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This paper describes work to estimate the age and growth of bigeye tuna (Thunnus obesus) in
the Indian Ocean from otoliths as part of the ‘GERUNDIO’ project1. The most recent stock
assessment for Indian Ocean bigeye tuna indicated that the stock is not overfished but
overfishing is occurring (Fu 2019; IOTC 2020). The stock assessment model used a fixed
growth function from Eveson et al. (2012), which was estimated using tag-recapture data and
daily age estimates from otoliths. A slightly updated (but very similar) growth curve was
presented in Eveson et al. (2015), which suggests a two-stanza growth model for bigeye tuna
where growth is slow for fish between 40 and 50 cm fork length (FL), then changes to faster
growth between 50 and 70 cm FL, before slowing again. The otolith-based daily age estimates
from Sardenne et al. (2015) that were used in the analysis varied considerably among readers
and there was a recommendation by Sardenne et al. (2015) to explore alternate ageing
methods, such as annual ageing (as opposed to daily ageing). More recently, Farley et al.
(2017; 2020) developed a new method to estimate the decimal age of bigeye tuna in the
western and central Pacific Ocean from validated counts of daily and annual growth zones in
otoliths. The aim of the current study was to apply this method to bigeye tuna in the Indian
Ocean to obtain new estimates of age and growth, and to attempt to validate the age
estimates using otoliths from bigeye tuna tagged and recaptured in the Indian Ocean Tuna
Tagging Programme (IOTTP).
Otoliths from 632 bigeye tuna collected in the current and previous projects were available
for analysis and otoliths from 108 fish were selected for ageing. The otoliths were collected
from bigeye tuna ranging in size from 18.5 to 178 cm FL. A combination of daily and annual
ageing was undertaken, and a final age was obtained for 107 of the 108 fish. The youngest
fish was aged 49 days and the oldest was 14.7 years. The preliminary age validation/
verification work using otoliths and data from the IOTTP provides evidence that the otolith
ageing method used in this study is accurate. However, we recommend that further age
validation work is undertaken.
Three growth models were fit to the age and length data (VB, Richards and VB log k). All three
provided very similar fits; however, the two-phase VB log k model provided a better fit to the
data for fish <40 cm FL. The length-at-otolith weight data (which is independent of the age
estimation method) showed a change in otolith growth at ~40 cm FL consistent with the
length-at-age data, which lends support to the VB log k model. Overall, our analysis shows
that growth is rapid in the first few years with fish reaching ~60 cm FL at age 1 and ~85 cm FL
at age 2. Mean asymptotic length was estimated to be ~168 cm FL.
The VB log k growth curve estimated in the current study is similar to the VB curve of Stéquert
and Conand (2004) for bigeye tuna in the western Indian Ocean, which was estimated using
counts of microincrements (daily age estimates) in otoliths, although we obtained higher age
estimates for fish >140 cm FL. Also, our VB log k curve suggests faster growth for fish up to
~40 cm FL.
Although our data suggests two-phase growth, our VB log k growth curve is quite different
from the integrated VB log k curves of Eveson et al. (2012; 2015). In particular, we do not see
the same very slow growth for fish < 50 cm, plus we estimate mean asymptotic length to be
higher (~168 cm FL in the current study compared to 152.5 cm FL in Eveson et al. 2015). This
may be due to the low number of fish >150 cm FL in the tag-recapture data available at the
time to be used in Eveson et al. (2015), which is likely related to the relatively short times at
liberty of fish included in the analysis (<6 years) compared to the current estimated longevity
of bigeye tuna of at least 14 years.
We recommend that additional otoliths are collected from the eastern Indian Ocean, and that
these otoliths and additional otoliths from those already collected in the GERUNDIO and
IOTTP projects are read/aged to provide further information on growth and longevity before
the next stock assessment for bigeye tuna in 2022.
