrounding wide 
translucent area, and, in 2 cases, an opaque edge. The 
sectioned otoliths of several similarly sized fish (151- 
211 mm TL) and of larger individuals (272-310 mm TL), 
caught in the same month, possessed the same character- 
istics but, in addition, contained 1 and 2 recently formed 
and diffuse opaque zones, respectively, that had become 
delineated from the otolith edge through the presence 
of a narrow, translucent zone at the otolith margin. The 
patterns of opacity within the otoliths of several small 
Indian halibut provide strong evidence that the first 
opaque zone is formed during the first winter of life, when 
Indian halibut are on average ~8 months old. Therefore, 
those Indian halibut caught in August with a wide trans- 
lucent zone surrounding the nucleus and with an opaque 
zone on, or just delineated from, the edge of the otolith 
were ~9 months old. Those fish with 2 opaque zones in 
their otoliths, the second being recently deposited, were 
therefore ~21 months old. 
The resultant CV of 1.85% indicates a high level of agree- 
ment between the counts of opaque zones for each otolith 
total body mass for their sex, 1773 and 752 g, 
respectively. 
Male Indian halibut were more abundant in 
the youngest and 3 oldest age classes in which 
both sexes were present, but females were more abun- 
dant in the age class of 4-5 years (Fig. 3). A single male 
constituted the oldest age class of 16-17 years. The ratio 
of females to males in each age class that contained >20 
individuals varied from 0.6:1.0 in the age classes of 6—7 
years and 8-9 years to 1.3:1.0 in the age class of 4-5 years. 
However, for no age class was the sex ratio significantly 
different from parity (y’=0.6—3.1, P>0.05). The maximum 
ages recorded for female and male Indian halibut were 11 
and 16 years, respectively. 
The length—mass relationships for the female and male 
Indian halibut were not significantly different (P=0.24); 
therefore, the length—mass data for both sexes were pooled 
and described by using this equation: 
InTM = 3.17(nL) — 12.20 (6) 
(coefficient of determination [r7]=0.99, sample size [n]=544). 
The von Bertalanffy growth curves for female and male 
Indian halibut provide good fits to the lengths at ages for 
the individuals of each sex (Fig. 5), as indicated by the 
high values for the r” for each sex (Table 1). The von Ber- 
talanffy growth curves for females and males, which were 
significantly different (P<0.001), increasingly diverged 
with increasing age, a pattern that is reflected in the val- 
ues for L,, and k (Table 1). On the basis of estimates from 
the VBGF, females attained lengths of 196, 322, 379, and 
392 mm TL at 1, 3, 6, and 9 years of age compared with 
