Connectivity and population structure of albacore tuna across southeast Atlantic and southwest Indian Oceans inferred from multidisciplinary methodology

Natacha Nikolic,Maxime Lalire,Nathalie Bodin,Wendy West,Jérôme Bourjea,Emmanuel Corse,Stéphanie Hollanda,Sven Kerwath,Sophie Arnaud-Haond,Sylvain Bonhommeau,Philippe Gaspar,Alexis Puech,Iratxe Montes

doi:10.1038/s41598-020-72369-w

Abstract

Albacore tuna (Thunnus alalunga) is an important target of tuna fisheries in the Atlantic and Indian Oceans. The commercial catch of albacore is the highest globally among all temperate tuna species, contributing around 6% in weight to global tuna catches over the last decade. The accurate assessment and management of this heavily exploited resource requires a robust understanding of the species’ biology and of the pattern of connectivity among oceanic regions, yet Indian Ocean albacore population dynamics remain poorly understood and its level of connectivity with the Atlantic Ocean population is uncertain. We analysed morphometrics and genetics of albacore (n = 1,874) in the southwest Indian (SWIO) and southeast Atlantic (SEAO) Oceans to investigate the connectivity and population structure. Furthermore, we examined the species’ dispersal potential by modelling particle drift through major oceanographic features. Males appear larger than females, except in South African waters, yet the length–weight relationship only showed significant male–female difference in one region (east of Madagascar and Reunion waters). The present study produced a genetic differentiation between the southeast Atlantic and southwest Indian Oceans, supporting their demographic independence. The particle drift models suggested dispersal potential of early life stages from SWIO to SEAO and adult or sub-adult migration from SEAO to SWIO.

Highlights

Albacore tuna (Thunnus alalunga) is an important target of tuna fisheries in the Atlantic and Indian Oceans
The results presented here indicate small but significant genetic and morphometric differences between the southeast Atlantic and southwest Indian Oceans, suggesting the existence of two independent populations, and identifying a spawning ground in South Western Indian Ocean waters
We consider these results in relation to simulations of larval dispersal through the main currents, and previous data on other species in the same geographic region, before discussing their implications under the current delineation of management units

Summary

Introduction

Albacore tuna (Thunnus alalunga) is an important target of tuna fisheries in the Atlantic and Indian Oceans. Regional Fisheries Management Organizations (RFMOs) currently manage albacore based on six hypothetical populations or stocks (Mediterranean, North Atlantic, South Atlantic, Indian, North Pacific, and South Pacific) The definition of these populations or stocks have been supported by some genetic studies[10,11,12,13,14,15] but the definition remains controversial, within an ocean (e.g. between albacore populations of northern and southern hemispheres). The scarce information on the population structure of albacore in the South Atlantic and Indian Oceans warrants further analyses of the population structure and connectivity of this species in these areas, in order to test Morita’s hypothesis of a significant exchange

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Conclusion