Six heavy metals (As, Cu, Cd, Zn, Cr, and Pb) in surface sediments (0–5 cm) from the twenty selected sites of the coastal Pearl Bay (South China Sea) were analyzed to assess the distribution pattern and potential ecological risk. Overall concentrations (mg/kg, dw) in the sediment samples were: As (10.88 ± 6.50), Cu (24.16 ± 18.63), Cd (0.55 ± 0.78), Zn (48.53 ± 30.06), Cr (35.78 ± 28.66), Pb (31.28 ± 18.50). Results showed that the overall mean values of Cd concentrations exceeded the standard of China Marine Sediment Quality, caused by significantly high levels of Cd contents in five sites (S8, S11, S13, S16, and S17) at the offshore area of Pearl Bay. Generally, the metal concentrations showed a decreasing trend from the offshore area to the inner bay. Various index values such as the geo-accumulation index (I_geo), the ecological risk index (E_ri), and the contamination factor (CF) demonstrated that the coastal Pearl Bay was not polluted by the examined metals except for Cd, which might cause contamination and ecological risk in the region. Principal component analysis (PCA) results indicated that Cu, Zn, and Cr might originate from natural sources inland, and Pb and As might come from the gasoline and diesel fuel from engine boats. It is recommended that further research should focus on detecting the acute source and transferring mechanisms of the toxic metal Cd.

The nearshore ecosystem of Hainan Island plays a crucial role as a breeding habitat for a wide variety of economically valuable fish species. Gaining insight into the structure of the fish community and the environmental factors that may impact them is highly significant in this marine region. This study collected fish data from 50 sampling sites using bottom trawling surveys. Methods including the swept area method, ecological indices, and abundance/biomass curve (ABC) analysis were employed to assess fish resource density and diversity. A comprehensive identification revealed a total of 363 fish species, encompassing 24 orders, 114 families, and 226 genera, with Perciformes being the dominant group. Spring exhibited higher fish abundance and biomass compared with autumn, with the southwestern waters as the primary concentration area. Acropoma japonicum, Decapterus maruadsi, and Navodon xanthopterus were dominant in spring, while Leiognathus bindus, Saurida tumbil, and Champsodon atridorsalis were dominant in autumn, indicating a seasonal shift towards smaller and lower-value fish species. A variability exceeding 80% was observed through SIMPER analysis, and a disrupted community structure was evident in the eastern and southern waters. Temperature and salinity were identified as primary environmental factors influencing the fish community. Overall, this study provides valuable insights into the nearshore fish community of Hainan Island, aiding in the understanding of its structure and dynamics.

Thirty samples of surface sediments (0–5 cm) from the northern Beibu Gulf were analyzed to determine the spatial distribution, potential risks and sources of six heavy metals (Cr, Cu, Zn, As, Cd and Pb). The concentrations (mg/kg, dw) of Cr, Cu, Zn, As, Cd and Pb were 15.38 ± 6.06, 6.54 ± 3.23, 41.86 ± 17.03, 6.92 ± 2.75, 0.04 ± 0.02 and 17.13 ± 6.38, respectively. Higher levels of Cr, Cu, Cd and Zn were observed in the western part of the study area. According to the potential ecological risk indexes and sediment quality guidelines, the measured metals were assessed at low contamination levels, with Pb posing the largest ecological risks. The results of positive matrix factorization (PMF) indicated that Cr and Zn mainly originated from natural geological background sources, while Cu, As, Cd and Pb were influenced by anthropogenic sources such as atmospheric deposition and anthropogenic activities. These three sources contributed 60.4%, 28.1% and 11.5% of the heavy metals, respectively. In addition, further research should be conducted focusing on the general relationships between As and various controls in sediments of the northern Beibu Gulf.

This study explored the ecological niche and interspecific relationships among the main fish species in the coastal waters of Hainan Island based on data from fishery stock surveys conducted in the spring and autumn of 2022. The methods of chi-square test, percentage of co-occurrence (PC), association coefficient (AC), cluster analysis, and redundancy analysis were used to analyze the interspecific associations and influence of environmental factors on the ecological niches of fish communities. According to the cluster analysis of niche breadth, 20 main fishes could be divided into broad, medium, and narrow niche species. There were four and eight broad niche species in spring and autumn, respectively. The ranges of niche overlap values were 0.001–0.91 in spring and 0–0.87 in autumn, indicating that the species differed and were similar in their ability to utilize resources, survive in habitats, and prey. According to the variance ratio and statistic value W, the main fishes in spring showed a significant positive association, whereas those in autumn showed a positive association, but not at a significant level, indicating that the main fishes in spring were more closely associated with each other. There were 56 species pairs in the 2 seasons that were significantly associated (χ² ≥ 3.841). AC and PC tests revealed that the interspecific association was strong and tended to be positive. According to the redundancy analysis, environmental factors such as surface temperature, water depth, and pH significantly affected the main fishes in spring, while environmental factors such as dissolved oxygen, bottom temperature, surface salinity, and pH significantly affected those in autumn.

Sexual growth dimorphism is a common phenomenon in teleost fish. However, the mechanism of this complex phenomenon remains unclear. The fine-patterned puffer (Takifugu poecilonotus; Temminck and Schlegel, 1850) exhibits female-biased sexual size dimorphism similar to other pufferfish. In this study, the transcriptomes of female and male T.poecilonotus were sequenced, 285.95 million raw read pairs were generated from sequence libraries. After identification and assembly, a total of 149,814 nonredundant unigenes were obtained with an N50 length of 3538 bp. Of these candidates, 122,719 unigenes (81.91% of the total) were successfully annotated with multiple public databases. The comparison analysis revealed 10,385 unigenes (2034 in females and 8351 in males) were differentially expressed between different sexes of T.poecilonotus. Then, we identified many candidate growth- and sex-related genes, including Dmrt1, Sox3, Spatas, Prl/Prlr, fabps, Ghr, and Igf1r. In addition to these well-known genes, Fabp4 was identified for the first time in fish. Furthermore, 68,281 simple sequence repeats (SSRs) loci were screened and identified in the transcriptome sequence of T.poecilonotus. The results of our study could provide valuable information on growth- and sex-associated genes and facilitate further exploration of the molecular mechanism of sexual growth dimorphism.

Fishery resource and environmental data from four surveys conducted in the Hainan Island offshore area from March 2021 to June 2022 were used to investigate the spatiotemporal distribution of round scad (Decapterus maruadsi). A generalized additive model was applied to explore the relationships among abundance and distribution, also biotic and abiotic factors (i.e., temperature, salinity, chlorophyll-a concentration, water depth, phytoplankton abundance, zooplankton abundance, and jack mackerel (Trachurus japonicas) abundance). Round scad abundance (average 67.17 kg/km²) showed distinct spatial and seasonal differences around Hainan Island, with highest abundance in summer (171.72 kg/km²) and lowest abundance in spring (3.06 kg/km²). The optimal model revealed that jack mackerel abundance, sea bottom temperature, sea surface temperature, and latitude very significantly (p < 0.01) affected round scad distribution. Jack mackerel abundance (mainly in the range 0–50 kg/km²) was positively correlated with round scad distribution. The distribution showed a dome-shaped relationship with bottom water temperature in the range 18–30 °C, with maximum abundance at 24 °C. The distribution was negatively correlated with surface water temperature in the range 22–30 °C. Sea surface chlorophyll-a concentration and longitude were significantly correlated (p < 0.05) with round scad distribution. The results provide theoretical support to further investigate the formation mechanism of round scad fishing grounds and to enrich knowledge of pelagic fish abundance in the continental-shelf waters of the northern South China Sea.