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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 skipjack tuna stock of the Indian Ocean is not overfished and is not subject to overfishing (Fu 2020, IOTC 2020). The assessment model used a maturity ogive for the western Indian Ocean from Grande et al. (2010), with length at 50% maturity at 38 cm FL and full maturity at around 42 cm FL. Grande et al. (2010), used the ‘cortical alveolar’ oocyte development stage and above as the threshold indicating a fish was mature. The aim 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) ‘advanced vitellogenic’ oocyte stage as the maturity threshold, which is a more reliable threshold for determining a female is mature and contributing to egg production (Schaefer 2001).
A total of 365 skipjack tuna were sampled in the 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 of 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 projects (862 females and 649 males) were obtained from previous (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 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 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 shape of the preliminary maturity ogive obtained in this study is similar to the ogive obtained by Grande et al. (2010). The estimate of length at 50% maturity was slightly higher than estimated by Grande et al. (2010) (39.3 cm versus 37.8 cm FL), which is expected given we used the slightly (older) ‘advanced vitellogenic’ oocyte stage as the 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.
