Fishery Bulletin 119(2-3) 
Table 2 
Maximum age, natural mortality (MV), and total mortality (Z) for female (F) and male (M) Indian hal- 
ibut (Psettodes erumei) caught by commercial trawlers between February 2014 and December 2015 
and during research surveys between August 2015 and September 2017 in northwestern Australia. 
Estimates of M were calculated by using the Pauly (1980), Hoenig (1983), and Then et al. (2015) equa- 
tions, and estimates of Z (provided with standard errors in parentheses) were calculated by using 
the catch curve method of Chapman and Robson (1960). Estimates of M from Gilanshahi et al. (2012) 
and Silvestre and Garces (2004) were based on estimates of von Bertalanffy growth function param- 
eters, asymptotic length and growth coefficient, derived from trends in monthly length frequencies, 
and those from Edwards and Shaher (1997) are based on individual fish aged by using vertebrae. 
Estimates of Z from Gilanshahi et al. (2012) and Silvestre and Garces (2004) were derived from 
length-converted catch curves. 
M Z 
Chapman and 
Robson (1960) 
Max. Pauly Hoenig ‘Then etal. 
Source age (1980) (1983) (2015) 
0.39 (0.09) 
0.32 (0.07) 
This study 11 1.02 0.40 0.55 
This study 16 1.31 0.28 0.39 
Edwards and Shaher (1997) Both 0.76 
Silvestre and Garces (2004) Both 0.73 0.85 
Gilanshahi et al. (2012) Both 0.51 1.20 
temperature from its winter minimum may provide a cue 
for the onset of gonadal development for Indian halibut, 
in the Bay of Bengal, it appears that spawning may be 
delayed until the post-monsoon period, when primary pro- 
ductivity is greatest (Choudhury and Pal, 2010) and when 
suitable prey for recently spawned larvae is abundant. 
Mortality 
The estimates of M for females, and particularly for 
males, derived in this study by using the Pauly (1980) 
equation were far higher than the values derived by 
using the same method in previous studies (Table 2). 
Because k is used for the Pauly (1980) equation, the 
resultant value for M reflects the accuracy of the esti- 
mates of that parameter. The results of the studies car- 
ried out by Silvestre and Garces (2004) and Gilanshahi 
et al. (2012) indicate that the growth of Indian halibut 
is slow and does not reach an asymptote (Fig. 4C), as 
reflected by lower values for k from those studies: 0.33 
year ' and 0.23 year |, respectively, in comparison with 
k values from our study. In the cases of those studies, the 
use of k to estimate M for Indian halibut is not suitable, 
firstly because & is not reliably estimated (i.e., monthly 
length-frequency data) and secondly because the esti- 
mated k values reflect a species that never reaches its 
asymptotic length (Kenchington, 2014). Although in our 
study k was estimated reliably by using individual length 
at (validated) ages, the fact that females and males grew 
rapidly in the first 2-3 years, after which growth slowed 
appreciably, also prohibits those k values for each sex 
from being employed to estimate M (Kenchington, 2014). 
Direct estimates of Z should always exceed the true 
M value. Results from this study indicate that indirect 
estimates of M for female and male Indian halibut, derived 
from the Hoenigyy;s equation of Then et al. (2015), exceed 
the estimates of Z and are therefore considered overesti- 
mates (Coulson et al., 2017). However, indirect estimates 
of M for each sex derived by using the equation of Hoenig 
(1983) are more consistent with those of Z. The similarity 
in values of Z and M, from the Hoenig (1983) equation, is 
consistent with the fact that, in northwestern Australia, 
the Indian halibut is a bycatch species. This similarity in 
mortality values parallels the mortality estimates for 4 of 
5 platycephalid species from southwestern Australia that 
are caught as bycatch in commercial fisheries that target 
other species, as part of the catch in multispecies commer- 
cial fisheries, or as catch in low numbers in a recreational 
fishery (Coulson et al., 2017). 
The use of mortality estimates that supposedly reflect a 
species that has high natural mortality, when those esti- 
mates are based on underestimates of age, may lead fish- 
eries managers to set catch limits higher than is suitable 
for a species that actually has far lower natural mortality 
rates. It is, therefore, imperative that the management of 
Indian halibut in those regions where this species accounts 
for a significant proportion of catch, and the management 
of species that have similar life history characteristics 
(i.e., sexually dimorphic growth, rapid initial growth, and 
medium longevity), is based on a sound understanding of 
their biology, determined by using proven and validated 
techniques. 
Conclusions 
This study is the first comprehensive investigation of the bio- 
logical characteristics of Indian halibut. The greater number 
