It is important to find a reliable method to estimate maximum sustainable yield (MSY) or total allowable catch (TAC) for fishery management, especially when the data availability is limited which is a case in China. A recently developed method (CMSY) is a data-poor method, which requires only catch data, resilience and exploitation history at the first and final years of the catch data. CMSY was used in this study to estimate the biological reference points for Largehead hairtail (Trichiurus lepturus, Temminck and Schlegel) in the Yellow Sea and Bohai Sea, based on the fishery data from China Fishery Statistical Year Books during 1986 to 2012. Additionally, Bayesian state-space Schaefer surplus production model (BSM) and the classical surplus production models (Schaefer and Fox) performed by software CEDA and ASPIC, were also projected in this study to compare with the performance of CMSY. The estimated MSYs from all models are about 19.7×10⁴–27.0×10⁴ t, while CMSY and BSM yielded more reasonable population parameter estimates (the intrinsic population growth rate and the carrying capacity). The biological reference points of B/B_MSY smaller than 1.0, while F/F_MSY higher than 1.0 revealed an over-exploitation of the fishery, indicating that more conservative management strategies are required for Largehead hairtail fishery.

Pacific saury is an important high-seas fishery resource in the Northwest Pacific Ocean for the Chinese Mainland. Reliable and accurate catch per unit effort (CPUE) plays a significant rule in Pacific saury stock assessment. Many statistical models have been used in the previous CPUE standardization research. Here, we compare the performance of Generalized Linear Models (GLMs) and Generalized Additive Models (GAMs) using CPUE data collected from Chinese saury fishery in the Northwest Pacific Ocean from 2003 to 2017 (excluding data from Chinese Taipei), and evaluate the influence of spatial, temporal, environmental variables and vessel length on CPUE. Optimal GLM/GAM models were selected using the Bayesian information criterion (BIC). Explained deviance and 5-fold bootstrap cross-validation results were used to compare the performance of the two model types. Fitted GLMs accounted for 21.57% of the total model-explained deviance, while GAMs accounted for 38.95%. Predictive performance metrics and 5-fold cross-validation results showed that the best GAM performed better than the best GLM. Therefore, we recommend GAM as the preferred model for standardizing CPUE of Pacific saury in the Northwest Pacific Ocean.

We quantified the δ¹³C and δ¹⁵N values in the lower beaks of Humboldt squid, Dosidicus gigas, collected from international waters off Costa Rica, Ecuador, Peru and Chile by Chinese squid jigging vessels during 2009, 2010 and 2013. There was a significant difference in the isotopic values among regions with the lowest value off Ecuador and the highest off Chile, which were interpreted as a function of trophic effects as well as baseline values. However, constant trophic level of D. gigas across its geographic range showed that spatial variation in the baseline of primary production is the main driver responsible for the observed geographic isotope variability. Inter-regional difference and intra-regional convergence of isotope values indicated squid off Costa Rica, Ecuador and Chile belong to different geographically segregated populations, which were previously proved by integrated population identifying method. In contrast, the higher variations in δ¹³C and δ¹⁵N values in a given size group suggest the squid off Peru move and forage in different places. Moreover, potential population exchange could be responsible for the overlap of the isotope values between the squid off Peru and off Chile. On the whole, the spatial difference in isotopic values of Humboldt squid beaks improves our understanding of potential geographic population connectivity and movement.

The migration route of oceanic squid provides critical information for us to understand their spatial and temporal variations. Mark-recapture and electronic tags tend to be problematic during processing. Cephalopod hard structures such as the beak, containing abundant ecological information with stable morphology and statolith-like sequences of growth increments, may provide information for studying spatio-temporal distribution. In this study, we developed a method, which is based on elemental concentration of beaks at different ontogenetic stages and sampling locations, to reconstruct the squid migration route. We applied this method to Ommastrephes bartramii in the North Pacific Ocean. Nine trace elements were detected in the rostrum sagittal sections (RSS) of the beak using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). For those elements, significant differences were found between the different ontogenetic stages for phosphorus (P), copper (Cu) and zinc (Zn). Sodium (Na), P and Zn were chosen as indicators of sea surface temperature (SST) and a regression model was estimated. The high probability of occurrence in a particular area represented the possible optimal squid location based on a Bayesian model. A reconstructed migration route in this study, combining all the locations at different ontogenetic stages, was consistent with that hypothesized in previous studies. This study demonstrates that the beak can provide useful information for identifying the migration routes of oceanic squid.

Euphausia superba is a key species in the Southern Ocean that serves as a link between primary production and higher trophic levels. To investigate the feeding strategies of E. superba from the eastern South Shetland Islands, fatty acid biomarkers, stable isotope signatures, and an incubation experiment were conducted. The results of the incubation experiment proved that adult E. superba mainly fed on 2–20 μm particles, demonstrating the importance of nanoplankton in their diet. Moreover, significant positive relationships between δ¹⁵N and body size demonstrated that size-related dietary shifts were present in E. superba. Evidence from principal component analysis and the C16:1ω7/C18:4ω3 ratio showed that juveniles preferentially fed on dinoflagellates and adults were more likely to feed on diatoms. Fatty acid profiles in adult E. superba roughly mirrored the different trophic conditions and feeding strategies between stations. Adult E. superba at Stas D2-07, D5-07, DA-01 and DA-02 exhibited elevated levels of C16:1ω7, C18:4ω3, C18:1ω9 and C18:1ω9/C18:1ω7, indicating higher levels of feeding on both phytoplankton and higher trophic diets. In contrast, adult E. superba at Stas D1-03 and D1-04 were characterized by high levels of polyunsaturated fatty acids/saturated fatty acids ratios and low levels of C16:1ω7, C18:1ω7, C18:4ω3, C18:1ω9 and total fatty acids. We inferred that adult krill at Stas D1-03 and D1-04 still suffered from difficult dietary conditions after overwintering. The different dietary conditions between stations suggest a highly plastic feeding strategy of E. superba in the eastern South Shetland Islands.

A prevailing, controversial hypothesis is that fishing pressure has played a greater role than climatic and environmental drivers, in changing fish species succession and biomass fluctuation in the South China Sea (SCS). Based on otter trawl survey data from 1959 to 2010 in the outer Beibu Gulf (OBG), northern SCS, large seasonal and interannual variation is reported for fish species composition, the proportional abundances of dominant taxa, and fish biomass. Generalized additive models are developed to quantify relationships between fish biomass and the external factors of fishing pressure and climate change. Fishing pressure proved to be the main driver of sharp declines in demersal fish stocks, with high-value species being replaced by low-value ones over time. Abrupt decreases in fish biomass during the years of 1993 and 1998 correspond to El Niño events, with climate change possibly the main driver of proportional representation of pelagic species in fisheries trawl catch. The need to differentiate impacts of fishing and environmental drivers on fish species with different life history strategies is stressed to better understand fish community dynamics.

With the decline of fish stocks, the proportions of economically important invertebrates like crustaceans and cephalopods have increased in the Bohai Sea. The community structure and trophic level of economically important invertebrates were analyzed using the bottom trawl survey data collected by the Yellow Sea Fisheries Research Institute in the Bohai Sea in May and August of 1958–1959, 1982, 1992–1993, 2004, 2009 and 2015. A total of 37 species of economically important invertebrates, belonging to 5 orders, 24 families, were captured. The biomass densities of economically important invertebrates in the Bohai Sea displayed an overall downward trend from 1982 to 2015. Oratosquilla oratoria and Loligo spp. were the most dominant species in the past 30 years, the biomass proportion of O. oratoria increased gradually in both May and August from 1982 to 2015. Moreover, biodiversity indices of economically important invertebrates in the Bohai Sea appeared to decline from 1982 to 2004 and then increased in 2015. Similarly, the mean trophic level of economically important invertebrates declined from 1982 to 2004 and increased slightly in 2015. Overall, although the proportions of invertebrates have increased, the biomass densities in the Bohai Sea have displayed an overall downward trend from 1982 to 2015. The increases in the biodiversity and trophic level of economically important invertebrates after the 2000s, possibly benefit from stock enhancement projects implemented by governments at different levels and national fishery management measures such as the “double-control” of the total number and engine power of fishing vessels and summer moratorium of fishing.

Based on a comparison of synchronized temperature and salinity data collected in the eastern Qiongzhou Strait and at coastal marine stations, this study finds that, in summer, the variation in salinity near the Weizhou Island in Guangxi is similar to that in the eastern and central portions of the Qiongzhou Strait. Additionally, the Beihai Station in Guangxi exhibits a small salinity variation, whereas the Longmen and Bailongwei Stations, both of which are located far from the Qiongzhou Strait, mainly exhibit continental hydrological characteristics in summer. Moreover, a comparison of the multi-year ocean current data from the Qiongzhou Strait and ocean current observations from the Weizhou Island Station and recently installed current-measuring stations shows that the residual current in the Qiongzhou Strait flows westward in winter and summer. The numerical simulation results also indicate that water from the eastern Qiongzhou Strait enters the Beibu Gulf. The characteristics of the temperature and salinity distributions and analyses of the residual currents further confirm that the western Guangdong coastal current is the main source of the westward transport of water in the Qiongzhou Strait. The primary driver of the formation of the western Guangdong coastal current is the westward flow of freshwater from the Zhujiang (Pearl) River. This water enters the Beibu Gulf via the Qiongzhou Strait and enhances the formation of the cyclonic circulation in the northern Beibu Gulf. In summer, the strong influence of the southwesterly wind leads to the formation of a strong northward coastal current along the western coast of the Beibu Gulf. This process promotes the transport of low-salinity diluted water toward the open ocean and the formation of larger-scale cyclonic circulation around Weizhou Island in the eastern Beibu Gulf. The results of this study regarding the effects of the water inflow from the eastern Qiongzhou Strait to the Beibu Gulf on the Guangxi coastal circulation directly challenge conventional conclusions concerning the transport direction of water through the Qiongzhou Strait in winter and summer.

A significant strong, warm “Blob” (a large circular water body with a positive ocean temperature anomaly) appeared in the Northeast Pacific (NEP) in the boreal winter of 2013–2014, which induced many extreme climate events in the US and Canada. In this study, analyses of the temperature and salinity anomaly variations from the Array for Real-time Geostrophic Oceanography (Argo) data provided insights into the formation of the warm “Blob” over the NEP. The early negative salinity anomaly dominantly contributed to the shallower mixed layer depth (MLD) in the NEP during the period of 2012–2013. Then, the shallower mixed layer trapped more heat in the upper water column and resulted in a warmer sea surface temperature (SST), which enhanced the warm “Blob”. The salinity variability contributed to approximately 60% of the shallowing MLD related to the warm “Blob”. The salinity anomaly in the warm “Blob” region resulted from a combination of both local and nonlocal effects. The freshened water at the surface played a local role in the MLD anomaly. Interestingly, the MLD anomaly was more dependent on the local subsurface salinity anomaly in the 100–150 m depth range in the NEP. The salinity anomaly in the 50–100 m depth range may be linked to the anomaly in the 100–150 m depth range by vertical advection or mixing. The salinity anomaly in the 100–150 m depth range resulted from the eastward transportation of a subducted water mass that was freshened west of the dateline, which played a nonlocal role. The results suggest that the early salinity anomaly in the NEP related to the warm “Blob” may be a precursor signal of interannual and interdecadal variabilities.

The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a ~14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m² of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs. From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.