LARGE PELAGIC FISHES

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Large pelagic fishes such as tunas, marlins, swordfish, and some sharks are predatory species inhabiting the world's open oceans. These fishes are broadly distributed and capable of making long distance movements spanning hundreds to thousands of miles. Many large pelagic fishes are commercially or recreationally important in regions across the globe; however,  limited information on species biology challenges the management of fisheries that interact with these species. 

My research on large pelagic fishes focuses on the application of genetic tools to explore spatial and temporal patterns of genetic variation among populations and species. This work provides insights into broad-scale genetic connectivity, barriers to gene flow, and regional adaptation in the marine environment, as well as practical information on stock structure and genetic diversity important for ensuring the short- and long-term persistence of populations. Research questions representative of this work are highlighted below.

Do large pelagic fishes characterized by broad spatial distributions and high dispersal capabilities display population structure? Across what spatial and temporal scales? 

My work on striped marlin (Kajikia audax) demonstrates the presence of five genetically distinct populations spanning the Pacific and Indian oceans (Figure 1). This pattern of population structure has persisted over at least four generations of striped marlin. In comparison, genetically distinct populations appear to be lacking for white marlin (K. albida), the closely related sister species of striped marlin found in the Atlantic Ocean.

Information on the presence of genetically distinct populations, and the degree of genetic connectivity among populations, will help regional fisheries management organizations in the Atlantic, Pacific, and Indian oceans develop scientifically-informed, population-specific management plans for white marlin and striped marlin. This is especially important for improving the effectiveness of recovery efforts for these overfished species.

Figure 2. STRUCTURE analysis of 14 microsatellite markers surveyed across roundscale spearfish (Tetrapturus georgii; RSS), longbill spearfish (T. pfluegeri; LBS), Mediterranean spearfish (T. belone; MSF), and shortbill spearfish (T. angustirostris; SBS). Each vertical bar represents an individual, and bars are colored according to genetic ancestry. A: analyses performed with all species included. B: analyses performed with RSS excluded, showing greater shared ancestry of LBS with SBS. Figure from McDowell et al. 2018.

Figure 1. A: Results from principal component analysis (PCA) of nearly 4,000 single nucleotide polymorphisms surveyed across striped marlin (Kajikia audax) sampled from the Pacific and Indian oceans (Mamoozadeh et al. In prep.). B: Results from PCA of 24 microsatellites surveyed across white marlin (K. albida) sampled from the Atlantic Ocean (Mamoozadeh et al. 2017). Each point represents an individual, and individuals are colored according to sampling location.

Can highly polymorphic molecular markers resolve evolutionary relationships among closely related and morphologically similar species?

Though four species of spearfish (genus Tetrapturus) are currently recognized, the validity of these species has been uncertain for several decades. This is primarily because spearfishes are infrequently observed, display similar morphological features, and previous genetic studies have not resolved each species as genetically distinct. My work on spearfishes employs traditional (e.g. microsatellites, mtDNA) and genomic (e.g. single nucleotide polymorphisms) molecular markers to resolve species boundaries within Tetrapturus.

So far, results from this work are consistent with the identification of roundscale spearfish (Tetrapturus georgii) and Mediterranean spearfish (T. belone) as distinct species. The genetic distinctiveness of longbill spearfish (T. pfluegeri) and shortbill spearfish (T. angustirostris) is less clear (Figure 2), potentially due to a comparatively more recent common ancestor. Information on species relationships helps to inform our understanding of speciation in the pelagic environment, and enables the conservation of genetically distinct evolutionary lineages.