Coulson and Poad: Biological characteristics of Psettodes erume/ from the Indian Ocean 169 
vertebrae (Edwards and Shaher, 1991), scales (Druzhinin 
and Petrova, 1980), and whole otoliths (Das and Mishra, 
1990), and none of those methods were validated; ages 
also have been estimated by examining trends displayed 
by monthly length frequencies (Silvestre and Garces, 
2004; Gilanshahi et al., 2012). The use of vertebrae to 
age fish has been associated with large discrepancies in 
age estimates between readers (e.g., Esteves et al., 1995; 
Filmalter et al., 2009) and interpretation difficulties 
(e.g., Baker and Timmons, 1991; Marriott and Cappo, 
2000; Khemiri et al., 2005). Although scales have been 
shown to be a useful aging structure for which removal is 
not lethal (e.g., Robillard and Marsden, 1996; Khan and 
Khan, 2009), the accuracy of ages derived from counts 
of growth zones on scales is questionable (Beamish and 
McFarlane, 1983). Even though whole otoliths are suit- 
able to age short-lived species (e.g., Fairclough et al., 
2000; Kornis et al., 2017), for many fishes, including a 
number of flatfishes, it has been acknowledged that ages 
based on whole otoliths are often underestimated (e.g., 
Campana, 1984; Forsberg”; Stevens et al., 2005; Albert 
et al., 2009). 
Previous estimates of the instantaneous rate of natu- 
ral mortality (M) for Indian halibut, calculated by using 
the Pauly (1980) empirical equation, range from 0.51 to 
0.76 year | (Edwards and Shaher, 1991; Silvestre and 
Garces, 2004; Gilanshahi et al., 2012) and are higher than 
the median value of 0.41 year ' for members of the Pleu- 
ronectidae (Froese and Pauly’). The fact that the land- 
ings of Indian halibut in waters of India have declined by 
~70% over the past 30 years, even as the overall catch of 
flatfishes has increased over the same period (Nair and 
Gopalakrishnan, 2014), indicates that the level of fishing 
pressure on this species may have been too high, possibly 
as a result of management advice based on unreliable 
age estimates. 
This study was undertaken to determine the biolog- 
ical characteristics, including age, growth trends, tim- 
ing and duration of spawning, and lengths and ages at 
maturity, of Indian halibut in waters off northwestern 
Australia. Because of the now known unreliability of the 
aging structures or methods that have previously been 
used to age Indian halibut in other regions, this study 
aimed to provide sound age estimates based on proven 
and validated aging methods, thereby facilitating accu- 
rate estimates of growth and of both natural and total 
mortality. The data were also used to test the hypothesis 
that, as reported for many other flatfishes (e.g., Stevens 
et al., 2005; Félix et al., 2011; Black et al., 2013), male 
Indian halibut grow faster than females, and females 
grow to a larger size. Furthermore, it has been hypoth- 
esized that the beginning of the spawning period for 
” Forsberg, J. E. 2001. Aging manual for Pacific halibut: proce- 
dures and methods used by the International Pacific Halibut 
Commission. Tech. Rep. 46, 50 p. [Available from website.] 
3 Froese, R., and D. Pauly (eds.). 2017. FishBase, vers. 12/2017. 
World Wide Web electronic publication. [Available from website, 
accessed January 2018.] 
Indian halibut, like that of other subtropical species, is 
associated with increasing day length and water tem- 
peratures (Lam, 1983). 
Materials and methods 
Sample collection and fish measurements 
Large (>250 mm in total length [TL]) Indian halibut 
were purchased whole from a wholesale market in Perth, 
Australia, in each month between February 2014 and 
December 2015, such that 20-50 fish were collected for 
each calendar month. These fish were taken as bycatch 
by commercial bottom trawlers that deployed standard 
stern trawling gear (a single net with extension sweeps) 
at depths of 50-200 m in the eastern Indian Ocean off 
the Pilbara coast in northwestern Australia (approxi- 
mately between 19°S, 120°E and 20°S, 116°E), where the 
catch is largely composed of lutjanid and lethrinid species 
(Wakefield et al.4; Newman et al.°). Additional smaller 
individuals, 35-215 mm TL), caught in August 2015 and 
September 2017, were provided by the Western Australian 
Department of Primary Industry and Regional Develop- 
ment. These fish were caught during research surveys off 
the Pilbara coast, by using an otter trawl with net mesh 
sizes of 229 mm (9 in) in the wings and 110 mm (4.3 in) in 
the pocket. 
The TL and total body mass of each Indian halibut was 
measured to the nearest 1 mm and 0.1 g, respectively. The 
eye side (i.e., right or left) of each fish was recorded, and 
the sagittal otoliths of each fish were removed, cleaned, 
dried, and stored in envelopes. 
Mean monthly sea-surface temperatures in waters off 
the Pilbara coast, for the 1° grid block of 19-20°S, 117- 
118°E, were calculated from monthly values in 2010-2015 
obtained from the Koninklijk Nederlands Meteorologisch 
Instituut Climate Explorer (available from website), which 
employs data from Reynolds optimum interpolation anal- 
ysis of sea-surface temperatures (Reynolds et al., 2007). 
Daily lengths of days for the town of Karratha (~22°S, 
117°E), on the Pilbara coast, obtained from timeand- 
date.com (day length data available from website), were 
employed to generate mean monthly day lengths for 2014 
and 2015. 
4 Wakefield, C. B., S. Blight, S. R. Dorman, A. Denham, 
S. J. Newman, J. Wakeford, B. W. Molony, A. W. Thomson, 
C. Syers, and S. O’Donoghue. 2014. Independent observations 
of catches and subsurface mitigation efficiencies of modified 
trawl nets for endangered, threatened and protected mega- 
fauna bycatch in the Pilbara fish trawl fishery. Fish. Res. 
Rep. 244, 33 p. West. Aust. Dep. Fish., North Beach, Australia. 
[Available from website.] 
> Newman, S., C. Wakefield, C. Skepper, D. Boddington, R. Jones, 
and P. Dobson. 2017. North coast demersal resource status 
report 2016. In Status reports of the fisheries and aquatic 
resources of Western Australia 2015/16: the state of the fish- 
eries (W. J. Fletcher, M. D. Mumme, and F. J. Webster, eds.), 
p. 144-152. West. Aust. Dep. Fish., Perth, Australia. [Available 
from website.] 
