Genetic connectivity between Atlantic bluefin tuna larvae spawned in the Gulf of Mexico and in the Mediterranean Sea.

Carolina Johnstone,Diego Lozano-Peral,Francisco Alemany,Trika Gerard,Montse Pérez,Alberto García,Raúl Laiz-Carrión,John Lamkin,Estrella Malca,José María Quintanilla

doi:10.7717/peerj.11568

Abstract

The highly migratory Atlantic bluefin tuna (ABFT) is currently managed as two distinct stocks, in accordance with natal homing behavior and population structuring despite the absence of barriers to gene flow. Larval fish are valuable biological material for tuna molecular ecology. However, they have hardly been used to decipher the ABFT population structure, although providing the genetic signal from successful breeders. For the first time, cooperative field collection of tuna larvae during 2014 in the main spawning area for each stock, the Gulf of Mexico (GOM) and the Mediterranean Sea (MED), enabled us to assess the ABFT genetic structure in a precise temporal and spatial frame exclusively through larvae. Partitioning of genetic diversity at nuclear microsatellite loci and in the mitochondrial control region in larvae spawned contemporarily resulted in low significant fixation indices supporting connectivity between spawners in the main reproduction area for each population. No structuring was detected within the GOM after segregating nuclear diversity in larvae spawned in two hydrographically distinct regions, the eastern GOM (eGOM) and the western GOM (wGOM), with the larvae from eGOM being more similar to those collected in the MED than the larvae from wGOM. We performed clustering of genetically characterized ABFT larvae through Bayesian analysis and by Discriminant Analysis of Principal Components (DAPC) supporting the existence of favorable areas for mixing of ABFT spawners from Western and Eastern stocks, leading to gene flow and apparent connectivity between weakly structured populations. Our findings suggest that the eastern GOM is more prone for the mixing of breeders from the two ABFT populations. Conservation of this valuable resource exploited for centuries calls for intensification of tuna ichthyoplankton research and standardization of genetic tools for monitoring population dynamics.

Highlights

Bluefin tuna are epipelagic oceanic species that have been exploited globally for centuries (Muhling et al, 2017)
In the Gulf of Mexico (GOM), two sample subsets were separated east (30 larvae) and west (32 larvae) of 90W according to oceanographic features known to influence Atlantic bluefin tuna (ABFT) catches (Teo & Block, 2010; Muller-Karger et al, 2015), with the eastern GOM (eGOM) dominated by the Loop Current shedding large anti-cyclonic eddies, which generate mesoscale cyclonic and anti-cyclonic eddies that are the key features of the western GOM (wGOM), preferred by breeding ABFT, and influencing larval fish distribution (Lindo-Atichati et al, 2012)
The mean number of larvae genotyped at eight microsatellite loci are shown in Table S2, for each of the two spawning areas (GOM and Mediterranean Sea (MED)), or for the three separate geographical regions

Summary

Introduction

Bluefin tuna are epipelagic oceanic species that have been exploited globally for centuries (Muhling et al, 2017). Despite additional spawning areas for ABFT have been recently discovered in the Slope Sea and the Bay of Biscay (Richardson et al, 2016; Rodriguez, Johnstone & Lozano-Peral, 2021), the GOM and the MED are still considered the main grounds to which the adults return to breed, with regional currents linking larval and juvenile nursery habitats (Muhling et al, 2017). Overexploitation of this valuable fishery resource resulted in sharp decreases in abundance from the 1960s onward.

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