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This paper describes work to investigate the potential of two fish hard structures, otoliths and first dorsal fin spines, to estimate the annual age of skipjack tuna (Katsuwonus pelamis) in the Indian Ocean, as part of the “GERUNDIO” project1. The 2020 stock assessment model for skipjack tuna in the Indian Ocean used a Richards curve approximation of the growth model in Eveson et al. (2012), with an asymptotic length (L∞) of 70 cm straight fork length (SFL), growth rate coefficient (k) of 0.34, and an inflexion parameter fixed at 2.96 (Fu 2020). To date, the inclusion of otolith age data into the skipjack growth model has been challenging due to considerable variation in daily age estimates among otolith readers and lack of validation that micro-increments are deposited on a daily basis (Eveson et al. 2012, 2015). Moreover, there was a recommendation by Sardenne et al. (2015) to explore alternate ageing methods such as annual ageing of otoliths (as opposed to daily ageing) or strucures (such as dorsal fin spines), which have rarely been studied for this species in the Indian Ocean (but see Romanov et al. 1995). The aim of the current study was to investigate and compare (assumed) annual age estimates from both otolith and fin spine of skipjack tuna.
A total of 943 otoliths and 613 fin spines from skipjack tuna collected in the current and previous projects in the Indian Ocean were available for analysis. The fish ranged in size from 25 to 77 cm straight fork length (SFL). Of the samples available, 159 otoliths and 120 spines were analysed, of which 86 were paired samples from fish ranging in size from 28 to 73 cm SFL. Comparison of annual ages (i.e., counts of growth zones) based on both structures resulted in very low agreement (only 8.14% of individuals were aged the same with both structures). Based on otolith opaque zone counts, analysed skipjack tuna were estimated to be between 0 and 3 years, while based on fin spine translucent zone counts fish were estimated to be between 0 and 6 years. Decimal age estimates were calculated for 159 fish based on the counts of opaque zones and otolith measurements; ages ranged between 0.14 and 3.59 years. Otolith-based age estimates showed greater variability in length-at-age than the fin spine-based estimates. Overall, the otolith age estimates suggest a very fast initial growth, with a transition to slower growth at around age two years, whereas the fin spine ageing method suggests skipjack tuna grow is linear. Preliminary age validation work using otoliths and data from the IOTTP provides some evidence that the otolith ageing method used in this study is accurate for fish tagged at 48 to 53 cm SFL and recaptured up to 1.65 year later.
Based on these results, we recommend that direct comparisons of otoliths and fin spines from the same fish are performed to help determine where the differences in counts occurred. We highly recommend that this work is undertaken when images of the sectioned structures, with the zones marked, are available. This is also necessary in order to determine the position of the first ring of the fin spines. This study highlights the current need to develop appropriate age validation and verification methods before conducting a large-scale age and growth study for skipjack tuna in the Indian Ocean.
