Figure 3: Coalescent Bayesian skyline plots of Trichiurus lepturus (upper left), T. nanhaiensis (lower left), and the NW Pacific population for the four study species (right).

Abstract

Background Species-specific information on distribution and demographic patterns provides important implications for conservation and fisheries management. However, such information is often lacking for morphologically-similar species, which may lead to biases in the assessments of these species and even decrease effort towards sustainable management. Here, we aimed to uncover the distribution range, population structure and demographic history for four exploited Trichiurus cutlassfishes using genetics. These cutlassfishes contribute substantial global fisheries catch, with a high proportion of catch harvested from the NW Pacific. Methods We chose the widely available mitochondrial 16S ribosomal RNA (16S) as the genetic marker for cutlassfishes. We compiled the 16S sequence data from both the GenBank and a survey of trawler catch samples along the NW Pacific coasts 22–39°N. Genealogical relationships within each species was visualized with haplotype networks and potential population differentiations were further evaluated with AMOVA. Demographic histories were estimated using neutrality test, mismatch analysis, and the Bayesian skyline plot. The reconstructed phylogenetic trees were used to delimit and estimate the divergence time of species and included populations. Results In each of two cosmopolitan species, T. lepturus and T. nanhaiensis, we observed distinct populations along the coasts of warm oceans; such population differentiation might result from historical geographic barriers in the Pleistocene. In the NW Pacific, four Trichiurus species vary in their distribution habitats, which reflect differential ecological niches among these species. The small-sized T. brevis are primarily found in nearshore habitats; the warm-affiliated T. nanhaiensis are present along the path of the Kuroshio Current; the cold-affiliated T. japonicus spatially diverged from the widely-distributed T. lepturus, with the latter mainly occupy in warmer regions. Despite these differences, a single well-mixing fish stock, thus one management unit, was identified in each of the four species, presumably due to expansion of their population sizes predated the Last Glacial Maximum and a lack of distribution barrier. The most dominant T. japonicus, which have at least one magnitude higher effective population size than the others, show a unique abrupt size expansion event at 75 to 50-kilo years ago when the low sea level occurred during the ice age. Main conclusions The demographic history revealed by our genetic analyses advances understanding of the current distribution and population structure for these congeneric species. Moreover, the uncovered population structure provides insight into the assessment and management of these species. Such information complements contemporary knowledge about these species and enables us to forecast their ability to resist future environmental and anthropogenic disturbances.