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Fishery Bulletin 119(2-3) 
Table 1 
Estimates of parameters of the von Bertalanffy growth function (VBGF) for female (F) and male 
(M) Indian halibut (Psettodes erumei) caught in the Indian Ocean off the Pilbara coast in north- 
western Australia by commercial trawlers as bycatch between February 2014 and December 2015 
and during research surveys with an otter trawl between August 2015 and September 2017. The 
parameters are the asymptotic length (L..) in total length, growth coefficient (k), and hypothetical 
age at which fish have zero length (t)). The upper and lower 95% confidence limits, coefficient of 
determination (r”), and sample size (n) are provided for each estimate. Values were derived from 
the total lengths at age of individuals. The VBGF parameters obtained for Indian halibut in other 
studies are provided for comparison. The values for L,, and k from Gilanshahi et al. (2012) and 
Silvestre and Garces (2004) were derived from trends in monthly length frequencies, and those 
from Edwards and Shaher (1997) and Das and Mishra (1990) are based on individual fish aged by 
using vertebrae and whole otoliths, respectively. 
VBGF parameter 
L 
= k to 
Source Value (mm) (year!) (years) 
This study Estimate 395 0.49 -0.38 
Upper 407 0.57 -0.22 
Lower 384 0.42 -0.55 
Estimate 319 0.66 -0.51 
Upper 324 0.76 -0.33 
Lower 313 0.56 -0.69 
Edwards and Shaher (1997) Both 
Silvestre and Garces (2004) Both 
Gilanshahi et al. (2012) Both 
Das and Mishra (1990) F 
M 
with mature or spawning gonads (stages 5 and 6) indicate 
that the spawning period for Indian halibut in northwestern 
Australia extends from September through February. 
Lengths and ages at maturity 
Because all female Indian halibut with TLs <225 mm 
that were caught during the spawning period were imma- 
ture and all those with TLs >300 mm had mature gonads 
(stages 3-8), no attempt was made to determine an L;, for 
this sex (Fig. 8). All male Indian halibut <250 mm TL that 
were caught during the spawning period, except 2 individ- 
uals (163 and 164 mm TL), were immature (Fig. 7). The 
percentage of mature males increased sharply to 50% and 
85% in the length classes of 250-274 and 275-299 mm TL, 
with all males >300 mm TL being mature. The L;, and Lo;, 
and their upper and lower 95% confidence intervals (CIs), 
for male Indian halibut were estimated to be 268 mm TL 
(95% CI: 250-288 mm TL) and 284 mm TL (95% CI: 255-— 
325 mm TL), respectively. 
All females and most males that were <1 year old and 
caught during the spawning period were immature (Fig. 7). 
In contrast, all females that were 2—9 years old and most 
males that were 2-10 years old, and caught during the 
spawning period, were mature. This result indicates that, 
generally, female and male Indian halibut do not mature 
until they are almost 2 years old. 
622 0.68 0.46 
545 0.33 
745 0.23 -0.61 
535 0.64 -0.61 
436 0.56 -0.72 
Mortality 
Estimates for M of female and male Indian halibut, derived 
by using the Pauly (1980) equation and the respective 
VBGF parameters, were far higher than those derived by 
using either the Hoenig (1983) equation or the Then et al. 
(2015) equation, both of which use the maximum age of each 
sex (Table 2). In addition, the M values estimated by using 
the equation of Then et al. (2015) were slightly greater 
than those derived by using the Hoenig (1983) equation, 
noting that, through cross validation, Then et al. (2015) 
established that their equation provided a better estimate 
of published values of M than other equations that they 
tested. The catch curve estimates of Z for both sexes were 
slightly less than the corresponding estimates of M derived 
from use of the Then et al. (2015) equation (Table 2). 
Discussion 
Method of aging 
The results of the comparison between the number of 
opaque zones in whole otoliths and the number in their 
corresponding sections indicate that ages of Indian hali- 
but estimated from counts of opaque zones in whole oto- 
liths, even when the numbers of such zones are few, are 
