Abstract:The large yellow croaker (Larimichthys crocea) is one of the most economically important marine fish species in China. Reducing water salinity can effectively prevent white spot disease, but it can also cause low-salinity stress and damage to fish. Low-salinity acclimation can improve the body’s tolerance to low salinity; therefore, it is necessary to explore the specific physiological mechanisms of low-salinity acclimation in fish under low-salinity stress. This study aimed to investigate the effects of low-salinity acclimation on energy metabolism and mitophagy in the large yellow croaker under low-salinity stress. Juvenile large yellow croakers with a body weight of (53.46±1.47) g were transferred from natural seawater (salinity 25) to water with a salinity of 20 and were cultured for 7 d and then exposed to low-salinity (salinity 12) water for 24 h. The results showed that low-salinity stress increased reactive oxygen species (ROS), adenosine triphosphate (ATP) content, tricarboxylic acid (TCA) cycle enzyme activities, and mitophagy gene expression levels, indicating that L. crocea improved aerobic metabolism and mitophagy under low-salinity stress but still suffered from oxidative damage. Compared with the low-salinity group, the low-salinity acclimation with low-salinity group showed increased ATP content, TCA enzyme activities, and mitophagy gene expression levels but reduced ROS content, indicating that low-salinity acclimation reduced oxidative damage by improving energy metabolism and mitophagy in large yellow croaker under low-salinity stress. During low-salinity stress, adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) activity was positively correlated with TCA and fatty acid β oxidation enzyme activities and negatively correlated with fatty acid synthesis enzyme activity, indicating that AMPK promoted energy-yielding metabolism and inhibited energy-consumptive metabolism in L. crocea, thereby improving energy generation efficiency. Forkhead box class O3 (FoxO3) mRNA levels were positively correlated with the expression levels of mitophagy genes, indicating that FoxO3 participates in the regulation of mitophagy gene expression during low-salinity adaptation. In summary, low-salinity acclimation improved aerobic metabolism and mitophagy, inhibited lipid synthesis, and enhanced the tolerance of L. crocea to low-salinity stress.

Abstract:As marine aquaculture expands from coastal regions to the deep seas, large-scale semi-submersible aquaculture platforms have become the primary engineering equipment in recent years because of their high stability and ease of installation. To reduce the difficulty of offshore construction, these platforms are typically installed on land and towed to the operating areas by tugboats. However, owing to the high center of gravity of the platform, there is a risk of capsizing during long-distance towing. In this study, a numerical analysis model was developed to assess the hydrodynamic performance of an aquaculture platform, including its frame and net, during wet-towing transportation. For a large-scale frame, including pontoons and columns, a three-dimensional potential flow theory was applied to solve the diffraction and radiation problems. For the thin net, a semi-empirical Morison equation was used to calculate the wave loads and a mesh group method was implemented to decrease the computational cost and time. The motion equation was solved in the time domain using the impulsive response function method, and analysis of nonlinear catenary towline was performed using the lumped-mass method. This model examines the impact of significant wave height, wave direction, towing speed, towline length, and mooring point position on the motion response and towline tension of a semi-submersible aquaculture platform. The results indicate that as the significant wave height rises from 1.25 m to 4.00 m, the heave acceleration and pitch angle of the aquaculture platform increase by 293% and 750%, respectively, whereas the towline tension increases by 358%. The aquaculture platform experienced more severe pitch motion in the following waves than in the head waves although the towline tension decreased. Furthermore, increasing the towing speed from 2.0 kn to 5.0 kn leads to a 56% reduction in heave acceleration and an 112% increase in towline tension. Tripling the towline length results in a 43% decrease in the pitch angle and 31% decrease in the towline tension. The height of the towing point has a significant impact on the pitch motion. For practical engineering applications, it is recommended to limit the maximum sea condition for towing aquaculture platforms to level 4, keep the towing speed below 5.0 kn, avoid towing in following waves, and instead opt for quarter head waves. Additionally, appropriately increasing the tow line length can improve the towing stability.

Abstract:The purpose of this study was to examine how Sebastes schlegelii responded to acute pH stress in terms of group behavior and physiological variations. The pH of the water column is lowered during industrial aquaculture due to the high shellfish mortality, excessive algae cultivation, and seawater red tides. Water’s pH rises as a result of the massive consumption of carbon dioxide in the water caused by photosynthesis. Numerous enzymes, including those involved in immunology, antioxidant, and metabolism in aquatic organisms, are impacted by pH variations. Under stress, fish frequently run, reduce their metabolism, and increase the activity of related enzymes such as SOD and CAT. When fish are subjected to external stimuli, physiological and biochemical indices can represent their overall health ability to adapt to their environment. In this study, the effects of acute pH stress on the immunity, liver function, antioxidant activity, and group behavior of juvenile S. schlegelii were investigated. Different pH values (pH 6.5, pH 7.5, pH 8.5, and pH 9.5) in seawater were studied to identify the effects of pH stress on immunity, liver function, antioxidant activity, and group behavior of juvenile S. schlegelii, The results show that S. schlegelii CAT activity significantly increased in the pH 7.5 and pH 9.5 experimental groups at 48 h (P＜0.05), SOD enzyme activity increased significantly (P＜0.05) at 72 h compared to 24 h in all experimental groups and started to decrease after 72 h. ACP activity showed a gradual decrease in the pH 6.5 and pH 9.5 experimental groups, with a rebound in the pH 9.5 experimental group at 96 h but the change was not significant (P＞0.05). At 48 h, AKP fell dramatically in the pH 6.5 and pH 9.5 experiment groups (P＜0.05), but rebounded somewhat in the pH 6.5 group at 72 h (P＞0.05). GPT decreased and then increased in all experimental groups with a significant drop in the pH 6.5 group after 24 h (P＜0.05). There was no significant difference among the pH 7.5 experiment groups (P＞0.05). Throughout the 48 h–72 h period, GOT activity gradually increased in the pH 7.5 and pH 9.5 experimental groups, although it significantly decreased at the start of the 72 h (P＜0.05). In comparison to the control group, S. schlegelii’s degree of aggregation increased greatly and its activity dramatically decreased under pH 6.5 and pH 9.5 (P＜0.05). The results showed that different pH levels influenced S.schlegelii’s physiological processes and behavioral decisions. The immunity of juvenile S.schlegelii was lower in the pH 6.5 and pH 9.5 experimental groups.

Abstract:Phenotypic parameter measurement is one of the most important methods for monitoring the growth of Larimichthys crocea and estimating its economic benefits. However, most current studies on phenotypic parameter measurements focus only on the length of the fish body, with few studies focusing on essential details such as the measurement of parameters at the caudal peduncle. Also, manual measurement is time-consuming and laborious. This study designed and developed an L. crocea phenotypic parameter measurement system using computer vision technology to measure the phenotypic parameters of L. crocea. First, an improved segmentation model, Mask-RCNN, was constructed to remove the fin from L. crocea. The improvement in the model includes two aspects: detection speed and accuracy. The model replaces the backbone network of the original network model with the MobileNetV2 network to improve detection speed. Subsequently, a hybrid attention module, CBAM, is introduced into the backbone network, and PointRend is imported into the head network. These improvements have enhanced the extraction accuracy of fish body edges. The accuracy of the optimal model was 87.94%, mAP was 83.21%, and average single-image detection time was 70.5 ms. We developed a L. crocea parameter measurement system for the greater amberjack based on PyQt5, which realized the fusion of fin removal images and complete contour images with the location of the key points and calculated the phenotypic parameters through the positional information. The average relative error for systematic measurements of the six parameters was 4.04%. The average absolute percentage error of each parameter for different sizes of L. crocea was under 7%. Overall, the process designed in this study provides a new method for measuring the phenotypic parameters of L. crocea and a new way of thinking to improve the efficiency of measuring these parameters.

Abstract:Copper is an important heavy metal and an essential trace element for aquatic organisms and is found at low concentrations in aquatic ecosystems. In aquaculture, copper sulfate is used as a therapeutic agent to reduce parasitic infections in cultured fish. However, exposure to heavy metals beyond a safe concentration range can cause a series of physiological and biochemical stresses in aquatic animals, including death. Fish are generally most sensitive to the effects of Cu2+ exposure during early life stages, especially during embryonic and larval development, which can result in reduced fertilization success, chromosomal abnormalities, DNA injury, increased incidence of yolk membrane ruptures, reduced hatching success and survival, altered time to hatching, embryonic and larval teratogenicity, inhibited growth, abnormal larval behaviors. Yellowtail kingfish (Seriola aureovittata) is a fast-growing marine species widely distributed in temperate and subtropical regions of the oceans. It has a large body size, superior flesh quality, high economic value, and can be cultured in sea cages, indoor recirculating aquaculture systems, aquaculture crafts, and other culture modes. Therefore, S. aureovittata is a promising candidate for open-ocean aquaculture in China. To investigate the effects of copper exposure on the early life stages of S. aureovittata, embryos and yolk sac larvae were exposed to copper concentrations of 0, 0.034, 0.060, 0.107, 0.192, 0.340, 0.600, and 1.070 mg/L for 60 h and 5 d under laboratory conditions in the present study. The half-lethal concentration (LC₅₀) and safe concentration (SC) were determined for embryos and newly hatched larvae of S. aureovittata, respectively. In addition, differential gene expression patterns and related signaling pathways were investigated in embryos and yolk-sac larvae to elucidate possible molecular mechanisms. The results showed that: (1) Low hatching rate of embryos, high malformation rate of newly hatched larvae, and high utilization speed of yolk sacs during the early developmental stage of S. aureovittata under copper exposure stress. (2) The 48 h-LC50 of copper for S. aureovittata embryos was 0.08 mg/L, which was much lower than that of yolk sac larvae (48 h-LC50 = 0.60 mg/L). The SC of Cu²⁺ in S. aureovittata yolk sac larvae was 0.034 mg/L. (3) Transcriptome sequencing analysis revealed that differentially expressed genes (DEGs) were mainly enriched in the oxidative phosphorylation pathway during the embryonic development stage of S. aureovittata, whereas in the yolk sac larval stage, DEGs were mainly enriched in the cell cycle and phototransduction pathways, indicating that excessive copper exposure may injure the antioxidative ability of embryos and cause DNA injury and phototransduction inhibition in larvae. The present study determined the safe concentration of copper sulfate during the early developmental stages of S. aureovittata and revealed the molecular responses of embryos and yolk sac larvae to copper exposure stress. These results could provide theoretical support for the scientific and staged application of copper agents in the aquaculture of S. aureovittata and technical guidance for monitoring aquaculture and marine environments.

Abstract:This study investigated the behavioral and physiological adaptation characteristics of Lateolabrax maculatus to different light colors at both population and individual levels. We analyzed the liver enzyme activity, retinal tissue morphology, and eye transcriptomics under preferred light colors. Five light colors (red, yellow, blue, green, and white) were used to explore population and individual color peference behavior. The results showed that the population color trend index (PCTI) and population retention time (PRT) of Lateolabrax maculatus were significantly higher under blue and white lights than those of other (P＜0.05). However, differences were observed in light-color preferences between populations and individuals; populations showed a stronger preference for blue light, whereas individuals preferred white light. Additionally, higher tail beat frequency (PTBF) was observed in red and yellow light regions. Physiological analysis indicated that hepatic antioxidant parameters such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), alkaline phosphatase (AKP), acid phosphatase (ACP), and malondialdehyde (MDA) content were significantly lower (P＜0.05) under blue light than those under green light. Retinal analysis showed an increased number of cone cells (C. C) under blue light compared to green and white light, though the pigment index (PI) was lower. The KEGG annotation of the eye transcriptome showed that differentially expressed genes (DEGs) in the green light group were mainly enriched in the calcium signaling pathway compared with that in the white light group, in which genes such as α7nAChR and casq were up-regulated within the pathway. Furthermore, green light upregulated motor protein signaling pathway genes (myh, tnn2, and tpm1) in the eyes of periwinkles compared to the blue and white light. In summary, Lateolabrax maculatus populations displayed a stronger tendency toward blue light and experienced lower stress levels under blue light compared to other light colors.

Abstract:Coilia nasus, a historically significant fish species in the Yangtze River, has experienced a significant decline because of overfishing and habitat degradation. As an anadromous fish, C. nasus traverses different habitats over large geographic areas, making effective conservation difficult. Tracing their natal origin and lifelong habitat useage at the individual and population levels will enhance our ability to protect critical habitats of this anadromous fish. The migratory life history of C. nasus across freshwater, estuarine, and marine environments can be determined by analyzing otolith Sr/Ca ratios. However, difficulties remain in tracking their natal origins and identifying transitions between freshwater habitats during the early stages of their development. The use of otolith 87Sr/86Sr isotopic ratios offers a robust approach for tracing the natal origins and migration patterns of C. nasus. The Sr isotope (⁸⁷Sr/⁸⁶Sr) composition of the otoliths was analyzed using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to confirm the hatching grounds and life histories of C. nasus in the Poyang Lake. In July and August 2023, a total of ten adult C. nasus and three adult C. brachygnathus were collected from the Duchang area of the Poyang Lake, Jiangxi Province, China. The otolith ⁸⁷Sr/⁸⁶Sr profiles from the core to the edge displayed minor fluctuations of approximately 0.71448, indicating that the three C. brachygnathus individuals reside in the Poyang Lake. The ⁸⁷Sr/⁸⁶Sr ratio of the otolith core region of nine C. nasus was close to 0.71448, indicating that they hatched in the Poyang Lake and subsequently returned to the lake (because they were caught in the Poyang Lake). Among these individuals, four migrated out of the Poyang Lake during their larval stage and five migrated during their juvenile stage. One individual exhibited an average ⁸⁷Sr/⁸⁶Sr ratio of 0.71078 in its first segment, suggesting that it was born in the main channel of the Yangtze River and then strayed into Poyang Lake after its sea life history. The results demonstrated the objectivity, accuracy, and stability of otolith ⁸⁷Sr/⁸⁶Sr ratio as a reliable geographical indicator for identifying the natal origin of C. nasus in the Poyang Lake. Furthermore, these findings shed light on habitat shifts between the Poyang Lake and the main Yangtze River channel, emphasizing the diverse timing of early life migration from the Poyang Lake. Consequently, despite being a preliminary study, this study demonstrates the potential of using Sr isotopes to distinguish between natal origins and freshwater habitats. This approach is significant within the context of a decade-long fishing prohibition in the Yangtze River, as it facilitates the assessment of species recovery and management while contributing to broader ecological and conservation insights.

Abstract:Kelp (Saccharina japonica) is one of the main seaweed aquaculture species in China and is crucial for environmental carbon sequestration and the development of the marine economy. However, longline aquaculture technology of kelp has been used for many years and there are many problems in the kelp aquaculture industry. Owing to the lack of mechanization modes, such as high labor costs and low profits, the mechanization of production operations is difficult. Rongcheng, in Shandong Province, is a prominent kelp aquaculture area. In the present study, longline aquaculture of kelp was the experimental object in the Sanggou Bay, Rongcheng, and a preliminary study on the mechanization of kelp aquaculture was conducted. There were two different single-rope aquaculture modes with kelp-seeding rope hanging spaces of 1 m and 2 m (A1 and A2) in the Sanggou Bay; the traditional aquaculture mode (zone B) was used as the control. By analyzing the differences in kelp growth under different aquaculture modes and the relationship between light intensity, water depth, and kelp growth in the single-rope aquaculture mode, we explored the optimal hanging space for kelp growth under this mode, which will provide important references for the mechanization and sustainability of kelp aquaculture. The results showed that the specific growth rate of kelp in the traditional aquaculture mode was higher than that in the single-rope aquaculture mode during the aquaculture period. The kelp in the upper layers of A1 and A2 grew well, with an average wet weight of 1005.35 g in the upper layer of A2, whereas the average wet weight of kelp in the traditional mode was 1254.09 g which was approximately 1.25 times of A2. The experiment lasted until the kelp was harvested. The temperature in the kelp aquaculture area was within the suitable range for the growth of kelp. Light intensity in zone A was mostly higher than that in zone B, which effected the growth of kelp to a certain extent, resulting in a lower wet weight of the kelp harvested in zone A compared to the traditional aquaculture mode. By analyzing the relationship between light intensity and water depth and the experimental results, it was found that insufficient light intensity in the lower layers of A1 and A2 led to poor growth of kelp, compared with those in the upper and middle layers. In this study, the average yield of A1 raft frame was about 690 kg whereas that of A2 raft frame was about 900 kg, which was 1.3 times that of A1; the average yield per raft under the traditional aquaculture mode was about 1420 kg, which was about 1.5 times that of A2. However, the maximum wet weight of a single kelp in the single rope aquaculture area could reach 1729.07 g, which is consistent with the maximum wet weight of a single kelp in traditional aquaculture. Although the yield of kelp in the single-rope aquaculture mode was not as high as that in the traditional aquaculture mode, the labor cost of the longline aquaculture mode was significantly reduced, which could compensate for the reduced yield. It is necessary to maintain a balance between kelp growth and improvement of comprehensive efficiency in aquaculture production. Therefore, by appropriately increasing the hanging spacing and reducing the water layer of the range, thereby allowing all kelp to receive uniform light intensity, the quality of kelp can be improved, technical support for kelp aquaculture can be provided, and the foundation for the realization of mechanized production operations for kelp aquaculture can be laid.

Abstract:The size spectra of fish communities can reflect the characteristics of their biomass, abundance, and size structure, and these parameters can be used to explore the effects of environmental changes and human disturbance on the ecosystem. Therefore, it is important to construct reasonable size spectra of fish communities to study their community structures. However, there are differences in the quantity and quality of data collected from different survey designs, which further affects the construction of the size spectra of fish communities. This simulation study examined the effects of sampling design on the estimation of parameters of normalized biomass size spectra of fish communities using a computer resampling method based on data collected from seasonal bottom trawl surveys in the southern waters off the Shandong Peninsula from 2016 to 2017. Simple random sampling (SRS), stratified random sampling with strata defined by depth (StRS_depth), and stratified random sampling with strata defined by size (StRS_size) were chosen as potential sampling designs for sample sizes ranging from 5 to 60. The relative estimation error (REE) and relative deviation (RB) were used to measure the performance of the different sampling designs. The results showed that under the three sampling methods, REE values of the slopes of the normalized biomass size spectra of the fish communities decreased with an increase in the sample size. As the sample size increased, the range of absolute values of RB decreased and gradually approached zero. Under the three sampling methods, the distribution of simulated values of the slopes of the normalized biomass size spectra of fish communities became more concentrated, and the estimation accuracy improved with an increase in sample size. StRS_size performed better compared with those of the other two sampling methods, which had lower REE and absolute RB values. In addition, different seasonal data showed different effects on the slope estimation of the normalized biomass size spectra of fish communities. Considering the sampling effects of different sample sizes during the four seasons under the three sampling methods, StRS_size was the optimal sampling method suitable for the estimation of biomass size spectra parameters of fish communities, and the number of sampling stations could be optimized.

Abstract:To investigate the pathological characteristics and pathogenesis of visceral white spot disease in Hapalogenys mucronatus, anatomical observations, tissue sectioning, and ultra-thin section electron microscopy were used to conduct a pathological analysis of the thymus, spleen, gills, liver, kidneys, intestines, and ovaries of affected fish. The results showed that clinical symptoms in H. mucronatus included ulceration and hemorrhage on the body surface, ulceration and bleeding at the base of the pectoral, caudal, and dorsal fins, visible whitish nodules in the gills, thickening and severe fibrosis of the ovarian membrane in females, and numerous white nodules in visceral organs such as the spleen, liver, and kidneys. Histopathological examination revealed that the spleen, thymus, liver, and kidneys were the main target organs for infection and injury, showing severe tissue degeneration and necrosis with pronounced vacuolation. Pathological nodules were observed in all the affected tissues. Ultrastructural pathology results showed severe damage to the cellular ultrastructure of the spleen, thymus, liver, and kidneys of the affected fish, particularly mitochondrial and nuclear damage. Mitochondrial swelling, cristae disruption, and vacuolization were observed, along with nuclear shrinkage, chromatin marginalization, and clustering of numerous pathogenic bacteria. The results indicate that tissue cellular pathology and ultrastructural pathological characteristics of visceral white spot disease in H. mucronatus demonstrate the invasion and harm caused by pathogenic bacteria, primarily resulting in damage to respiratory-, immune-, and reproductive-related tissue organs, ultimately leading to death, while potentially exerting a negative impact on reproductive capacity.

Abstract:Numerical computer simulation is an important method to study the hydrodynamics of fishing gear, providing visualization of motion, morphology, loading, and surrounding flow characteristics under different operating conditions. Here, the research progress of hydrodynamics and the application of fishing gear based on numerical simulation were reviewed. Additionally, the development and simulation methods of numerical models, as well as solving algorithms and efficiency improvement approaches of numerical analyses were summarized in detail. Finally, the disadvantages and suggestions of numerical research on fishing gear hydrodynamics were put forward. The summary showed that the numerical analysis of fishing gear primarily involves the dynamics model based on the topology of physical structure and the hydrodynamic model based on computational fluid dynamics (CFD) software. Both explicit and implicit algorithms were used to calculate the rigid motion equations and reducing the number of elements and optimizing the programming algorithm were the main methods used to improve the computational efficiency. The review suggests that the numerical model and related parameters could be improved by combining with actual working conditions such as non-constant currents or irregular waves at sea, the accumulation of catch in the codend, as well as the mechanical properties affected by the braided structure and non-linear deformation of the mesh. Moreover, numerical computation efficiency could be improved by the modification of solution algorithms, optimization of node topology, and improvement of calculation frameworks. In addition, the coupled motion and flow simulation of fishing gear, fishing methods, sea condition, and catches should be strengthened, providing an efficient approach for designing netting configurations, improving operational technology, and optimizing fishing gear hydrodynamics.