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Two surveys using a combination of midwater trawl and acoustics were undertaken in October 2014 and June 2015 to investigate species composition and biomass density of mesopelagic nekton over the continental slope of the South China Sea (CSSCS). In two surveys, diversity and dominant species of nektons (Chondrichthyes, Osteichthyes and Cephalopoda) showed obviously diel and depth–dependent difference. In 2014, in general species richness and diversity of nektons in the 0–100 m at night were greater than that in the 350–650 m during the day and 350–650 m at night. In Jun. 2015, species richness and diversity of nektons in the 350–650 m during the day were much greater than that in the 350–650 m at night and 0–100 m at night. Myctophidae and Diaphus are the most common fishes in the CSSCS mesopelagic zone. Jellyfishes could cause the nautical area scattering coefficient (NASC) to increase, but the effect of jellyfishes on NASC could vary over the light–night cycle and trawl depth. Catch rates of different nektonic categories varied by time of day and depth. Night–time catch rate at trawl depths

Mesopelagic fishes are major components contributing to deep-sea scattering layers in the open ocean. They have the potential to support a substantial higher trophic level harvest, but knowledge about this possible resource is still rudimentary. Some of the mesopelagic fishes undertake diel vertical migration (DVM) over large depths by feeding in shallow water during darkness and migrating to depth to avoid visual predators during daylight. The vertical distribution and diel migration pattern of the mesopelagic fishes on the northern slope of the South China Sea were investigated during two multidisciplinary surveys carried out in October 2014 and June 2015. Based on acoustic and biological sampling, a distinct deep scattering layer (DSL) consisting mainly of mesopelagic fishes was identified in deep water (>300 m). Extensive diel vertical migration for hundreds of meters to form dense aggregations in the epipelagic zone (0–150 m) at night was observed in both seasons, but the migration was highly species dependent. While most of the Myctophiformes and the Perciformes undertook DVM, most of the Salmoniformes and Beryciformes remained in deep water both day and night. Based on paired day-night differences of acoustic density in the DSL, the migrating proportion of mesopelagic backscatter varied markedly in different locations, ranging from 28% to 63%, with a mean of 48% in two seasons. The extensive vertical migration occurred in separate groups at different periods during sunset and sunrise hours. Additional analysis showed that the center of gravity of the mesopelagic backscatters (200–1000 m) and their night distribution in the subsurface layer (10–150 m) were positively correlated with the mean temperature in respective zones. Moreover, some of the mesopelagic communities on the slope were found to undertake pronounced diel horizontal migration in a pattern of towards the shore in the evening and away from the shore after midnight. The horizontal migrant species were mainly Myctophidae, in particular Diaphus chrysorhynchus and Diaphus garmani. Simultaneous analysis on typical tidal currents indicated that the horizontal migration, over a large distance with high speed and biomass, could not be explained by passive transport due to tidal currents alone, and the active swimming of the mesopelagic fishes may also play an important role.

To estimate the age and growth of a dominant species of lanternfish, Myctophum asperum, in the South China Sea, 399 fish were collected by a midwater trawl in spring and autumn, 2014–2016. Age was estimated using counts of daily growth increments laid down across the sagittal section of otoliths (n = 52 fish). Three regions of deposition in the otolith microstructure were clearly evident: the larval, post-larval and post-metamorphic zones. Increments in the larval zone (average of 30) and the post-metamorphic zone appeared brighter under a light microscope than the increments in the post-larval zone. The age of the sampled fish ranged between 183 and 405 days. The von Bertalanffy growth model provided the best fit for the post-metamorphosis fish and was expressed as: SL = 85.3[1-exp{− 0.008(t − 28.1)}]. The maximum rate of growth occurred at 183 days after metamorphosis and 58 mm SL among the age determined samples, and the growth rate decreased with age. The length-weight relationship for the entire sample of fish was W = 0.000037234 SL2.7935, whereas for females and males it resulted Wf = 0.000050140 SL2.7255 and Wm = 0.000035459 SL2.8022, respectively. Fish reached the adult size by 74 mm SL and 5–8 g, and this size group was numerically the most and constituted the greatest portion of the biomass.

The Pleistocene climate oscillations have been proven to have profound influences in the evolutionary processes of marine species and left significantly tractable footprints in genomes of marine fauna. However, such history remains unknown for the Japanese grenadier anchovy Coilia nasus, a relatively large fish found in marine, freshwater and brackish water. In order to provide insight into the phylogeographic pattern and historical demography of C. nasus, we employed the mtDNA D-loop and Cyt b as molecular markers to estimate the genetic variation among populations of C. nasus. We took samples from 18 populations throughout the distribution of C. nasus. Partial sequences of mtDNA D-loop (n = 424) and Cyt b (n = 129) were obtained. Four phylogeographic lineages were revealed by the mtDNA D-loop analysis. Frequency changes of the four lineages throughout the species’ distribution indicated limited genetic exchange among three major geographic regions. Patterns of isolation-by-distance were detected among the populations. Neutrality test, mismatch distribution, and network analyses consistently showed that all the four lineages had populations with relatively stable over time and then subjected to sudden expansion during the late-Pleistocene era (c. 20 000 – 75 000 years ago). The phylogeographic and demographic history of C. nasus was highly influenced by climatic fluctuations in the Pleistocene. Geographic isolation resulted from Pleistocene climatic fluctuations might have profound impacts on the distribution pattern of genetic variation of C. nasus. Our result also indicated that the population expansion event for C. nasus occured along with habitat continuous expansion during the inter-glacial period.

Seventy-one Sardinella zunasi individuals collected from 5 locations in the Northwest Pacific were used in an AFLP analysis to examine the population genetic structure of S. zunasi . Using 4 selective primer combinations, 414 putative loci were detected, including 340 polymorphic loci (82.13%). Both AMOVA and pairwise F ST analysis detected significant population genetic structure between Chinese and Japanese populations, supporting separate stocks in this species. The UPGMA trees also detected significant spatial structure within S. zunasi . Pleistocene glaciations are hypothesized to be responsible for the genetic differentiation. The significant genetic differentiation found among S. zunasi populations might also be due to geographic isolation, because a pattern of isolation-by-distance was observed. The weaker but significant structures between the Japanese Aichi and Kagawa populations might be impacted by the semi-enclosed coastal topography there. No significant genetic structure was detected among the Chinese populations, perhaps because of S. zunasi ’s migratory behavior and ocean current patterns.

Knowledge of population genetic structure plays an important role in fisheries management. In this research, Inter-Simple-Sequence-Repeat (ISSR) markers were employed to evaluate the genetic structure of Japanese sardinella (Sardinella zunasi) populations in the Northwest Pacific. Eighty seven individuals from 5 locations were screened using 4 highly polymorphic primers. A total of 173 polymorphic loci were detected out of 191 loci amplified. Small but significant genetic differentiation was detected between the Chinese and Japanese populations by both AMOVA and pairwise F ST analyses, which was further supported by cluster analysis. We consider that climate change during glaciations should be responsible for the genetic differentiation. Isolation by geographic distance among populations was observed, indicating that the distance might also lead to the genetic differentiation. However, no genetic structure was found within the populations off both the Chinese and Japanese coasts, indicating a high-level along-coast gene flow, which might result from ocean current transport and common ground for over-wintering.

The gizzard shad (Konosirus punctatus) is a common pelagic fish in the East China Sea. To evaluate the influence of Kuroshio Current in shaping the genetic structure of marine species, 10 populations of K. punctatus from Chinese and Japanese coastal waters were collected for analysis based on the mitochondrial DNA marker. All the Japanese populations exhibited higher nucleotide diversity than did Chinese populations. Two distinct clades were identified by the neighbour-joining tree based on haplotypes of Cyt b and the control region. Both AMOVA and pairwise Fst strongly supported the significant genetic divergence between Chinese and Japanese clades, which suggested strictly limited gene exchange. The mismatch distribution of Chinese clade and Japanese Clade B appeared to be unimodal, and Tajima’s D and Fu’s Fs statistics were significantly negative, and, therefore, the hypothesis of selective neutrality was rejected. The results indicated that the Kuroshio Current may not be the strong barrier for the dispersal of K. punctatus in the East China Sea. The climate of Pleistocene periods had played an important role in phylogeographical patterns of K. punctatus and the dispersal strategy of coastal species may be the major current physical barrier for the gene flow among populations from Chinese and Japanese coastal waters.