Genetic Population Structure of the Recently Validated Billfish, the Roundscale Spearfish, and Its Genetic Diversity and Demographic History Compared to Its Istiophorid Congener, the White Marlin

The recent validation of the roundscale spearfish (Tetrapturus georgii) as a distinct species within the Atlantic, coupled with the facts that it is sympatric with and strongly resembles the overfished white marlin, has clouded management and conservation efforts for both species. It is now realized that historical misidentifications of roundscale spearfish as white marlin have occurred and have therefore complicated management efforts for the latter, since stock assessments have unknowingly been conducted on a species complex. Furthermore, the population status of roundscale spearfish is largely unknown. To aid management of both billfish species, we delineated the genetic population structure of roundscale spearfish in the western Atlantic and assessed its genetic diversity and demographic history relative to the white marlin. We genotyped 203 roundscale spearfish collected from the Northwest (NWA) and Southwest Atlantic (SWA) (n = 144, and n = 59, respectively) at 11 microsatellite loci, and sequenced a 585 bp segment of the mitochondrial control region (mtCR) for a subset (n = 83) of these individuals (NWA n = 42, SWA n = 41). The combined marker sets provided mixed evidence for population differentiation in the western Atlantic. Microsatellite population-level comparisons demonstrated some evidence of genetic differentiation among individuals from the NWA and SWA (F_ST = 0.004; P = 0.05); however, individual-based, multi-locus cluster analyses grouped these individuals into a single genetic unit. Analysis of mtCR data also showed mixed evidence of differentiation (Ô_ST = 0.0037, P = 0.284; S_nn = 0.618; P = 0.015). Demographic analyses using roundscale spearfish mtCR data and a previously published white marlin mtCR dataset demonstrated substantial historical population growth for both species. Estimates of genetic diversity (Ɵ) appeared to be higher for the white marlin relative to the roundscale spearfish [0.758 (0.532-1.197) and 0.433 (0.295-0.730), respectively], although there was some overlap in confidence intervals. The estimate of lower roundscale spearfish effective female population size, reflected by Ɵ, is consistent with a smaller roundscale population size relative to white marlin as evidenced by fishery observer catch data in the NWA. The mixed signal of population genetic differentiation of roundscale spearfish coupled with a seemingly smaller population size compared to the already depleted white marlin stock, suggests that in the absence of clear contradictory evidence, a two-stock (NWA vs SWA) approach may be the prudent precautionary management strategy for this newly recognized billfish.