Disentangling a species complex: using genetic tools to guide management of a recently validated billfish, the roundscale spearfish (Tetrapturus georgii)

Guy Harvey Oceanographic Center Facility

Misidentifications between exploited species may lead to inaccuracies in population assessments, with potentially irreversible conservation and managements ramifications if overexploitation of either species is ongoing. A notable showcase of this management challenge is provided by the recent validation of the roundscale spearfish (Tetrapturus georgii), a large pelagic billfish that has been historically misidentified as the morphologically very similar and severely overfished white marlin (Kajikia albida) (IUCN listing: Vulnerable). In light of the roundscale spearfish’s validation, it is now realized that stock assessments for the white marlin have unknowingly been conducted on a species complex, thereby exacerbating management concerns for both species. Thus, to assist in the management of both these species and western Atlantic billfishes in general, we developed a suite of genetic tools and approaches to: (1) rapidly and accurately identify Atlantic billfishes to species, and (2) resolve the roundscale spearfish’s distribution, population status, genetic connectivity, and evolutionary history – and present our findings in a comparative context by re-evaluating existing white marlin DNA sequence data. Our genetic findings confirm a broad western Atlantic distribution for roundscale spearfish, sympatric with white marlin as well as another morphologically similar billfish, the longbill spearfish (Tetrapturus pfluegeri). Across the western Atlantic, we found mixed evidence for roundscale spearfish population differentiation between north and south Atlantic regions using microsatellite and mitochondrial DNA and strong evidence indicating historical population growth. Furthermore, comparative analyses suggest that genetic diversity and female effective population size is 1.5X-1.9X higher for the white marlin – a species already of management concern – than the roundscale spearfish. Together, these findings suggest that the first step towards sustainable management for billfishes, is the accurate identification of distinct ‘management units’ – whether it be at the level of species or population, and that precautionary approaches and global cooperation are required to ensure their persistence.