This is what we know: Assessing the stock status of the data-poor skipjack tuna (Katsuwonus pelamis) fishery in the South China Sea

Skipjack tuna (Katsuwonus pelamis) is a highly migratory species of significant value in global marine fisheries. The South China Sea (SCS) is the largest marginal sea in the northwestern Pacific Ocean, and many tuna and tuna-like species occur within it. Although a recent Western and Central Pacific Fisheries Commission report identified skipjack tuna in the western and central Pacific to be sustainably fished, the exploitation status of skipjack tuna in the SCS is still unclear, due to limited data. We apply two data-poor methods, length-based Bayesian biomass estimation (LBB) and length-based spawning potential ratio (LBSPR), to assess the status of skipjack tuna stock in the SCS. We use electronic length frequency analysis to estimate von Bertalanffy growth parameters (asymptotic length Linf and growth coefficient K) to serve as priors for LBB and LBSPR estimation. Estimates are calculated for SCS skipjack tuna Linf (68.3 cm), K (0.325 year−1), natural mortality (0.49 year−1) and mean fork length at 50% sexual maturity (36.7 cm). LBB analysis reveals the estimated relative stock size (0.29) indicates that the SCS skipjack tuna stock maybe heavily overfished. LBSPR analysis reveals the estimated spawning potential ratio for this fishery to be 3%, which is significantly below the limit reference point of 20%. Accordingly, for the SCS skipjack tuna stock we identify a need to reduce fishing mortality by controlling fishing effort and increasing catchable size

Trophic Interactions of Mesopelagic Fishes in the South China Sea Illustrated by Stable Isotopes and Fatty Acids

As the most abundant fishes and the least investigated components of the open ocean ecosystem, mesopelagic fishes play an important role in biogeochemical cycles and hold potentially huge fish resources. There are major gaps in our knowledge of their biology, adaptations and trophic dynamics and even diel vertical migration (DVM). Here we present evidence of the variability of ecological behaviors (migration and predation) and trophic interactions among various species of mesopelagic fishes collected from the South China Sea indicated by isotopes (δ13C, δ15N), biomarker tools [fatty acids (FAs), and compound- specific stable isotope analysis of FAs (CSIA)]. Higher lipid contents of migrant planktivorous fishes were observed with average values of 35%, while others ranged from 22 to 29.5%. These high lipids contents limit the application of δ13Cbulk (bulk–tissue δ13C) as diet indicator; instead δ13Cextraction (the lipid extracted δ13C) values were applied successfully to reflect dietary sources. The δ15N values of non-migrant planktivorous and piscivorous fishes varied in a narrow range (10.0–10.8‰). This small difference may be caused by the low temperature in deep ocean, which results in low metabolic rates of nitrogen. The piscivorous fishes had higher FA ratios of DHA (22:6n-3)/EPA (20:5n-3) than planktivorous fishes, which mirrored their potential carnivory. CSIA can reveal small changes in biochemical composition and distinguishes between possible dietary sources such as between the non-migrant piscivorous and the semi-migrant piscivorous fishes with significantly different δ13C values of the 20:4n-6 and 20:5n-3 FAs (p < 0.01). Mesopelagic fishes exhibit complex trophic interactions revealed by molecular and CSIA tools and play a vital role in the marine “biological pump.” This knowledge is important to comprehensively evaluate the role of mesopelagic fishes in global carbon budgets

Fish Composition and Diversity of Four Coral Reefs in the South China Sea Based on Hand-Line Catch

To improve the overall understanding of the fish diversity and spatial patterns of major coral reefs in the South China Sea, fish assemblage composition, dominant species, biodiversity indices, and multivariate analysis of community structure were reported for four major coral reefs based on hand-line survey data in May and September 2018. A total of five orders, 21 families, 45 genera and 121 species of fish were recorded with Perciformes (78.5%) being the most diverse. The highest number (5) of dominant species was found near Chenhang Island while the lowest (2) number was detected at Zhubi Reef. The highest abundance index (7.21) occurred at Zhubi Reef, while the Shannon–Wiener diversity (4.80), Pielou’s evenness (0.81), and Simpson’s dominance (0.95) indexes were all highest at Qiliangyu Island. Based on cluster analysis and non-metric multi-dimensional scaling (NMDS), fish communities varied more spatially than seasonally. Our results led us to hypothesize that the habitat complexity and level of anthropogenic disturbance were the main factors affecting the composition of reef-dwelling fish on each coral reef. Topography was likely responsible for most variation in the spatial pattern of fish diversity

Fish Composition and Diversity of Four Coral Reefs in the South China Sea Based on Hand-Line Catch

To improve the overall understanding of the fish diversity and spatial patterns of major coral reefs in the South China Sea, fish assemblage composition, dominant species, biodiversity indices, and multivariate analysis of community structure were reported for four major coral reefs based on hand-line survey data in May and September 2018. A total of five orders, 21 families, 45 genera and 121 species of fish were recorded with Perciformes (78.5%) being the most diverse. The highest number (5) of dominant species was found near Chenhang Island while the lowest (2) number was detected at Zhubi Reef. The highest abundance index (7.21) occurred at Zhubi Reef, while the Shannon–Wiener diversity (4.80), Pielou’s evenness (0.81), and Simpson’s dominance (0.95) indexes were all highest at Qiliangyu Island. Based on cluster analysis and non-metric multi-dimensional scaling (NMDS), fish communities varied more spatially than seasonally. Our results led us to hypothesize that the habitat complexity and level of anthropogenic disturbance were the main factors affecting the composition of reef-dwelling fish on each coral reef. Topography was likely responsible for most variation in the spatial pattern of fish diversity

Climate-related changes in seasonal habitat pattern of Sthenoteuthis oualaniensis in the South China Sea

A habitat suitability index model (HSI) was established to examine seasonal habitat pattern of Sthenoteuthis oualaniensis in the South China Sea and assessed the impacts of the El Niño event on habitat variations based on sea surface temperature (SST), chlorophyll-a (Chla) and sea surface height (SSH). Environmental conditions and habitat patterns showed significant seasonal and spatial variations. Spatial pattern of favorable environmental conditions played important roles in regulating the ranges and longitudinal and latitudinal distributions of suitable habitats (areas with HSI≥0.6) of S. oualaniensis across seasons. Significant positive correlation was found between SST and HSI, whereas negative relationship was revealed between HSI and Chla as well as SSH by correlation analysis. Comparing to normal climate condition in spring 2014, the El Niño events in spring 2015 and 2016 yielded lower SST and higher Chla and SSH, which were unfavorable for the formation of high-quality habitats. Favorable ranges of environmental conditions showed decreasing trend from spring 2014 to 2016. Consequently, the suitable habitats in spring 2015 and 2016 largely contracted and became discontinuity. Our findings indicated that seasonal habitat patterns of S. oualaniensis in the South China Sea were strongly affected by the El Niño events and local environmental conditions

Nitrogen supply in the upwelling of mid-west region of the South China Sea: The case of mesopelagic fishes

Coastal upwelling zone hosts most fishing grounds worldwide and nitrogen dynamics in those systems mainly have been addressed through the nutrients replenished the ecosystem. Compared with coastal and upper-layer ecosystems, relatively limited knowledge has been explored by nitrogen isotope imprints through the food webs of mesopelagic fishes and their links to upwelling characteristics. Here, we collected several species of mesopelagic fishes in the mid-west region of the South China Sea (SCS) and analyzed them by biochemical methods (fatty acids, δ13C, δ15N, and δ13C of fatty acids). Due to the occurrence of upwelling, we want to evaluate the variation of nitrogen isotope compositions among these mesopelagic species and their links to the upwelling nitrogen supply. The δ15N of mesopelagic fishes in the non-upwelling area was depleted by 2‰ when compared with those in upwelling area, which indicated different nitrogen sources impacting for fishes in the two areas. A combination of multi-biochemical proxies was used to divide mesopelagic fishes into six groups and indicated that the feeding behaviors and upwelling were the main factors to affect the biological composition of mesopelagic fishes, even in such small regions. The differences in δ15N values between the upwelling and non-upwelling areas allowed us to estimate that N2 fixation supported about 37% of the N-demand of food sources in the non-upwelling area, which was larger than the 10% in the upwelling area. These results illustrate that nutrients from the deep layers in upwelling systems might be the main factors that cause differences in δ15N in mesopelagic fishes between upwelling and non-upwelling areas. Thus, more studies of N dynamics in mesopelagic fishes should be conducted in the upwelling systems to illustrate the impact of climate changes on fish biomass in the future

Bayesian analysis on stock-recruitment relationship with an application to the southern Atlantic albacore (<em>Thunnus alalunga</em>)

2025-2032SR data is fitted using three two-parameter SR models based on the least square method and then verified the posterior distribution of the &ldquo;degree of compensation&rdquo; parameter in the Shepherd SR model. Results of Akaike information criterion (AIC) showed that the Cushing SR model was the best SR model for this stock. According to the results of the Bayesian analysis, we had estimated the uncertainty on Maximum Sustainable Yield (MSY). For the B-H SR model, the 80% confidence interval of MSY was (19168, 25860) t, with a median of 22085 t. For the Shepherd SR model, the 80% confidence interval was (21414, 29044) t, with a median of 24799 t. In comparison with the other studies, the result of the Shepherd SR model was similar but the result of the B-H SR model was conservative

Variations in Fish Community Structure at the Lagoon of Yongshu Reef, South China Sea between 1999 and 2016&ndash;2019

Owing to climate change and anthropogenic stressors, the abundance and diversity of reef fishes have globally decreased. However, we know little about the long-term change in reef fishes in the South China Sea (SCS). To reveal the response of reef fishes to these threats in the SCS, based on the fish specimens at Yongshu Reef lagoon collected in 1999 and 2016&ndash;2019, this study investigated variations in fish composition and community structure. Additionally, we inferred the changes in the coral cover of Yongshu Reef based on the change of the coral cover of near Meiji Reef in the SCS, sea surface temperature (SST) in Yongshu Reef, and total annual catch of Epinephelinae from China in the SCS. We found that compared with 1999, the number of dominant species in 2016&ndash;2019 decreased. The relative dominance of the first dominant species increased from 24.46 in 1999 to 39.44 in 2016&ndash;2019. Indices of catch per unit effort, species diversity and richness declined from 1999 to 2016&ndash;2019. Community pattern changed with some species with a higher trophic level (3.7&ndash;4.5) decreased, while those with lower trophic level (2.2&ndash;4.1) increased greatly. Some species with high economic value and important ecological functions decreased or disappeared locally. Correlation analysis indicated that declining coral cover, increasing fishing and rising SST were clearly associated with changes in fish community structure at Yongshu Reef. We speculated that declining coral cover and increasing fishing had a direct impact on the degradation of fish communities at Yongshu Reef lagoon, but increasing SST had an indirect impact on that. To conserve coral reef fish, it is recommended to take rigorous measures for improving habitat and protecting resource

Transcriptomic analyses of gastrointestinal function in the "dwarf" and "medium" forms of Sthenoteuthis oualaniensis during sexual maturation.

Sthenoteuthis oualaniensis (SA) is an important squid species in the South China Sea. Based on SA samples collected in 2016, SA was divided into the "dwarf" form (DF) and "medium" form (MF). To understand the changes in gastrointestinal function in SA during sexual maturation, we undertook transcriptomic analyses of the stomach and intestine tissues of the mature and immature DF and MF of SA using the deep-sequencing platform Illumina HiSeq™. We exploited a high-throughput method to delineate differentially expressed genes (DEGs) in the DF and MF of SA. A total of 135464 unigenes (68627 unigenes of the DG and 66837 unigenes of the MF) were generated. We identified 7965 and 4051 relative DEGs in the intestine and stomach tissues of the mature DF of SA compared with those of the immature DF of SA; and 22138 and 18460 DEGs in the intestine and stomach of the mature MF of SA compared with those of the immature MF of SA. Gastrointestinal function related to the metabolism of lipids, amino acids, glucose, and energy were changed in SA during sexual maturation. This work is the first to identify a set of genes associated with gastrointestinal function during sexual maturation in SA