Seasonal fishing closures are often used in fisheries management to conserve overfished stocks. As one of the unintended consequences, fishermen often contend for maximizing catches immediately after reopening fisheries. The resultant large catch landings in a short time period (i.e., pulse fishing) may undermine the benefit of closure. We implemented an end-to-end model OSMOSE-JZB (Object-oriented Simulator of Marine ecOSystem Exploitation OSMOSE) modelling ecosystem in the Jiaozhou Bay located in China to evaluate the impact of pulse fishing on the effectiveness of seasonal closure at levels of fish community, population, and individual. Our study demonstrated that the three-month closure was successful in conserving fish stocks. There were small variations on ecological indicators (i.e., total biomass of the community, mean trophic level of the community, mean trophic level of the catch, and Shannon-Wiener biodiversity index) when pulse fishing occurred. Pulse fishing seemed not to result in a great shift in community structure. Compared to other species, the biomass of two large predatory fishes were more susceptible to pulse fishing. Pulse fishing could change the pressure of predators to fish stocks via food webs, especially for young individuals. Our simulations indicate that we can improve the effectiveness of seasonal closure by managing pulse fishing. Although the results derived in this study may be specific to the target ecosystem, the general approach is applicable to other ecosystems when evaluating fishing impacts.

China (herein referred as China’s mainland, and excluding Hong Kong, Macau and Taiwan) ranks as the world’s leading fishing nation, with approximately 11.1 million tons of domestic marine catch acquired in 2017. Marine fisheries resources in China are mainly exploited by its 11 coastal provinces and municipalities, and the development of fishing industry varies among them. However, few studies have examined the exploitation history of the 11 coastal provinces and municipalities. In this paper, we systematically quantified the exploitation history of marine fishery resources in China and then measured the vulnerability of the 11 coastal provinces and municipalities of China to a reduction in marine catches. Our analysis suggested that Chinese marine fisheries experienced rapid growth from the mid-1980s to the end of the 20th century, and this rapid increase in marine catches were mainly promoted by increased fishing effort. The total primary production required level amounted to approximately 80% of the average primary productivity in 2017, and Zhejiang, Fujian, Shandong, Hainan and Guangdong provinces were the main fishing provinces in China. By assessing three dimensions of vulnerability (exposure, sensitivity and adaptive capacity) to the impacts of a reduction in marine catches in the 11 coastal provinces and municipalities, we found that Hainan, Guangxi, Zhejiang and Fujian provinces had high or very high vulnerability, while the municipalities of Shanghai and Tianjin had low vulnerability. Identifying suitable adaptation policies and management plans based on the differences in vulnerability among coastal provinces is important in sustainable fisheries management.

Most of reported harmful algal blooms (HABs) of microalgae (75%) have been caused by dinoflagellates. Studies on the negative effects of HABs have generally focused on animals, valuable organisms in particular, and environmental factors such as dissolved oxygen and nutrients, but relatively fewer on community level, particularly that using metagenomic approach. In this study, we reported an investigation on the effects of a HAB caused by the dinoflagellate Prorocentrum donghaiense on the species diversity and community structure of the dinoflagellate sub-community via a pyrosequencing approach for the samples taken before, during, and after the bloom season of P. donghaiense in the East China Sea. We sequenced partial 28S rRNA gene of dinoflagellates for the field samples and evaluated the species richness and diversity indices of the dinoflagellate community, as a sub-community of the total phytoplankton. We obtained 800 185 valid sequences (categorized into 560 operational taxonomic units, OTUs) of dinoflagellates from 50 samples and found that the biodiversity of dinoflagellate community was significantly reduced during the blooming period in comparison to that in pre- and after-blooming periods, as reflected in the four diversity indices: the species richness expressed as the number of OTUs, Chao1 index, Shannon index (evenness), and Gini-Simpson index. These four indices were all found to be negatively correlated to the cell density of the bloom species P. donghaiense. Correlation analyses also revealed that the P. donghaiense cell abundance was correlated negatively with ${\rm{NO}}_3^- $-N, and ${\rm{NO}}_2^- $-N, but positively with total nitrogen (TN) and total phosphorus (TP). Principal coordinates analysis (PCoA) showed that the community structure of dinoflagellates was markedly different among the different sampling periods, while the redundancy analysis (RDA) revealed P. donghaiense abundance, salinity, ${\rm{NO}}_3^- $-N, and ${\rm{SiO}}_3^{2-} $ were the most four significant factors shaping the dinoflagellate community structure. Our results together demonstrated that HABs caused by the dinoflagellate P. donghaiense could strongly impact the aquatic ecosystem on the sub-community level which the blooming species belongs to.

Epizoic diatoms on marine copepods are common in nature and may have a special ecological relationship with their hosts. However, this special ecological group is not well known, and it has only rarely been studied in the China seas. To address this knowledge gap, the species diversity and classification of epizoic diatoms on planktonic copepods were studied with samples collected from the East China Sea. In the present study, a marine araphid diatom genus Protoraphis and its type species, Pr. hustedtiana, were observed and identified by light and electron microscopy, thus representing the first record of this genus and its type species in China. This genus is characterized by a median sternum strongly bent to opposite sides and terminate in two transapical grooves at the valve ends. Protoraphis hustedtiana was found to be epizoic on the posterior body appendages and segments of the marine calanoid copepod Candacia bradyi. An internal view shows a complex, ear-shaped process that is close to the apical slit field. The ecological habitats and geographical distributions of Protoraphis were also discussed, and, together with complementary morphological studies, our results have increased the number of records for marine epizoic diatoms to three genera with three species in China, including Pseudohimantidium and Pseudofalcula.

Current characteristics and vertical variations during summer and autumn in the southern East China Sea were investigated by measuring current profile, tide, wind, and wave data for 90 d from July 28 to October 25, 2015. Our results are: (1) The current was mainly a (clockwise) rotating flow, displaying reciprocating flow characteristics, and vertically the current directions were the same throughout the vertical profile. (2) The horizontal current speed was strongest during August (summer) with an average speed of 51.8 cm/s. The average current speeds during spring tides were highest in August and weakest in September, with speeds of 59.9 and 42.8 cm/s, respectively. (3) Considerable differences exist in average current speeds in different layers and seasons. The highest average current speeds were found in the middle–upper layers in August and in the middle–lower layers in September and October. (4) The residual current speed was highest in August, when the speed was 12.5–47.1 cm/s, whereas the vertical average current speed was 34.3 cm/s. The depth-averaged residual current speeds in September and October were only 50% of that in August, and the residual current direction gradually rotated in a counter-clockwise direction from the lower to surface layers. (5) Typhoon waves had a significant influence on the currents, and even affected the middle and lower water layers at depths of >70.0 m. Our results showed that the currents are controlled by the dynamic interplay of the Taiwan Warm Current, incursion of the Kuroshio Current onto the continental shelf, and monsoonal changes.

The impacts of Kuroshio intrusion (KI) optimization on the simulation of meso-scale eddies (MEs) in the northern South China Sea (SCS) were investigated based on an eddy-resolving ocean general circulation model by comparing two numerical experiments with differences in their form and intensity of KI due to the optimizing topography at Luzon Strait (LS). We found that a reduced KI reduces ME activities in the northern SCS, which is similar to the observations. In this case, the biases of the model related to simulating the eddy kinetic energy (EKE) west of the LS and along the northern slope are remarkably attenuated. The reduced EKE modeling bias is associated with both the reduced number of anti-cyclonic eddies (AEs) and the reduced amplitude of cyclonic eddies (CEs). The EKE budget analysis further suggests that the optimization of the KI will change the EKE by changing the horizontal velocity shear and the slope of the thermocline, which are related to barotropic and baroclinic instabilities, respectively. The former plays the key role in regulating the EKE in the northern SCS due to the changing of the KI. The EKE advection caused by the KI is also important for the EKE budget to the west of the LS.

Using AVISO satellite altimeter observations during 1993–2015 and a manual eddy detection method, a total of 276 anticyclonic rings and 242 cyclonic rings shed from the Kuroshio Extension (KE) were identified, and their three-dimensional (3D) anomaly structures were further reconstructd based on the Argo float data and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruise and buoy data through an interpolation method. It is found that the cyclonic (anticyclonic) rings presented consistent negative (positive) anomalies of potential temperature; meanwhile the relevant maximum anomaly center became increasingly shallow for the cyclonic rings whereas it went deeper for the anticyclonic rings as the potential temperature anomaly decreased from the west to the east. The above deepening or shoaling trend is associated with the zonal change of the depth of the main thermocline. Moreover, the composite cold ring between 140° and 150°E was found to exhibit a double-core vertical structure due to the existence of mode water with low potential vorticity. Specifically, a relatively large negative (positive) salinity anomaly and a small positive (negative) one appeared for the composite cyclonic (anticyclonic) ring at the depth above and below 600 m, respectively. The underlying driving force for the temperature and salinity anomaly of the composite rings was also attempted, which varies depending on the intensity of the background current and the temperature and salinity fields in different areas of the KE region, and the rings’ influences on the temperature and salinity could reach deeper than 1 000 m on average.

The continental slope in the northern South China Sea (SCS) is rich in mesoscale eddies which play an important role in transport and retention of nutrients and biota. In this study, we investigate the statistical properties of eddy distributions and propagation in a period of 24 years between 1993 and 2016 by using the altimeter data. A total of 147 eddies are found in the continental slope region (CSR), including 70 cyclonic eddies (CEs) and 77 anticyclonic eddies (ACEs). For those eddies that appear in the CSR, the surrounding areas of Dongsha Islands (DS) and southwest of Taiwan (SWT) are considered as the primary sources, where eddies generated contribute more than 60% of the total. According to the spatial distribution of eddy relative vorticity, eddies are weakening as propagating westward. Although both CEs and ACEs roughly propagate along the slope isobaths, there are discrepancies between CEs and ACEs. The ACEs move slightly faster in the zonal direction, while the CEs tend to cross the isobaths with large bottom depth change. The ACEs generally move further into the basin areas after leaving the CSR while CEs remain around the CSR. The eddy propagation on the continental slope is likely to be associated with mean flow at a certain degree because the eddy trajectories have notable seasonal signals that are consistent with the seasonal cycle of geostrophic current. The results indicate that the eddy translation speed is statistically consistent with geostrophic velocity in both magnitude and direction.

The effect of river runoff over the northern Indian Ocean (NIO) especially over the Bay of Bengal (BoB) has been studied using global Nucleus for European Modelling of the Ocean (NEMO). Two sensitivity experiments, with and without river runoff are conducted and the influence of river runoff on the Indian Ocean hydrography, stratification and circulation features are studied. It is found that due to river runoff surface salinity over the northern BoB decreases by more than 5 and the East India Coastal Current strengthens by 2 cm/s during post monsoon season. The fresh river water reaches up to 15°N in the BoB and is the main cause for low salinity there. Sea surface temperature in the northwestern BoB increases by more than 0.2°C due to the river runoff in summer monsoon while surface cooling upto 0.2°C is seen in north-west part of BoB in winter season. The seasonal mixed layer depth in the region is found to be dependent on river runoff. The effect of vertical shear and Brunt Vaisala frequency on stratification is also examined. The ocean water becomes highly stratified up to 3 035 m due to the river runoff. It is found that the energy required for mixing is high in the northern and coastal BoB.

The ocean and land color instrument (OLCI) and synthetic aperture radar altimeter (SRAL) installed aboard the Sentinel-3 satellite have been in orbit for operational uses. In this study, data collected from Sentinel-3 are used to investigate internal waves in the South China Sea. An internal wave is detected using an OLCI image with a resolution of 300 m, and an analysis was performed with a quasi-synchronous moderate-resolution imaging spectroradiometer (MODIS) image. The opposite characteristics of OLCI and MODIS images of the same internal wave are explained by the critical angle in brightness reversals. The unique observational geometry of the OLCI image and its influence on observations of internal waves are discussed. The distribution of σ₀ and sea surface height anomalies (SSHAs) induced by internal waves are studied using SRAL records. The σ₀ records of SRAL occasionally show less sensitivity to the modulation of internal waves, which may be attributed to the observational geometry, while SSHAs show obvious variations. The synchronous pairing of OLCI images and SRAL records are analyzed to extract the three-dimensional sea surface signatures induced by internal waves. The analysis demonstrates that the profile of SSHAs in the surface shows an opposite phase to the profiles of internal waves in the ocean. The opposite phase relationship, observed in the remote sensing view, is also confirmed with a laboratory experiment.