This article is a part of WIOMSAs video series “Voices of MARG Grantees”.
Marine Research Grant I (MARG I) is a capacity building programme of WIOMSA that targets young and upcoming scientists with the aim of providing them with opportunities to undertake independent studies. Some of this research has led to publications in international peer reviewed journals and also produced innovative solutions to address issues at the local level. “Voices of MARG Grantees” is a series of videos that showcases some outstanding MARG I case studies that WIOMSA has funded over the years. We will feature 4 case studies in the video series. This time we feature Zahirah Dhurmeea.
Study generates new information about albacore tuna
A WIOMSA-supported study has shed new light on the reproductive biology of albacore tuna, Thunnus alalunga, a species that is an important food and economic resource for a number of countries in the western Indian Ocean.
The study “Reproductive biology of albacore tuna (Thunnus alalunga) in the Western Indian Ocean”, was published in the open access journal Plos One in 2016. It was followed by a second paper “Lipid and fatty acid dynamics in mature female albacore (Thunnus alalunga) in the Western Indian Ocean,” which was published in 2018. Both studies form part of the PhD thesis of Zahirah Dhurmeea, a Scientific Officer at the Ministry of Fisheries in Mauritius.
Dhurmeea, was awarded her PhD from the University of Mauritius in November 2020. Her PhD study is entitled “Energy allocation, trophic ecology and reproductive biology of albacore tuna (Thunnus alalunga) in the western Indian Ocean.”
Globally, albacore is one of the most important commercial tuna species representing around 6% of the total catch. The fisheries scientist says that, in the Indian Ocean, albacore tuna have not been thoroughly studied in comparison to other tuna species, in spite of the fact that albacore is one of the most important species of tuna in the region. In the western Indian Ocean, albacore are targeted by longline and pole fishing boats from Reunion Island and South Africa, while island states such as Dhurmeea’s home country, Mauritius, allocate fishing licenses to distant water fishing fleets, most of which target albacore tuna using longlines. Artisanal fishers from Mauritius also target albacore, using fish aggregating devices which are maintained along the outer-reefs of Mauritius and Rodrigues island by the Ministry of Fisheries.
“We didn’t have a lot of (biological) information on this fish,” says Dhurmeea, “so in 2014 I took a three year leave without pay from the Ministry so that I could undertake a study of albacore tuna on a full-time basis at the University of Mauritius.” Her PhD study was undertaken in collaboration with the Commonwealth Scientific and Industrial Research Organisation and the French Institute of Research for Development (IRD). The study was funded by the IRD and WIOMSA.
The study of the reproductive biology of albacore tuna was the first step and formed the basis of her PhD. It provided core information about the sex ratio, spawning season, length-at-maturity, spawning frequency and fecundity for a sample of 923 female and 867 male albacore. Laboratory analysis of the samples was conducted in the Seychelles, with the assistance of the Seychelles Fishing Authority.
“During three months in Seychelles, I collected samples from a cannery and I did the laboratory work, including the histological analysis,” explained Dhurmeea. Additional samples were collected in Mauritius, Reunion and South Africa. “Seychelles was the place where we had all the samples from the different fisheries converging so that we could analyse them.”
Dhurmeea received training to process the samples, help her understand and interpret the hundreds of samples she looked at under the microscope, with each slide revealing the stage of reproduction of the sampled tuna. This information was recorded and later included in a large database containing environmental and biological data, as well as fishing information. The data gathered was then analysed statistically.
Dhurmeea was also trained to conduct a fecundity analysis of spawning tuna, a rather tedious and time-consuming process that involves counting fish eggs under a stereomicroscope.
“This was the most tedious part of my study,” she recalls, “to get the data it was very intensive, you had to process hundreds of ovaries and look at them under the microscope.”
The use of histology over macroscopic observation allows a highly accurate classification for ovarian development in a species like albacore tuna.
Dhurmeea’s histological and gonad analyses revealed new information about the reproductive development and classification of albacore in the western Indian Ocean. Specifically, sex ratio analysis showed a predominance of males in the larger length classes, as has been observed for albacore in the Pacific and Atlantic oceans, as well as for other species of tuna.
Histological analyses revealed that the latitudes south of 30°S are inhabited mainly by immature fish, and that spawning takes place between 10°S and 30°S, mainly to the east of Madagascar from October to January, although spawning activity was also observed on the western side of Madagascar in the Mozambique Channel. The spawning period corresponded to sea surface temperatures above 24 °C, as reported from other ocean regions and other species of tuna. In the western Indian Ocean, albacore spawn on average every 2.2 days within the spawning region and spawning months. Importantly, large females contribute more to reproduction because they spawn for a longer period than smaller-sized fish.
The new information about the reproductive biology of albacore tuna in the western Indian Ocean will improve future stock assessments for the species. For instance, stock assessments can take into account the bias in sex ratio towards males in the larger size classes, and the importance of large females.
In 2016, the results of Dhurmeea’s study were presented to the Working Party on Temperate Tunas of the Indian Ocean Tuna Commission (IOTC) – the regional fisheries management organisation that is responsible for the management of tuna and tuna-like species in the Indian Ocean. She also received a WIOMSA Travel Grant which enabled her to present the results of the reproductive study, plus a follow-up study on lipids, at the WIOMSA Symposium in 2017.
Newly graduated with a PhD, Dhurmeea looks forward to using the skills she has gained to study and assess other fish species that are also important to Mauritius.
“I would have liked to have continued to learn more about albacore tuna,” says Dhurmeea, “but I can use my knowledge to study other species in Mauritius that are also important, not necessarily tunas, but fish from the banks fishery or the demersal fishery. You can assess those species in the same way.”