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This paper describes preliminary work to estimate reproductive parameters for bigeye tuna (Thunnus obesus) in the Indian Ocean as part of the ‘GERUNDIO’ project1. The 2019 stock assessment for bigeye tuna in the Indian Ocean (IOTC) indicated that the stock is not considered to be overfished but is subject to overfishing (Fu et al. 2019; IOTC 2021). The assessment model used a maturity ogive equivalent to that used by Shono et al. (2009), for which length at 50% maturity was 110.9 cm fork length (FL) and full maturity was around 125 cm FL. The source of this maturity ogive is unclear. The aim of the current project was to produce updated estimates of key biological reproductive parameters for bigeye tuna in the Indian Ocean, including informaion on length at maturity.
A total of 162 bigeye tuna were sampled in the project (101 females and 61 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 small number of ovary samples (i.e., females only), which were read using an agreed classification system. Additional ovaries collected in the current project will be processed soon to update the current analysis.
Data from an additional 485 bigeye tuna (230 females and 255 males) were obtained from previous projects (EMOTION database, see Bodin et al. 2018), which included histological data from 158 females classified using a similar classification scheme to that agreed in the project. The individuals were collected between 2009-2019 and were also predominantly from the western Indian Ocean.
Here, preliminary estimates of sex ratio, spawning periodicity, batch fecundity and length at maturity are provided for bigeye tuna, mainly from the west region of the Indian Ocean. Further work is required to finalize these 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.
The shape of the preliminary maturity ogive (i.e., the proportion of mature individuals at age or length) obtained in this study is very different to the ogive currently used in the stock assessment, although the estimates of length at 50% maturity are similar (112.7 cm versus 110.9 cm FL). The proportion mature at length does not reach 100% as expected in the larger length classes and requires further investigation. There was insufficient data to examine region-specific reproductive parameters in this project since most of the gonads were sampled 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 >60 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. We also recommend collecting additional gonad samples from different fishing gears (e.g., longline) to improve the size coverage and have better representation of the population spatial range. Continuing to collect and analyse gonads over time will be particularly important for assessing inter-annual variation in reproductive parameters
