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This paper describes preliminary work to estimate reproductive parameters for skipjack tuna (Katsuwonus pelamis) in the Indian Ocean as part of the ‘GERUNDIO’ project. The most recent stock assessment of skipjack tuna in the Indian Ocean shows that the stock is not overfished and is not subject to overfishing (Fu 2020, IOTC 2020). The assessment model used a maturity ogive (i.e., the proportion of mature individuals at age or length) for the western Indian Ocean from Grande et al. (2010), with length at 50% maturity (L50) at 37.8 cm fork length (FL) and full maturity at around 42 cm FL. A slightly updated ogive was presented in Grande et al. (2014) with L50 estimated at 39.9 cm FL. Grande et al. (2010; 2014), used the ‘cortical alveolar’ oocyte development stage and above as the threshold indicating a fish was mature. One of the aims of the current project was to produce updated estimates of key biological reproductive parameters for skipjack tuna in the Indian Ocean. This included updating information on length-at-maturity using the (older) ‘early vitellogenic’ oocyte stage as the maturity threshold, which is considered a more reliable threshold for determining a female is mature and contributing to egg production (Schaefer 2001). A total of 635 skipjack tuna were sampled in the GERUNDIO project (296 females and 339 males). The individuals were collected in 2020-2021 predominantly from purse seine fisheries unloading at canneries in the western Indian Ocean. Histological sections were prepared from a subset of 84 ovary samples, which were read using an agreed classification system. Additional ovaries collected in the project will be processed soon to update the current analysis.
Data from an additional 1151 skipjack tuna (862 females and 649 males) were obtained from previous projects (EMOTION database, see Bodin et al. 2018), which included histological data from 756 females classified using a similar classification scheme to that agreed in the GERUNDIO project. The individuals were collected from 2009-2019 and were also predominantly from the western Indian Ocean.
Preliminary estimates of sex ratio, spawning periodicity, batch fecundity and length at maturity are provided for skipjack tuna predominantly from in the western Indian Ocean. Further work is required to finalize the analyses, particularly the spawning periodicity and maturity results. The analysis is currently based on data from a subset of the ovaries collected in the GERUNDIO project as it was not possible to process all the ovaries collected or to undertake the required cross-checking (re-reading) of histological sections within the project timeframe. In addition, it was not possible to cross-check the histological data obtained from the EMOTION database. Postovulatory follicles were not recorded in this study to estimate spawning fraction (the proportion of females spawning per day), but it may be possible to estimate when the data are available.
The estimate of L50 (41.3 cm FL) obtained in this study is slightly higher than that obtained by Grande et al. (2010; 2014), probably affected by different maturity thresholds used in both studies. Our L50 estimate was slightly lower than estimated by Stequert and Rammcharrun (1996) and Timohino and Romanov (1996) (42 and 43 cm FL respectively) using the same maturity threshold.
There was insufficient data to examine region-specific reproductive parameters in this project since most of the gonads were samples in the western Indian Ocean. We recommend that additional gonad samples are collected and analysed from all regions of the Indian Ocean, but particularly from the northern and eastern areas (from all size classes and months) to improve the reproductive parameters obtained in this project. Fish >30 cm FL (~minimum size at maturity) are particularly important to increase the sample size available for maturity, fecundity and spawning fraction analyses. Monthly sampling is important in reproductive studies to obtain reproductive data throughout the year. Continuing to collect and analyse gonads over time will be particularly important for assessing inter-annual variation in reproductive parameters.
