Rabaoui et al.: Life history of 7henus orientalis in the Arabian Gulf 
2 
ro) 
Ts 
| 
Relative frequency 
[@) 
(2) 
wo 
Carapace length (mm) 
Figure 3 
Aggregated length-frequency data (bars) and the fit of the model 
with 2 age cohorts (lines) for flathead lobsters (Thenus orientalis) 
collected from 2013 through 2016 in the waters of Saudi Arabia in 
the Arabian Gulf. 
Lobster fishery and stock assessment 
The landed catch has been increasing substantially since 
2000, amounting to 43 metric tons (t) in 2008, the last year 
before the ban (Fig. 4A). Considering the mean weight of 
flathead lobsters in our sampling during 2016, that catch 
translates to over 200,000 individual lobsters. Dhow fleets 
had a dominant role in contributing to the landings, from 
50% in 1995 and >98% in 2008 (Fig. 4A). The rise in land- 
ings is totally attributable to the dhow fleet, with approx- 
imately 15,000 trips per year (Fig. 4B) or 70,000 days 
per year (Fig. 4C) after 2003. The catch rate in 
kilograms per day of fishing was similar in both 
fleets, 0.486 kg/d for the dhow fleet and 0.300 
kg/d for the tarad fleet (Fig. 4D), or around 2 flat- 
head lobsters per day. 
The best fit of the generalized depletion model 
to the data and the predicted biomass per month 
are shown in Supplementary Figure 5 (online 
only) (also see Supplementary Material) (online 
only). Estimates from the model closely follow 
the observed catch of both fleets (Figs. 5 and 6), 
although there are signs of deviation from dis- 
tributional assumptions in the residual cloud 
for the dhow fleet, and this deviation tends to 
widen toward the end of the time series. Pre- 
dicted biomass on the other hand indicates wide 
fluctuations driven by recruitment (Fig. 7) and 
intense M (Table 3), but the prediction is fairly 
imprecise, with wide statistical margins. Param- 
eter estimates and their precision are presented 
in Table 3. Natural mortality is very high, at an 
annualized rate of 1.8 year !, and abundance 
inputs (No and recruitment) vary widely from 
nearly 1 to 100 million individuals. Estimates of 
M, No, and all recruitment pulses higher than 
50 million individuals have fairly good precision 
(coefficient of variation <50%). The operations of 
the dhow fleet, which accounts for over 97% of the land- 
ings in the latest years, are proportional with respect to 
effort and are hyperstable with respect to abundance. 
Discussion 
With the results of this study, we have provided for the first 
time the necessary scientific information for managing a 
sustainable fishery for flathead lobsters in waters of Saudi 
Arabia in the Arabian Gulf. We determined the spatial 
Table 2 
Published estimates of parameters of the von Bertalanffy growth 
function, growth coefficient (K) and asymptotic length (L..), 
and estimates of natural mortality (M) for species of Thenus in different regions around the world. Also provided are mean 
values from this study for comparison, with standard errors given 
in parentheses. The estimates for this study in the Arabian 
Gulf are based on data pooled for both sexes. Values of L,, for male and female T: unimaculatus are not provided in the consulted 
reference. CL=carapace length. 
K (year™') 
M (a) L.. (mm CL) 
Species Country Males Females Males Females Males Females Source 
T. australiensis Australia 0.511 0.584 0.0022 0.0023 77.45 89.04 Courtney* 
T. parindicus Australia 0.949 0.834 0.0035 0.0034 61.23 72.44 Courtney* 
T. unimaculatus India 1.387 1.059 0.0017 0.0019 - = Radhakrishnan 
et al. (2013) 
T. orientalis Kuwait 1.789 1.679 0.0068 0.0071 71.6 81.7 Hussain (1996) 
(max: 74) (max: 85) 
T. orientalis Saudi Arabia 0.840 (1.497) 0.0048 (0.0012) 106.52 (11.06) This study 
