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This paper describes preliminary work to estimate the age and growth of swordfish in the Indian Ocean as part of the ‘GERUNDIO’ project[1]. The most recent stock assessment for Indian Ocean swordfish was undertaken in 2020 using Stock Synthesis. The base case model used otolith-based growth estimates for swordfish from the southwest Pacific Ocean from Farley et al. (2016), and the sensitivity models used fin spine-based growth estimates for swordfish in the northern Indian Ocean from Wang et al. (2010). Farley et al. (2016) found that age estimates from fin spines from Pacific Ocean swordfish are likely to underestimate age of older swordfish, so the current project was undertaken to assess the suitability of otoliths to estimate age and growth for swordfish in the Indian Ocean.
A total of 317 swordfish otolith samples collected in the current and a previous project were available for analysis from the western and eastern Indian Ocean. In the western Indian Ocean, 128 otoliths were available from fish ranging in size from 59-251 cm lower jaw fork length (LJFL), although the majority were from fish <200 cm LJFL. Sex was recorded for 86 of these fish. In the eastern Indian Ocean, 189 otoliths were available from fish ranging in size from 51-309 cm LJFL, but sex was not recorded for any of these fish.
Since swordfish have sexually dimorphic growth, only samples from the western Indian Ocean were selected for analysis. Otoliths for this preliminary work were selected from fish for which the sex was reported (n=84 out of 86, since two arrived at the lab too late to be included). An additional 21 otoliths were selected from fish <150 cm LJFL with unknown sex, since sexual dimorphism in growth is not expected for fish under this size (see Farley et al. 2016).
A combination of daily and annual ageing was undertaken. The maximum age was 11 years for both males (183 cm LJFL) and females (251 cm LJFL). The youngest fish was aged 76 days. There was insufficient data to model growth as otoliths from large male/females were not available. However, our preliminary length-at-age data suggests that swordfish in the western Indian Ocean have similar, although perhaps slightly slower, growth to swordfish in the southwest Pacific. Fish length to otolith weight (which may be an indicator of age) data are consistent with this finding. However, the sample size is too small to be conclusive. The otolith-based length-at-age data for the western Indian Ocean does not align well with the fin spine-based growth curves from Wang et al. (2010) for swordfish from the northern Indian Ocean, particularly for small/young fish (<150 cm LJFL, ~3 years old), where we have the most data to compare.
We recommend that additional otoliths are collected and analysed from fish >200 cm LJFL, from which sex is known, in the Indian Ocean to provide further information on growth and longevity. It would be desirable to collect samples from across the range of the species including northern and southern regions to provide potential to clarify hypothesis about stock structure from the PSTBS-IO project (see Grewe et al., 2020). It may be possible to genetically determine the sex of fish with otoliths currently available, if a genetic-based sex-specific marker can be found in studies currently underway. We also recommend age validation studies to determine if the otolith age estimates obtained are accurate.
[1] “Collection and analysis of biological samples of tropical tunas, swordfish, and blue shark to improve age, growth and reproduction data for the Indian Ocean Tuna Commission (IOTC)”, FAO Contract No. 2020/SEY/FIDTD/IOTC - CPA 345335.
