National Marine 
Fisheries Service 
NOAA 
Abstract—The Indian halibut (Pset- 
todes erumei) is an important flatfish 
species in tropical and subtropical 
waters of the northern Indian Ocean. 
However, inappropriate aging methods 
used in previous studies of Indian hal- 
ibut limited the use of age and growth 
data from those studies. This study 
highlights the importance of correct 
aging methods and identification of 
sexually dimorphic growth, particu- 
larly if growth characteristics are used 
to estimate mortality rates. Counts of 
opaque zones in whole otoliths of Indian 
halibut were almost always fewer than 
those in their corresponding sections, 
particularly for older individuals. Mul- 
tiple validation methods have been 
used to demonstrate that opaque zones 
form annually in otoliths of Indian hal- 
ibut. Although the frequency of females 
increased with size, because of sexu- 
ally dimorphic growth, males attained 
a greater maximum age (16 versus 11 
years). Because of the greater ages and 
rapid early growth of Indian halibut 
described in this study, natural mor- 
tality estimates calculated by using 
the Pauly equation, particularly for 
males, were higher than estimates 
derived from the same equation in pre- 
vious studies, which employed growth 
parameter estimates based on ages 
derived from whole otoliths, vertebrae, 
or monthly length—frequency trends 
and, typically, did not account for sexu- 
ally dimorphic growth. In northwestern 
Australia, spawning in Indian halibut 
commences when water temperature 
and day length begin to increase and 
occurs between early spring and late 
summer. 
Manuscript submitted 22 December 2020. 
Manuscript accepted 20 July 2021. 
Fish. Bull. 119:168—183 (2021). 
Online publication date: 30 August 2021. 
doi: 10.7755/FB.119.2-3.6 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Fishery Bulletin 
@ established in 1881 << 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Biological characteristics of the primitive flatfish 
Indian halibut (Psettodes erumei) from the 
tropical northeastern Indian Ocean, including 
implications of the use of incorrect aging 
methods on mortality estimates 
Peter G. Coulson (contact author) 
Jake A. Poad 
Email address for contact author: peter.coulson@dpird.wa.gov.au 
Centre for Sustainable Aquatic Ecosystems 
Murdoch University 
90 South Street 
Murdoch, Western Australia 6150, Australia 
Present address for contact author: Department of Primary Industries and Regional Development 
Western Australian Fisheries and Marine Research Laboratories 
P.O. Box 20 
North Beach, Western Australia 6920, Australia 
Globally, flatfish species (flounders, hal- 
ibuts, and soles) are an important group 
of fish species, contributing over 1 mil- 
lion metric tons (t) to world fisheries 
production in 2014 (FAO!; Nelson et al., 
2016). The Indian halibut (Psettodes 
erumel) iS a moderate-sized flatfish 
distributed in tropical and subtropical 
waters of the Indian and western Pacific 
Oceans, where it is an important com- 
ponent of trawl fisheries (Mathew et al., 
1992; Vivekanandan et al., 2003). The 
annual catch of Indian halibut from 3 of 
the tropical fishing regions designated 
by the Food and Agricultural Organi- 
zation of the United Nations ranged 
from ~15,000 to ~30,000 t between 
2000 and 2011 (Gibson et al., 2015). In 
waters of India, where the majority of 
previous research on Indian halibut has 
been carried out (e.g., Pradhan, 1962; 
Hussain, 1990; Mathew et al., 1992; 
Gilanshahi et al., 2012), this species 
' FAO (Food and Agriculture Organization of 
the United Nations). 2016. FAO yearbook. 
Fishery and aquaculture statistics 2014, 
76 p. FAO, Rome. [Available from website.] 
constituted ~1300 t of the total flatfish 
catch in 2012 (Nair and Gopalakrish- 
nan, 2014). Although most flatfish have 
eyes on the right-hand (dextral) or 
left-hand side (sinistral) of the mouth 
(Nelson et al., 2016), a few species, 
including the Indian halibut and its 
2 congeners, have varying proportions 
of dextral to sinistral polymorphism 
(Hubbs and Hubbs, 1945; Policansky, 
1982). In addition, spiny dorsal and pel- 
vic fin rays, vertebrae number, mouth 
structure, and upright swimming 
behavior are characteristics that sepa- 
rate Psettodes species from other flat- 
fishes and why they are considered the 
most primitive form of flatfish (Hubbs 
and Hubbs, 1945; Platt, 1983). 
The importance of choosing the 
appropriate aging structure and 
then validating the periodicity of 
growth increments has been made 
clear (Beamish and McFarlane, 1983; 
Campana, 2001), as have the implica- 
tions of not doing so (c.f. Mace et al., 
1990; Smith et al., 1995). Previous age 
estimates for Indian halibut have been 
derived from counting growth rings in 
