Analyses of Nuclear ldhA Gene and mtDNA Control Region Sequences of Atlantic Northern Bluefin Tuna Populations

Abstract

There has been considerable debate about whether the Atlantic northern bluefin tuna exist as a single panmictic unit. We have addressed this issue by examining both mitochondrial DNA control region nucleotide sequences and nuclear gene ldhA allele frequencies in replicate size or year class samples of northern bluefin tuna from the Mediterranean Sea and the northwestern Atlantic Ocean. Pairwise comparisons of multiple year class samples from the 2 regions provided no evidence for population subdivision. Similarly, analyses of molecular variance of both mitochondrial and ldhA data revealed no significant differences among or between samples from the 2 regions. These results demonstrate the importance of analyzing multiple year classes and large sample sizes to obtain accurate estimates when using allele frequencies to characterize a population. It is important to note that the absence of genetic evidence for population substructure does not unilaterally constitute evidence of a single panmictic population, as genetic differentiation can be prevented by large population sizes and by migration.

Facts and uncertainties about the genetic population structure of Atlantic bluefin tuna (Thunnus thynnus) in the Mediterranean. Implications for fishery management

“…On the contrary, the lack of genetic population differentiation found within the Mediterranean Sea in several studies (this study, Alvarado Bremer et al 2005b;Ely et al 2002;Pujolar et al 2003) can be explained by several factors including sampling design and the type of genetic analysis, but also by the nature and behaviour of the Atlantic bluefin tuna. For instance, Ely et al (2002) only examined temporal samples.…”

“…On the contrary, other genetic studies conform to the alternative hypothesis of a single panmictic unit of Atlantic bluefin tuna throughout the Mediterranean. Ely et al (2002) failed to find genetic differences between several temporal Mediterranean samples. This initial result of lack of differentiation was further supported by a study of nuclear allozymes by Pujolar et al (2003).…”

“…For instance, Ely et al (2002) only examined temporal samples. Pujolar et al (2003) and Alvarado Bremer et al (2005a, b) analyzed a combination of adult and young individuals using either nuclear allozymes or a short segment of the mtDNA CR respectively, which in both cases might not have the resolution to observe genetic differentiation for such closely related populations.…”

“…Furthermore, we compared the obtained mtDNA CR sequence with the previously published sequences reported in the studies by Ely et al (2002) and Alvarado Bremer et al (2005b). This complete data set represents a total of 516 characterized individuals distributed along the Mediterranean and over a range of twenty years from 1990 to 2010.…”

Population structure of istiophorid billfishes

“…While conventional tagging demonstrated some trans-Atlantic movements of bluefin, these were believed to occur at a low frequency. Early genetic studies of eastern and western bluefin typically included juveniles and adults of several age classes, and were unable to discriminate between the putative stocks (Edmunds and Sammons, 1973;Thompson and Contin, 1979;Broughton and Gold, 1997;Takagi et al, 1999;Ely et al, 2002;Pujolar et al, 2003). Subsequently, results from electronic tagging studies (Block et al, 2001(Block et al, , 2005Wilson et al, 2005), otolith stable isotope analyses (Rooker et al, 2008), and investigations of tissue organochlorine pollutant ratios (Dickhut et al, 2009) provided new perspectives on movements of bluefin tuna between the western and eastern North Atlantic.…”