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This paper describes preliminary work to estimate reproductive parameters for yellowfin tuna (Thunnus albacares) in the Indian Ocean as part of the ‘GERUNDIO’ project1. The 2018 stock assessment for yellowfin tuna in the Indian Ocean (IOTC) indicated that the stock is overfished and subject to overfishing (Fu et al. 2018; IOTC 2020). The assessment base case model used a maturity ogive for the western Indian Ocean from Zudaire et al. (2013), which used the ‘cortical alveolar’ oocyte development stage and above as the threshold indicating a fish was mature. Zudaire et al. (2013) also estimated a maturity ogive 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). The two length based ogives from Zudaire et al. (2013) were converted to age based ogives in Langley (2015) and age at 50% maturity was estimated to be ~9 quarters and ~12 quarters, respectively. Sensitivity analyses conducted in Langley (2015) using the alternate maturity ogive gave a slightly more pessimistic outlook for the status of the stock relative to the base case model.
The aim of the current study was to: (i) identify and compile gonad samples, histological sections or histological data from previous studies in the Indian Ocean; (ii) design a gonad sampling plan to collect representative samples across the Indian Ocean, particularly where gaps currently exist; (iii) develop and apply a standardized histological based classification method to all ovary samples; (iv) provide training on sample collection and reproductive analysis; and (iv) produce updated estimates of key biological reproductive parameters.
A total of 1145 yellowfin tuna were sampled in the project (476 females and 669 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 for 212 ovary samples (i.e., females only), which were read by project partners using an agreed classification system after receiving training at an online workshop in July 2021. Additional ovaries collected in the current project will be processed soon to update the current analysis.
Data from an additional 921 yellowfin tuna (476 females and 445 males) were obtained from previous projects (EMOTION database, see Bodin et al. 2018), which included histological data from 388 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.
Preliminary estimates of sex ratio, spawning periodicity, batch fecundity and length at maturity are provided for yellowfin tuna predominantly from 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. 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. Genetic studies indicate that the population structure of yellowfin tuna within the Indian Ocean is complex (Kunal et al. 2013; Barth et al. 2017), and a recent study found evidence for genetic structure north and south of the equator (Grewe et al. 2020).
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 fork length (~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.
