Date Awarded

Summer 2018

Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


John E. Graves

Committee Member

Michael A. Banks

Committee Member

Bongkeun Song

Committee Member

Bruce B. Collette


The istiophorid billfishes (marlins, spearfishes, and sailfish) are highly migratory pelagic fishes exhibiting broad and continuous spatial distributions in the Atlantic, Pacific, and Indian oceans. These species are targeted by a number of recreational, commercial, artisanal, and subsistence fisheries worldwide, and are also caught as bycatch in pelagic longline fisheries targeting tunas and swordfish. Though stock assessments have not been conducted for all istiophorids, assessments available for some species indicate that many istiophorid stocks are overfished and/or experiencing overfishing. However, the development of stock-specific recovery efforts is often impeded by a lack of information on basic species biology, including stock structure. The species status of some istiophorids is also uncertain, further complicating management efforts as well as strategies to conserve genetic diversity characteristic of distinct evolutionary lineages. In this dissertation, a molecular approach is used to address questions currently contributing uncertainty to the conservation and management of two istiophorid billfishes, white marlin (Kajikia albida) and striped marlin (K. audax). These closely related sister species are distributed in the Atlantic and Indo-Pacific oceans, respectively. Previous assessment of genetic population structure for white marlin based on mitochondrial (mt) DNA and five nuclear microsatellite markers suggested the possibility of population structuring for this species; however, results from the evaluation of mtDNA and 24 microsatellites across a larger number of samples, including a collection of larvae, are consistent with the presence of a single genetic stock (Chapter II). This result highlights the importance of analyses based on large numbers of molecular markers and samples, as well as a biologically informed sampling design, for studies of population structure in highly migratory pelagic species. Compared to the apparent lack of genetic population structure for white marlin, analysis of nearly 4,000 single nucleotide polymorphism (SNP) molecular markers across collections of striped marlin from the Pacific and, for the first time, Indian oceans resolved multiple genetically distinct populations (Chapter III). These populations correspond with striped marlin sampled from the western Indian Ocean, Oceania, North Pacific Ocean, and eastern central Pacific Ocean. Results from individual-based cluster analyses also suggest the presence of a second genetically distinct population in the North Pacific Ocean. Comparisons of replicate sample collections for some regions demonstrate the stability of allele frequencies across multiple generations. Finally, the uncertain species status of striped marlin and white marlin was evaluated using over 12,000 genome-wide SNPs surveyed across large numbers of exemplars per species (white marlin: n = 75, striped marlin: n = 250; Chapter IV). Results from individual-based cluster and maximum likelihood phylogenetic analyses suggest the presence of distinct evolutionary lineages for striped marlin and white marlin. This result is consistent with levels of genetic differentiation between striped marlin and white marlin which are an order of magnitude higher than those calculated between populations of striped marlin. Collectively, results of this dissertation provide practical insights for improving the conservation and management of white marlin and striped marlin, including revised stock structures which should be recognized in assessment and management plans for striped marlin. Future genomic studies should focus on addressing uncertainties regarding rangewide stock structure and species relationships for other istiophorids. Additionally, studies which continue to improve the genomic resources available for istiophorid billfishes and other large pelagic fishes may ultimately facilitate the evaluation of questions previously unexplored for the pelagic marine environment, such as localized adaptation and speciation.



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