The Indian Ocean tuna fishery is based on three species whose populations have been under increasing pressure for 30 years. In order to ensure that this fishery remains sustainable, scientists - including those of the UMR MARBEC - have evaluated the current knowledge on these species to identify points for improvement.

Skipjack, bigeye tuna and yellowfin tuna... these three species of tropical tuna together represent 95% of the world's tuna catches, hence the importance of managing this resource sustainably.

Distribution potentielle des trois espèces de thons tropicaux dans l’océan mondial, sur la base de critères d’habitat

© IRD - Artetxe-Arrate et al., 2022

Fish of high market value

Among the fish consumed by humans, tuna is king! The Japanese won't tell you otherwise: a 211 kg tuna was sold for 129,000 euros at the traditional New Year's Eve sale in Tokyo... The Atlantic bluefin tuna (Thunnus thynnus) is the most prized for sushi, but three other species – Katsuwonus pelamis, Thunnus albacares, Thunnus obesus?Skipjack (or bonito), yellowfin tuna, bigeye tuna – Three other species make up most of the world's tonnage. They mainly end up in cans or in raw specialties that have become very trendy. The Indian Ocean fisheries are based on these three species, which are also present in the Pacific and Atlantic oceans and are traded internationally. Tuna fishing provides food, employment and economic resources to the populations that practice it, so it is essential that this activity is sustainable. The growing global demand is putting pressure on natural tuna populations, whose stocks are declining. « Bluefin tuna in the Pacific and Southern ocean is at 20% of its 1950s biomass, and yellowfin tuna in the Indian ocean is at around 30%, levels that call for aggressive measures to restore these stocks, says Francis Marsac, a fisheries researcher at UMR MARBEC and co-author of the summary article published in Advances in Marine Biology. To ensure better management, we need to learn more about the biology, ecology and distribution of these species. ».

Etagement vertical des niches trophiques des thons adultes dans l’océan Indien

© IRD - Artetxe-Arrate et al., 2022

Essential parameters for a reliable valuation of stocks

Evaluation of the available literature regarding the three species reveals gaps that will need to be filled in order to elaborate on stock assessment models with reliable data, that are as complete as possible. Important parameters for effective modeling include those related to life traits?Life span, sex ratio, sexual maturity, natural mortality rate, etc., i.e. biological and behavioral indicators. « We lack data on growth, reproduction periods and sites, vertical movements, trophic relationships », lists Iraide Artetxe-Arrate, first author and researcher in the Fisheries department of AZTI. In addition, studies on life history traits are often limited to a few areas, whereas they should be extended to the entire Indian ocean, to cover all the interactions between the environment and these species. All these parameters must be taken into account for a fair assessment of the vulnerability of the three species according to the types of fishing gears used (longline, seine, fish aggregating device).

Pêche industrielle et artisanale, Seychelles

© IRD - Thibaut Vergoz

Recommendations for sustainable fisheries

Based on this assessment, the authors of this review article make recommendations to the Indian Ocean Tuna Commission (IOTC). The international organization, which is based in the Seychelles, is in charge of the management of tuna resources in the Indian ocean. It would be wise to improve the monitoring capacities of the countries in the area, in order to increase the collection of statistical and biological data on the three target species and to obtain a more accurate assessment of their dynamics and resilience with respect to climate impact or human activities. This improved knowledge of the state of the tuna populations would legitimize the measures recommended by the IOTC, for example the limitation of the annual fishing quotas. In terms of scientific methods, the authors believe that combining different approaches – genetic markers?Short segments of DNA that help to discriminate between subpopulations within a stock, otolith analysis?mineral concretion in the inner ear, pest inventory, marking – with basic fisheries information will provide better indicators for sustainable stock management.

Publication : Artetxe-Arrate I., Fraile I., Marsac F., Farley J. H., Rodriguez-Ezpeleta N., Davies C. R., Clear N. P., Grewe P., Murua H. 2021. A review of the fisheries, life history and stock structure of tropical tuna (skipjack Katsuwonus pelamis, yellowfin Thunnus albacares and bigeye Thunnus obesus) in the Indian Ocean. Advances in Marine Biology, 88, 39-89.

Contacts science : Francis Marsac, IRD, MARBEC

Iraide Artetxe-Arrate, AZTI

Contacts communication : Fabienne Doumenge, Julie Sansoulet