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of each gonad was next embedded in paraffin wax, cut 
into 6-ym transverse sections, and stained with Mallory’s 
trichrome. 
The TLs at which 50% and 95% of males reached 
maturity, together with their 95% confidence limits, were 
determined by using logistic regression analysis, as has 
been done in similar studies for other species (Coulson 
et al., 2005, 2009). Fish were considered mature (i.e., 
about to spawn, spawning, or just spawned) if they pos- 
sessed gonads at a maturity level between stage 3 and 
stage 8. The spawning period is defined as the consecu- 
tive months in which >50% of fish possessed gonads at 
stage 5 or 6, together with elevated mean monthly GSIs. 
Logistic regression analysis was restricted to males 
obtained during the spawning period, from September 
through February (see the “Reproductive biology” sub- 
section in the “Results” section). The form of the logistic 
model relating the probability that a male Indian halibut 
is mature to its TL is as follows: 
Pe=vileeete [= sin (1.9)| (isan) (een em) in (5) 
where P = proportion mature; 
L = total length in millimeters; 
L59 = total lengths in millimeters at which 50% of fish 
were mature; and 
Lo, = total lengths in millimeters at which 95% of fish 
were mature. 
Mortality estimates 
In northwestern Australia, the Indian halibut is consid- 
ered a bycatch species and is caught in very low numbers 
in a fishery that targets larger bodied, more valuable lut- 
janids, lethrinids, and serranids (WADF’). This low level 
of catch is indicated, for example, by the total weight of 
Indian halibut available on market days at the whole- 
sale market in Perth typically ranging only between 5 
and 15 kg (senior author, personal observ.), with all fish 
available on a particular day, in some instances, being 
purchased for this study. Therefore, the sizes and ages of 
fish collected for this study are assumed to be represen- 
tative of the population of this species in northwestern 
Australia. 
Estimates of the M of females and males were calculated 
from their maximum individual ages, by using both the 
Hoenig (1983) and Hoenigyy;s (Then et al., 2015) equations, 
with the estimates from the latter equation recognized as 
the more reliable (Maunder and Piner, 2015). The estimates 
of M, derived with the Hoenig (1983) equation, are provided 
to make comparisons possible because this equation has 
been widely used in previous studies. To further enable 
comparisons with previous studies (Edwards and Shaher, 
1991; Silvestre and Garces, 2004; Gilanshahi et al., 2012), 
M was also estimated by using the Pauly (1980) empirical 
7 WADF (Western Australia Department of Fisheries). 2010. 
A bycatch action plan for the Pilbara fish trawl interim man- 
aged fishery. Fish. Manage. Pap. 244, 24 p. West. Aust. Dep. Fish., 
Perth, Australia. [Available from website.] 
equation for length and by using a water temperature of 
27°C (i.e., the mean annual sea-surface temperature for 
waters off the Pilbara coast). 
For catch curve analysis, only those age classes con- 
sisting of individuals 1 year older than the age at full 
recruitment for each sex (i.e., 4 years) were used (Ricker, 
1975), with the remaining age classes assigned an age 
relative to the age at full recruitment (i.e., the first fully 
recruited age was assigned the age of zero). Total mortal- 
ity (Z) for fully recruited fish was estimated by using the 
Chapman and Robson (1960) approach, as implemented 
in the chapmanRobson function of the FSA package 
(vers. 0.8.6; Ogle, 2016) in R. 
Results 
Whole versus sectioned otoliths 
Although a single opaque zone was visible in all of 
the whole otoliths whose sections also possessed a single 
opaque zone, the level of agreement between the opaque 
zone counts for whole otoliths and for their correspond- 
ing sections decreased rapidly (Fig. 2). For example, the 
same number of opaque zones were visible in only 4 of 
the 41 otoliths in which 5 opaque zones were visible in 
their sections. There was no agreement in opaque counts 
when the number of opaque zones in sections was 27. In 
the most severe case, only 4 opaque zones were clearly 
visible in a whole otolith whose corresponding section 
revealed 16 opaque zones (Fig. 2). 
Validation of aging methods 
The mean monthly MIs on sectioned otoliths of Indian 
halibut with 2—4 opaque zones remained <0.41 between 
January and March before increasing to 0.49 in May and 
to a maximum of 0.57 in July (Fig. 3). The mean monthly 
MI remained at elevated levels in August and Septem- 
ber, before declining rapidly to a minimum in December. 
The mean monthly MIs for otoliths with =>5 opaque zones 
in other months of the year followed a similar trend, 
increasing from low levels in January to a maximum in 
September, before declining to lower levels in December 
(Fig. 3). The single pronounced decline and then progres- 
sive increase of the mean monthly MIs during the year 
for otoliths with different numbers of opaque zones indi- 
cates that a single opaque zone is formed annually in the 
otoliths of Indian halibut. The validity of the notion that 
the mean monthly MIs followed a single cycle during the 
year is substantiated by the results in this study from 
circular distribution models (Okamura and Semba, 
2009; Coulson et al., 2016). For otoliths with 2—4 opaque 
zones and >5 opaque zones, the accompanying AIC val- 
ues of 302 and 169, respectively, for an annual cycle were 
less than the AIC values of 309 and 183 for no cycle and 
312 and 182 for a biannual cycle; in addition, the differ- 
ence between the former value and the AIC values of the 
latter 2 cycles exceeded 2, the number required to 
