The size grading during fish farming is beneficial for growth management and feed control, improving aquaculture efficiency. A guided live fish grading device was designed to solve the problem of high labor intensity and low efficiency in domestic live fish grading. The structural composition and working principle of the grading device were introduced, and the mechanical analysis and parameter design of the fish conveying mechanism were carried out. Based on the relationship between fish body mass and body shape parameters, the morphological data of experimental crucian were measured, and the power function fitting method was used to obtain the equation of the relationship between crucian body thicknessand body weight, which serves as the basis for setting the distance between grading channels.When grading, the spacing between the guide plates on both sides of the grading channel is adjusted, and the grading channel is set to meet the fish specification requirementssothat fish's body through the grading channel. Live fish grading performance experimentalwas carried out, with fish weight range of 0.14~0.33kg and thickness range of 30~39mm. Grading tests and effect analysis were conducted under three conditions of fish conveying speeds of 0.15 m/s, 0.20 m/s, and 0.25 m/s. The experimental results show that under normal conditions, the grading accuracy can reach over 90%, and the appropriate grading speed and grading channel setting have higher efficiency. This guided grading device can quickly and accurately grade live fish, improving the efficiency of mechanized operations in the breeding process.

This study aimed to estimate the optimal light conditions for indoor cultivation of juvenile tiger puffer (Takifugu rubripes), The juvenile fish were exposed to five intensities of green light (G1:300 mW/m2, G2:500 mW/m2, G3:750 mW/m2, G4:1 000 mW/m2 and G5:1 500 mW/m2), white light (W) and dark (D) for 60 days. At the end of the experiment, the retinal structure of the juvenile fish was observed, and measurements were taken for retinal thickness, expression of visual protein genes, and changes in antioxidant enzyme activity. The results showed that different intensities of green light had no significant effect on the retinal structure of juvenile fish. Different degrees of melanin migration occurred in the retina of all treatment groups. Melanin granules were more aggregated in the G1 and D groups, while in the G4 and G5 treatment groups, the density of melanin granules was relatively large. The total retinal thickness (TT)decreased with the increase of green light intensity. The ratio of the thickness of each retinal layer to the total thickness was further analyzed. The results showed that the changes in retinal thickness showed different trends among different regions, with the G3 group having the highest proportion of photoreceptor layer (PRos/is/TT) (P<0.05), and the proportion of outer plexiform layer (OPL/TT) was significantly higher than those in other groups except G1 (P<0.05). The proportion of outer nuclear layer (ONL/TT) of juvenile fish in the G2 and G3 groups was significantly higher than that in the G1 and G4 groups (P<0.05). There was no significant difference in the proportion of pigment epithelium (RPE/TT) and ganglion cell layer (GCL/TT) among different treatment groups (P>0.05). Regarding the expression of opsins genes, Rod opsin expression was highest in groups exposed to white light (W)and dark conditions (D)(P<0.05). SWS2 expression in the G3 group was the highest (P<0.05). The expression of Opsin3 was the highest in the W group and the G5 group (P<0.05). For antioxidant enzymes, the activities of Superoxide Dismutase (SOD), Catalase (CAT) and Glutathione Peroxidase (GPX) in the green light treatment groups increased regularly with the increase of light intensity, and there was no extreme decrease. In view of the effects of different light intensities on retinal thickness and oxidative stress in eye tissues of juvenile tiger puffer in this study, it is recommended to use a white-colored cultivation substrate and green light in indoor cultivation environments. Additionally, it is advised to control the light intensity within the range of 500 to 750 mW/m2.

Crassostrea arakensis is one of the main fouling organisms in the ocean, which has a significant impact on marine aquaculture facilities and marine engineering equipment. Short wavelength and high-energy ultraviolet radiation (UV-C, 200-280 nm) can damage the DNA and protein structure of shellfish larvae, affecting their survival and attachment, and can be used to prevent biological fouling. This study selected the larvae of Crassostrea arakensis as the research object. The single variable method was used to study the lethal response of larvae under different intensities and doses of UV-C, as well as the differences in attachment rates of larvae under different UV-C irradiation frequencies and distances. The research results showed that there were significant differences (P <0.05) between some treatment groups and the control group during the observation time period in the 10 W shortwave UV-C irradiation experiment. After 24 hours, there were no surviving larvae in all treatment groups in the experimental units, and the survival rate of the control group was (26.3±1.9)%; In the 20W UV-C irradiation test, there was a significant difference in survival rates between the treatment group and the control group at all observation time periods (P <0.05). After 24 hours, there were no surviving larvae in all treatment groups in the experimental units, and the survival rate of the control group was (22.3±1.3)%; when the irradiation frequency reached 5 minutes/h, the average adhesion rate decreased from (12.83±1.17)% to (3.31±0.69)%, significantly reducing the adhesion rate; There was no significant difference in the irradiation effect of the attachment rate between the distance of 15 and 20 cm, and both groups were able to effectively reduce attachment compared to the control group. This study can provide references for the design of equipment for UV-C prevention and control of biological fouling.

Tilapia farming effluent contains a large amount of suspended particles such as uneaten feed and feces, which are the main carriers of nitrogen and phosphorus nutrients. Discharging the effluent into surrounding water bodies can lead to eutrophication. The common effluent treatment methods currently have disadvantages such as large land occupation, high investment, and low efficiency, which affect their widespread adoption. To address this issue, this paper explores the use of flocculation process to treat tilapia farming effluent, selecting four flocculants: polyferric sulfate, ferric chloride, chitosan, and magnesium carbonate. By comparing the removal effects on turbidity, total suspended solids (TSS), and chemical oxygen demand (COD), a suitable flocculant for effluent treatment is selected. The results show that polyferric sulfate outperforms the other three flocculants (ferric chloride, chitosan, and magnesium carbonate). With a dosage of 0.4 g/L, pH = 8, and flocculation time of 15 min, polyferric sulfate achieved removal rates of 96.7% for turbidity, 92.6% for TSS, and 95.1% for COD. Evaluation of the flocculated flocs revealed that the height of floc settling has a certain correlation with the turbidity removal rate, and the growth of the flocs is affected by nucleation. The zeta potential before and after effluent treatment suggests that polyferric sulfate purifies the effluent by disrupting the stable structure of suspended particles in the water. Therefore, polyferric sulfate can be selected as a flocculant in the treatment of tilapia farming effluent.

In order to investigate the mechanical performance of semi-submersible net cages under wave action, a parameter sensitivity analysis was conducted on the outer diameter of the cage components. A numerical hydrodynamic model of the semi-submersible net cage was established using the boundary element method, and after validating the accuracy of the model, a finite element method was introduced to calculate the stress distribution of the net cage. Subsequently, based on the Latin hypercube sampling method, sample schemes were extracted, and the Spearman coefficient of each design parameter was calculated to comprehensively analyze the sensitivity of each design parameter. Further individual analysis of parameter sensitivity was conducted, and recommended ranges for these parameters during structural optimization were provided. The results showed that the design parameters of the semi-submersible net cage were negatively correlated with the maximum stress within their range of values. Specifically, the sensitivity of the outer diameter of the lower support pipe was the highest, while the sensitivity of other parameters was relatively close. The recommended ranges for the outer diameter of the upper and lower arch pipes were relatively large, ranging from 36 cm to 56 cm and from 46 cm to 76 cm, respectively. The recommended ranges for the outer diameter of the middle arch pipe, inclined pipe, and slender columns were relatively small, ranging from 23 cm to 28 cm, 38 cm to 40 cm, and 50 cm to 60 cm, respectively. The outer diameter of the lower support pipe and upper support pipe should not be less than 70 cm and 50 cm, respectively. The research results can provide reference for the structural optimization of semi-submersible net cages under wave action.

In order to obtain accurate identification and real-time monitoring of mesh damage location, a new modeling method for mesh damage location based on acceleration measurement was proposed by combining physical test and numerical simulation. The method increases the data difference between the monitoring points in the broken area and the non-broken area, so as to determine the location of the broken fishing net. The fishing net physical tests and numerical simulation calculations were combined to study the fishing net breakage localization. Firstly, we carry out the physical model test of mesh coat damage, validate the numerical model of mesh coat, and use the validated numerical model of mesh coat to deeply explore the influencing factors affecting the modeling of mesh coat damage location detection, which serves as the basis of the mesh coat damage location detection model proposed in this study. The damage localization model includes four monitoring areas, and eight damage conditions are listed to test the localization detection ability of the model. The model uses artificial neural networks to localize and identify the damages. The Y-direction acceleration data collected from the monitoring points in the four regions were processed as inputs, and the eight types of damage and one intact case were used as outputs. The training data are the data obtained from numerical simulation and the test data are the sensing data from the physical model. (1) The results show that under the effect of regular waves, when the center of the mesh line is selected as the monitoring point, the data of the monitoring points in the broken and non-broken areas almost overlap, and the difference is within 1%. When the mesh node of the mesh coat is used as the monitoring point, the data change trends of the monitoring points in the two monitoring regions are different, and the data peak and change range of the monitoring point in the broken region at 210-240 s are three times of that of the monitoring point in the non-broken region. (2) Y-direction acceleration component comparing the acceleration, in addition to this broken when the data difference changes the same, with the increase of the degree of broken, the acceleration data changes in the intact state in the range of 0.75-1.2 times, while the Y-direction acceleration changes in the range of 0.75-23 times, and comparing the other direction of the acceleration component, the intact region and the broken region of the data difference is greater. (3) The accuracy of the model in locating the breakage in the training set is 98.5 % and the test accuracy is 97.9 %, which can locate the breakage accurately. In this study, we proposed a modeling method for detecting breakage of mesh garments based on region division, proved that using mesh nodes as monitoring points can increase the difference between monitoring points that are in the intact and broken regions, and the selection of acceleration component as an indicator parameter can increase the ability of the monitoring points to perceive the breakage. The results of this study provide some guidance for the modeling of digital twin of net clothing, and at the same time provide theoretical basis for the sensor arrangement and type selection of fishing nets.

Floats are the primary buoyancy component of plastic fish cages in offshore aquaculture. They can increase fish cage stress and deformation under the action of sea currents, and the rise of the float's surface liquid level can cause waves on the fish cage walkway. Although plastic fish cages are a recent development in aquaculture, little research has been conducted on the arrangement of floats. Therefore, studying the resistance performance of floats is significant for safe offshore aquaculture production. This article first studies the resistance of single and double floats through on-site measurement methods. Then, based on viscous flow theory, numerical simulations are carried out to simulate the resistance and waveforms of single and double floats under different flow velocities. The simulation results are compared with the on-site measurement results to verify the accuracy of the numerical simulation. Using the same numerical simulation method as above, numerical simulations are carried out for square and diamond two typical float arrangement forms, to analyze liquid level and resistance under different flow velocities. The results show that the numerical model agrees with the on-site measurement results, with an error of 9.5%. Three aspects are used to analyze the advantages and disadvantages of square and diamond arrangements. (1) Liquid level rise: When the flow velocity is the same, the total liquid level rise of the diamond-shaped floats arrangement is less than that of the square arrangement. (2) Flow field distribution in the middle of two floats: The flow field between the two floats of square arrangement is more chaotic than that of diamond arrangement. (3) Average resistance: When the flow velocity is greater than 1.5m/s, the resistance of diamond arrangement is 29% less than that of square arrangement. The results of the on-site measurement of the small boat towing test are compared with the results of the numerical simulation output. The resistance and waveforms agree with the test results, proving that the numerical simulation method used in this article is accurate and reliable and can provide technical support for subsequent numerical studies of floats. Analyzing the liquid level situation and liquid level cloud images of the two arrangements shows that the diamond arrangement has more stable liquid level changes and better air gap performance than the square arrangement, so it is more suitable for complex offshore environments. The diamond arrangement can provide a better working environment for offshore aquaculture. By sorting out and analyzing the results of numerical simulation, it is found that the average resistance of square arrangement and diamond arrangement is less than that of individual floats. Further comparing average resistance shows that diamond arrangement has less average resistance than square arrangement, so it is more suitable for complex offshore conditions. Floats are the primary buoyancy components of plastic fishing rafts used in offshore aquaculture. Under the influence of sea currents, the resistance generated by floats can result in increased force and deformation of the fishing raft. Additionally, the elevation of the liquid level on the float's surface can induce waves on the raft platform. Therefore, studying the resistance performance of floats is crucial for ensuring the safety of offshore aquaculture production. The resistance performance of both single and double floats was investigated through practical measurements at sea. Numerical simulations based on viscous flow theory were employed to analyze the resistance and waveform of single and double floats at various flow rates. The accuracy of the numerical simulations was validated by comparing the results with actual sea measurements. Furthermore, numerical simulations were conducted to assess the placement of two typical floats in square and diamond configurations. Liquid surface heights and resistance at different flow rates were analyzed. The findings indicate that numerical simulations align well with actual sea measurements, with an error rate of less than 9.5%. The diamond-shaped float configuration corresponds to an elevation in the liquid surface, creating a more stable flow field between the two floats. This results in a smaller average resistance, with values below 29% in flow velocities exceeding 1.5 m/s. Consequently, it is recommended that this placement configuration be adopted for production operations.

Inertia force coefficient CM and resistance coefficient CD of monofilament and rope used in tuna longline gear directly affect the computational speed and accuracy of numerical simulations of fishing gear. In this study, wave forces acting on seven types of monofilament and rope materials for tuna longline gear under regular wave conditions were investigated through wave tank experiments. Utilizing the Morison equation and employing the least squares method, CM and CD for respective materials were calculated. The variations of CM and CD with Keulegan-Carpenter number KC and Reynolds number Re were analyzed. The results indicate that: (1) Under the same wave height, the wave forces acting on experimental materials gradually increase with increasing wave period. (2) Under the same wave period, the wave forces acting on experimental materials gradually increase with increasing wave height. (3) With increasing diameter, the wave forces acting on experimental materials also increase. (4) For three types of nylon monofilament, CD gradually increases with the increasing of KC number, while CM remains relatively stable, and CM and CD exhibit an increasing trend with the increasing of Re . (5) For three types of polypropylene ropes, CM increases with the increasing of KC and Re number, while CD stabilizes at lower KC numbers and decreases gradually with the increasing of Re . (6) Polypropylene ropes coated with asphalt exhibit relatively stable CM and CD values. The obtained CD and CM values from this experiment can be applied to the numerical simulations of longline fishing gear.

The results of hydrodynamic model experiment of otter board will have different degree of test deviation due to various factors such as processing method and structure simplification. In order to explore the influence mechanism of the processing materials of the otter board model on the hydrodynamic experiment, the double-slit rectangular curved otter board was selected as the research object. Based on the fluid-structure coupling algorithm combined with the flume tank model experiment, the influence on the hydrodynamic experiment and its structural response difference between the three 3D printing materials otter board and the traditional steel materials otter board was analyzed. The results show that there are significant differences in structural response and hydrodynamic performance between the traditional metal (Steel) and non-metal (ABS) models. With the increase of the angle of attack and the flow rate, the maximum deformation of the otter board increases, and the difference of lift coefficient and drag coefficient also increases. Due to the large elastic modulus and strong rigidity of PVC material, the structural response is close to that of metal material, so it has little influence on the results of hydrodynamic experiment. Based on the above results, PVC can be used as the 3D printing material to ensure the accuracy of the model and reduce the influence of the model material on the accuracy of the hydrodynamic experiment results.

In marine biology research and fishing industries, ships using underwater robots for capturing and identifying marine organisms may face limited communication bandwidth and computing resources, making lightweight network models a better fit for such conditions.To address this issue, a modified YOLOv5 model for marine organism detection was proposed, incorporating improvements in the neck network, model pruning, and knowledge distillation techniques. By leveraging the Gsconv lightweight convolution module to replace standard convolutions in the YOLOv5n Neck section, the model was effectively reduced. Additionally, a novel α-giou loss function was adopted to enhance bounding box regression accuracy. L1-norm regularization pruning was applied to eliminate unnecessary channels and associated convolutional kernels based on weight coefficients. Finally, retraining and L2 knowledge distillation were employed to fine-tune the model accuracy close to pre-pruning levels. Experimental results demonstrated a 53% reduction in computational load and a 51% decrease in parameters compared to the original YOLOv5n baseline network. The proposed algorithm ensures the effectiveness of lightweight processing while maintaining model performance, offering a promising approach for lightweight handling of marine organism recognition models for underwater robots.

 With the development of the prefabricated shellfish food industry, precision processed pre made foods based on shellfish are gradually being promoted in the market. Although there are many types of cleaning devices in the market, there is no suitable equipment for cleaning various types of shellfish products, which has problems such as low cleaning efficiency, poor cleaning rate, and easy damage. Based on the mechanism of submerged water jet, this article designs a shellfish cleaning device using submerged water jet as the cleaning power, and conducts Fluent-EDEM coupled numerical simulation on the cleaning device to determine the key structural parameter range of the cleaning device. The rationality of the cleaning device was demonstrated through two-phase experiments. In the first phase, through orthogonal cleaning experiments, it was found that the submerged water jet cleaning method had the best cleaning effect when the jet diameter was 10 mm, the cleaning amount was 14 kg, and the cleaning time was 6 minutes, respectively. In the second phase, a production comparison experiment was conducted, and the cleaning rate and damage rate were used as evaluation indicators to compare the cleaning rates of three cleaning devices. The experimental results show that when using scallop skirts as cleaning objects, the average cleaning rate of the submerged water jet cleaning device is 97.77%, and there is no damage phenomenon. The cleaning rate of the submerged water jet cleaning device is 14.4% higher than that of the propeller cleaning device, and 7.3% higher than that of the ultrasonic cleaning device. When using Cockle meat as the cleaning object, the average cleaning rate of the submerged water jet cleaning device is 98.23%, which is 12.49% higher than the propeller cleaning device and 6.7% higher than the ultrasonic cleaning device, and there is no damage. This indicates that the submerged water jet device has a good cleaning effect and has industrial promotion value.