Design and Experimental Optimization of an Automated Longline-Suspended Oyster Spat Insertion Device

Abstract

To address the challenges of labor-intensive and costly manual oyster spat insertion in longline-suspended farming, an automated oyster spat insertion device was designed based on negative pressure suction and bundling fixation technologies. Using this device as the experimental platform, a three-factor, three-level Box–Behnken experiment was conducted, with fixation mechanism inclination, negative pressure suction cup span, and horizontal distance between turnover and fixation mechanisms as the experimental factors. The performance of the device was evaluated using the effective fixation rate and damage rate as the experimental indicators. The quadratic polynomial regression models were established to assess the impact of these factors on operational performance, while the response surface method was employed to analyze the interaction effects between factors. Parameter optimization and experimental validation were also performed. The results indicate that the factors affecting the effective fixation rate, in order of significance, are as follows: fixation mechanism inclination, horizontal distance between turnover and fixation mechanisms, and negative pressure suction cup span. For the damage rate, the order of significance is as follows: fixation mechanism inclination > negative pressure suction cup span < horizontal distance between turnover and fixation mechanisms. The optimization results show that when the fixation mechanism inclination is set at 43°, the negative pressure suction cup span at 27 mm, and the horizontal distance between turnover and fixation mechanisms at 179 mm, the effective fixation rate reaches 92.08%, and the damage rate is 4.71%. The relative errors between the measured and model-predicted values are less than 5%, indicating that the regression models are reliable. This research provides valuable insights for advancing the mechanization of the oyster farming industry and replacing manual labor with mechanized equipment.

1. Introduction

Oysters are the world’s most widely farmed shellfish, prized for their economic and ecological values [1]. Oyster farming is prevalent in nearly all coastal nations [2], with major production concentrated in China, Japan, and Korea in Asia, France and Denmark in Europe, and the U.S. and Canada in North America [3]. Globally, China holds the largest share of oyster production [4]. In 2023, China produced approximately 6.67 million tons of oysters, accounting for 40.53% of total global shellfish mariculture [5], making it a significant contributor to both the domestic and international oyster farming industries [6,7].

There are various methods for oyster cultivation, and as the industry continues to evolve, new techniques and technologies are constantly emerging [8]. Common farming methods include raft culture and stake culture, with raft culture further categorized into longline suspension, raft suspension (rigid raft), and cage culture [5,9]. In recent years, longline-suspended oyster farming has gained popularity due to its fast growth, high yield, and excellent economic returns [10,11]. This method involves stringing oyster spat carriers—usually made of perforated scallop or oyster shells—along a longline, which is then suspended below a buoyant raft structure for cultivation [12]. The spat insertion process is a critical step in this method. Automating and mechanizing spat insertion can significantly reduce costs, enhance efficiency, and boost industrial growth. However, the current process heavily relies on manual labor to thread spat carriers onto the culture ropes, the lack of operating equipment limiting the industry’s overall mechanization [13]. As a result, there is a pressing need for mechanized equipment to overcome the challenges of high labor intensity and low mechanization in longline-suspended oyster farming.

While much of the research on oyster farming has focused on spat collection [14,15], environmental assessments [16,17], seedling breeding [18], and genetic traits [19], less attention has been given to the spat insertion process. In China, studies have largely concentrated on the preparation, perforation, cleaning, and conveying of adhering substrates for spat insertion [13,20]. Although some scholars have explored mechanized solutions for longline-suspended oyster spat insertion and clamping [21,22,23], these efforts remain in the theoretical and experimental stages. There is a noticeable lack of applied research and case studies on mechanized spat insertion equipment. Negative pressure suction technology, which offers reliable adsorption to uneven or cracked surfaces [24,25,26], has been widely used in industries such as aerospace, manufacturing, and biomedicine [27,28,29]. Similarly, bundling technology [30,31], which clusters single or multiple items together, has seen widespread application across various sectors, with bundling machinery advancing towards standardization, modularization, and smart technology [32,33,34]. Thus, applying mature technologies such as negative pressure suction and bundling to automate oyster spat insertion holds great promise for advancing the mechanization of the oyster industry.

This study, based on the structure of longline-suspended oyster spat strings, developed an automated oyster spat insertion device that integrates functions such as spat carrier separation, turnover, fixation, conveying, and storage. Using this device as the experimental platform, the performance tests and parameters optimization were conducted to understand the effects of these parameters on the effective fixation rate and damage rate of the spat carriers. This paper aims to enhance the quality of mechanized oyster spat insertion and provide a reference for the development of mechanized oyster farming equipment.

2. Materials and Methods

2.1. Equipment Structure and Operating Principle

After oyster spats have attached to their carriers in the seedling nursery, these carriers are used for subsequent spat insertion. One of the challenges is effectively separating individual spat carriers from those connected by a glass wire rope, as conventional mechanical structures struggle with this task. To address this, the study employs negative pressure suction technology [25], where suction cups are used to detach each spat carrier. Given the uneven surface of the spat carriers, which could compromise the vacuum environment within the suction cups, optimizing the structure and size of the suction cup is crucial for ensuring stable separation. Based on common suction cup designs and their application scenarios [29], this study opted for the corrugated suction cup. The base of the cup is fitted with a soft texture and airtightness sponge, which conforms to the shape of the spat carrier, enabling a better seal and improving the stability of the suction process. In manual spat insertion, complex actions such as unwinding, perforating, and threading the glass wire rope—on which the spat carriers are strung—through the breeding ropes are required. This process prevents the spat carriers from loosening during growth, which could impede the oysters’ development. However, these intricate steps significantly complicate the mechanization of spat insertion. To simplify the mechanical structure of the oyster spat insertion device, this study references existing bundling technologies. It proposes a method where the glass wire rope, carrying the oyster spat carriers, is bundled together with the breeding rope, forming an integrated oyster seedling rope for spat insertion, as illustrated in Figure 1.

The automated longline-suspended oyster spat insertion device (hereinafter referred to as the “spat insertion device”) consists primarily of a conveying and separating mechanism, a turnover mechanism, a sorting and feeding mechanism, a fixation mechanism, and a seedling rope storage mechanism. This spat insertion device is capable of completing the entire process—material feeding, oyster spat carrier separation, turnover of the spat carrier, bundling, and conveying and storage—in a single operation. During operation, under the vibration of the sorting and feeding mechanism, cable ties are continuously delivered to the fixation mechanism. The spat insertion device, through the conveying and separating mechanism, separates individual oyster spat carriers from a string of spat carriers. With the coordinated operation of the conveying and separating mechanism and the turnover mechanism, each separated oyster spat carrier is lifted between the two turnover claws of the turnover mechanism, which then sequentially transports the carriers to the fixation mechanism. The fixation mechanism works to bundle the glass wire rope threaded through the spat carriers together with the breeding rope positioned above the fixation mechanism using cable ties, securing the spat carriers between two cable ties on both the glass wire rope and the breeding rope. After bundling and securing, the completed breeding rope is clamped by the seedling rope storage mechanism, conveyed backward by a set distance, and then stored in a rope basket. This allows the device to achieve fixed-distance spat insertion and storage for longline-suspended oyster seedling ropes.

2.2. Control System

The spat insertion device can connect and secure individual oyster spat carriers, separated from the spat strings, at specific intervals to produce longline-suspended oyster seedling ropes according to actual production requirements. Its automatic control system is implemented through PLC logic programming, integrating functions such as material feeding, conveying and separating, turnover, bundling, and seedling rope storage. To enable the device’s automated spat insertion function, limit signals are applied to the corresponding actuators. When a mechanical fault occurs, the limit signal can’t reach its position, and the next driver will not be executed; the corresponding command will only be executed if the limit signal is detected, ensuring precise control of the actuator’s position during operation. As shown in Figure 2, Figure 2a illustrates the circuit diagram of the automatic control system for the oyster spat insertion process, and Figure 2b shows the wiring diagram of the stepper motor driver.

Before starting operation, the system needs to be reset. At this point, all actuators are in their initial positions: the seedling rope is guided into position above the fixation mechanism, and the glass wire rope, threaded with oyster spat carriers, is positioned by the turnover mechanism, securely tied to the seedling rope, and clamped and tightened at the storage device. Once these steps are completed, the oyster spat insertion process can begin. The control system operates as follows:

The sorting and feeding motor (Q1.0) starts, and the negative pressure suction cup extends (Q8.0), reaching its limit (I8.0). The oyster spat string conveying motor moves forward, elevating the oyster spat string to the upper limit (I0.3), and then stops. The vacuum generator is activated (Q8.2), and the suction cup begins to lift the spat carrier (Q8.1), reaching the upper limit (I8.2). The oyster spat string conveying motor reverses, and the vacuum generator halts. The oyster spat string descends to the preset position (I0.4), and the conveying motor stops. The negative pressure suction cup retracts (Q8.0) to its limit (I8.1) and descends (Q8.1) to the lower limit (I8.3), after which the negative pressure suction cup extends again (Q8.0).

Next, the turnover motor is activated, and the spat carriers are turned between the turnover claws, reaching the turnover claw limit (I0.5). The spat carriers are then transferred to the fixation mechanism for bundling, and the turnover motor stops. The cable tie feeding motor starts, and the feed stopper returns to its initial position (I0.6), prompting the motor to stop. The cable tie guiding motor moves forward to the preset position (I0.7) and then stops. The guiding motor reverses to its initial position (I1.0), after which it stops. The cable tie tightening motor engages, moving to its present position (I1.1) before stopping. The cable tie cutting motor then starts, and once the cutter returns to its initial position (I1.2), the motor stops. At this point, the seedling rope, glass wire rope, and spat carriers are bundled together. The seedling rope storing cylinder descends (Q8.4) until it reaches its limit (I8.5). The seedling rope clamping cylinder opens (Q8.3) to its limit (I8.7), allowing the seedling rope storing cylinder to ascend (Q8.4) to its upper limit (I8.4). The seedling rope clamping cylinder then closes (Q8.3) to its limit (I8.6) and grips the seedling rope, completing the oyster spat insertion. This process is repeated to achieve automated oyster spat insertion.

Key synchronization points include:

• The continuous action of the sorting and feeding motor (Q1.0).
• The extension of the negative pressure suction cup (Q8.0), and the forward movement of the oyster spat string conveying motor, which are both triggered by the suction cup descending limit (I8.3).
• The vacuum generator activation (Q8.2), the seedling rope storing cylinder descends (Q8.4) and turnover motor action, triggered by the upper limit switch (I0.3).
• The synchronization of the cable tie guiding motor forward motion and the cable tie tightening motor action, triggered by the feed stopper position (I0.6).

2.3. Performance Testing on Oyster Spat Insertion Device

2.3.1. Experimental Materials

During the performance testing of the oyster spat insertion device, common nylon cable ties with dimensions of 3.6 × 120 mm were used for automatic bundling. The seedling ropes used in the tests were polyethylene ropes with a diameter of 6 mm, while glass wire ropes with a diameter of 1.5 mm were employed to connect the oyster spat carriers. Split-half, center-perforated scallop shells, which are commonly used as an adhering substrate during spat collection, were used to form oyster spat strings for this performance testing.

2.3.2. Experimental Factors and Indicators

The key to successful oyster spat insertion lies in effectively securing the oyster spat carriers, glass wire rope, and seedling rope together at a specified distance without damaging the spat carriers. This is reflected in two critical performance indicators: the effective fixation rate and the damage rate, which measure the operational performance of the spat insertion device. Several experimental factors can influence the performance of automated oyster spat insertion. Based on preliminary experimental research, three key operational parameters were identified as having a significant impact on the effectiveness of the spat insertion: the fixation mechanism inclination, the negative pressure suction cup span, and the horizontal distance between turnover and fixation mechanisms. These parameters were chosen as the experimental factors, with the effective fixation rate and damage rate of the spat carriers as the performance indicators. The performance testing and parameter optimization were conducted based on these factors. The formulas for calculating the experimental indicators are as follows:

E = N_b/N_a × 100%

(1)

where E represents the effective fixation rate of the spat carriers, N_a is the total number of spat insertions in each test group, and N_b is the number of tests in which the seedling rope and glass wire rope were successfully bundled, with the spat carriers strung on the glass wire rope and bundled between two cable ties at the preset distance.

D = N_c/N_a × 100%

(2)

where D represents the damage rate of the spat carriers, and N_c is the number of tests in which the spat carriers were damaged (e.g., cracked or broken) due to extrusion, impact, or other factors, rendering them unsuitable for subsequent normal oyster growth.

2.3.3. Experimental Design and Method

Building on the results of previous experimental studies, it was found that when the fixation mechanism inclination exceeds 50°, the cable ties delivered by the sorting and feeding mechanism are prone to tail overlap, which obstructs the fixation process. Conversely, when the inclination is less than 40°, it facilitates sequential feeding of the cable ties, but the distance between the seedling rope and the glass wire rope at the bundling point becomes too large, making it difficult for the cable ties to gather the glass wire rope towards the seedling rope, resulting in failed operation. To ensure stable suction of the spat carriers, the conveying and separating mechanism was designed with negative pressure suction cups having a diameter of 20 mm. The span between the two suction cups should be no less than 20 mm, and if the span exceeds 40 mm, one of the suction cups may extend beyond the edge of the spat carrier during operation, compromising the stability of the suction and increasing the risk of mid-operation drops, which can disrupt the continuity of the spat insertion process. When the horizontal distance between turnover and fixation mechanisms is less than 160 mm, spat carriers conveyed by the turnover mechanism tend to get blocked above the fixation mechanism, which could lead to the breakage of the spat carrier and unsuccessful fixation. However, when the horizontal distance exceeds 200 mm, although the spat carriers are conveyed smoothly, the glass wire rope is too far from the fixation mechanism, making it difficult for the cable ties to bundle the glass wire rope to the seedling rope, thus preventing normal operation. Based on these experimental observations, the experimental parameters and their respective ranges were defined as follows: fixation mechanism inclination: 40° to 50°; negative pressure suction cup span: 20 to 40 mm; horizontal distance between turnover and fixation mechanisms: 160 to 200 mm.

A Box–Behnken experiment is simpler and more economical in studying the effects of multiple experimental factors on the response variable, therefore, a three-factor, three-level Box–Behnken experiment was designed and conducted based on these experimental factors and performance indicators [34]. The experimental factors, codes, and levels are presented in

Table 1. Experimental factors, codes, and levels.

Factors	Code	Levels
		−1	0	1
fixation mechanism inclination (°)	X1	40	45	50
negative pressure suction cup span (mm)	X2	20	30	40
horizontal distance between turnover and fixation mechanisms (mm)	X3	160	180	200

, while the experimental design and response values for the performance indicators are shown in

Table 2. Test design and response values.

Test	X1	X2	X3	Response Values
				E (%)	D (%)
1	0	−1	1	88.62	4.79
2	−1	−1	0	90.19	3.92
3	0	1	1	84.05	3.88
4	1	1	0	84.51	6.39
5	0	−1	−1	90.22	8.91
6	0	0	0	93.35	5.53
7	0	0	0	92.16	4.78
8	0	0	0	91.67	4.47
9	−1	1	0	84.76	3.36
10	−1	0	1	82.34	2.56
11	0	0	0	94.33	5.61
12	−1	0	−1	86.96	5.26
13	0	1	−1	88.92	5.37
14	1	0	1	83.13	5.91
15	1	0	−1	81.78	12.21
16	1	−1	0	82.53	11.05
17	0	0	0	93.29	5.05

. The orthogonal test design includes 17 experimental points, consisting of 12 factor analysis points and 5 zero-point error estimates. (X₁, X₂, and X₃ represent the coded values for the fixation mechanism inclination, the negative pressure suction cup span, and the horizontal distance between turnover and fixation mechanisms, respectively).

3. Results and Discussion

3.1. Regression Models and Significance Analysis

Based on the experimental design and response values in Table 2, multivariable regression fitting was performed [35,36]. The following quadratic polynomial regression models were established for the effective fixation rate (E) and the damage rate (D) corresponding to the coded values of the fixation mechanism inclination (X₁), the negative pressure suction cup span (X₂), and the horizontal distance between turnover and fixation mechanisms (X₃):

E = 92.96 − 1.54X₁ − 1.16X₂ − 1.22X₃ + 1.85X₁X₂ + 1.49X₁X₃ − 0.82X₂X₃ − 5.93X₁² − 1.53X₂² − 3.48X₃²

(3)

D = 5.09 + 2.56X₁ − 1.21X₂ − 1.83X₃ − 1.03X₁X₂ − 0.90X₁X₃ + 0.66X₂X₃ + 0.92X₁² + 0.17X₂² + 0.48X₃²

(4)

Variance analysis was conducted for these regression models, with the results presented in

Table 3. Analysis of variance of regression models.

Source	Effective Fixation Rate (E, %)				Damage Rate (D, %)
	Sum of Squares	Degree of Freedom	F-Value	p-Value 1	Sum of Squares	Degree of Freedom	F-Value	p-Value
Model	293.53	9	28.53	0.0001 **	104.86	9	33.17	<0.0001 **
X1	18.91	1	16.54	0.0048 **	52.33	1	148.95	<0.0001 **
X2	10.86	1	9.50	0.0178 *	11.69	1	33.27	0.0007 **
X3	11.86	1	10.37	0.0146 *	26.68	1	75.95	<0.0001 **
X1X2	13.73	1	12.01	0.0105 *	4.20	1	11.96	0.0106 *
X1X3	8.91	1	7.79	0.0268 *	3.24	1	9.22	0.0189 *
X2X3	2.67	1	2.34	0.1701	1.73	1	4.92	0.0620
X12	148.13	1	129.56	<0.0001 **	3.56	1	10.14	0.0154 *
X22	9.87	1	8.64	0.0218 *	0.12	1	0.36	0.5697
X32	50.88	1	44.51	0.0003 **	0.96	1	2.73	0.1425
Residual	8.00	7			2.46	7
Lack of fit	3.56	3	1.07	0.4561	1.51	3	2.13	0.2389
Pure error	4.44	4			0.95	4
Total	301.53	16			107.32	16

¹ Asterisks indicate significance level: ** = p < 0.01 (highly significant), and * = p < 0.05 (significant).

. As indicated in Table 3, the significance levels (p-values) for both the effective fixation rate (E) and the damage rate (D) models are less than 0.01, demonstrating that both models are highly significant. The lack-of-fit significance levels for both models are greater than 0.05 (0.4561 for E and 0.2389 for D), indicating that the models exhibit a high degree of fit. As such, the regression models can be reliably used in place of the actual experimental data points for analysis. The determination coefficients (R²) for both models exceed 0.97 (0.9735 for E and 0.9771 for D), indicating that the models explain more than 97% of the variability in the evaluation indicators. Therefore, these models can be utilized to optimize the operational parameters of the automated oyster spat insertion device.

Furthermore, the p-values in Table 3 reveal the significance of each regression model’s impact on the experimental indicators. Specifically: X₁ (fixation mechanism inclination) has a highly significant impact on both the effective fixation rate and the damage rate; X₂ (negative pressure suction cup span) and X₃ (horizontal distance between turnover and fixation mechanisms) significantly influence the effective fixation rate and have a highly significant impact on the damage rate; the interaction terms X₁X₂ and X₁X₃ have significant effects on both indicators, while the interaction between X₂ and X₃ is not significant for either index; the quadratic term X₁² has a highly significant effect on the effective fixation rate and a significant effect on the damage rate. X₂² has a significant effect on the effective fixation rate but no significant impact on the damage rate. X₃² has a highly significant impact on the effective fixation rate but does not significantly affect the damage rate. The regression models were further optimized by eliminating the insignificant terms. The results of this optimization are as follows:

E = 92.96 − 1.54X₁ − 1.16X₂ − 1.22X₃ + 1.85X₁X₂ + 1.49X₁X₃ − 5.93X₁² − 1.53X₂² − 3.48X₃²

(5)

D = 5.38 + 2.56X₁ − 1.21X₂ − 1.83X₃ − 1.03X₁X₂ − 0.90X₁X₃ + 0.96X₁²

(6)

After analyzing the optimized models, it was found that both the effective fixation rate and the damage rate models remained highly significant, with p-values of less than 0.01. The lack-of-fit p-values were 0.3753 for the effective fixation rate and 0.1482 for the damage rate. The determination coefficients (R²) of the optimized models were 0.9646 and 0.9505, respectively. These results confirm that the optimized regression models are highly significant and exhibit a good fit, making them reliable for further use.

The significance of the parameters’ impact on the regression model can be evaluated by their p-values. The ranking of the significance of the three experimental factors’ impact on the effective fixation rate is as follows: fixation mechanism inclination, horizontal distance between turnover and fixation mechanisms, negative pressure suction cup span. For the damage rate, the order of significance is as follows: fixation mechanism inclination > negative pressure suction cup span < horizontal distance between turnover and fixation mechanisms.

3.2. Response Surface Analysis

The 3D response surface plots illustrating the significant interactions between factors and their impact on effective fixation rate and damage rate are shown in Figure 3 and Figure 4. These response surface diagrams allow us to analyze how the interaction of experimental factors affects the response values of the experimental indicators.

Figure 3a depicts the response surface of the interaction between X₁ (fixation mechanism inclination) and X₂ (negative pressure suction cup span) on the effective fixation rate, where the horizontal distance between turnover and fixation mechanisms is fixed at the central level. From the contour map, we observe that when the horizontal distance is 180 mm, the effective fixation rate increases with the fixation mechanism inclination, but after reaching a peak, it begins to decline. When the inclination is below the central level, the effective fixation rate decreases gradually as the negative pressure suction cup span increases. Conversely, when the inclination is above the central level, the effective fixation rate initially increases and then decreases as the negative pressure suction cup span increases. This indicates that the effect of the negative pressure suction cup span on the effective fixation rate varies at different levels of the fixation mechanism inclination, highlighting the interaction between these two factors. Figure 3b shows the response surface of the interaction between X₁ (fixation mechanism inclination) and X₃ (horizontal distance between turnover and fixation mechanisms) on the effective fixation rate, where the negative pressure suction cup span is set at 30 mm. Similarly, the effective fixation rate initially rises and then decreases as both the inclination and the horizontal distance increase. When the inclination is low, the effective fixation rate increases slightly with the horizontal distance, but then declines significantly, showing a more pronounced change. At higher inclination, the effective fixation rate also rises and then decreases, but the change is more gradual, indicating that the impact of the horizontal distance on the effective fixation rate varies at different levels of the fixation mechanism inclination, demonstrating the interaction between these two factors.

Figure 4a illustrates the response surface of the interaction between X₁ (fixation mechanism inclination) and X₂ (negative pressure suction cup span) on the damage rate, with the horizontal distance between turnover and fixation mechanisms set at the central level. From the contour map, it can be seen that when the horizontal distance is 180 mm, the damage rate increases gradually with the inclination. When the inclination is low, the damage rate decreases slightly as the negative pressure suction cup span increases, with a gentle change. However, at higher inclination, the damage rate decreases significantly as the negative pressure suction cup span increases, indicating that the impact of the negative pressure suction cup span on the damage rate varies at different levels of the inclination, demonstrating the interaction between these two factors. Figure 4b presents the response surface of the interaction between X₁ (fixation mechanism inclination) and X₃ (horizontal distance between turnover and fixation mechanisms) on the damage rate, with the negative pressure suction cup span set at 30 mm. As the inclination increases, the damage rate rises gradually, while it decreases with an increase in the horizontal distance. When the inclination is low, the damage rate decreases slightly with an increase in horizontal distance, showing a gentle change. At higher inclination, the damage rate decreases more significantly as the horizontal distance increases, indicating that the horizontal distance’s impact on the damage rate varies at different levels of the fixation mechanism inclination, illustrating the interaction between these two factors.

3.3. Parameter Optimization

To achieve optimal performance for the automated oyster spat insertion device, the goal is to maximize the effective fixation rate while minimizing the damage rate. From the factor level effect analysis, it was shown that a higher effective fixation rate could be obtained with moderate fixation mechanism inclination, lower negative pressure suction cup span, and moderate horizontal distance between turnover and fixation mechanisms, while lower damage rate could be obtained with lower fixation mechanism inclination, higher negative pressure suction cup span, and higher horizontal distance between turnover and fixation mechanisms. It is clear that each factor, along with the interactions between factors, influences the experimental index response values to varying degrees. To find the optimal parameter combination for the device, a comprehensive multi-objective parameter optimization is necessary, taking into account the effects of all factors on performance indicators.

Based on the analysis of the regression models and the operating conditions for the automated oyster spat insertion device, the constraint conditions for the experimental factors were set as follows: fixation mechanism inclination: 40° to 50°, negative pressure suction cup span: 20 to 30 mm, horizontal distance between turnover and fixation mechanisms: 160 to 200 mm. The effective fixation rate (E) was set as the maximum value (100%) of the objective function, and the damage rate (D) was set as the minimum value (0). The optimal results from the analysis were as follows: fixation mechanism inclination: 43.12°, negative pressure suction cup span: 26.67 mm, horizontal distance between turnover and fixation mechanisms: 179.08 mm. With these parameters, the model predicted an effective fixation rate of 93.22% and a damage rate of 4.89%.

3.4. Verification Testing

To verify the reliability of the model’s predictions, five verification tests were conducted using the optimized parameters, and the average of the results was calculated. Considering the actual operational conditions of the device, the optimized parameters were rounded as follows: fixation mechanism inclination: 43°, negative pressure suction cup span: 27 mm, horizontal distance between turnover and fixation mechanisms: 179 mm. The oyster spat insertion verification tests were performed according to the adjusted parameters. The testing site is shown in Figure 5, and the results of the tests are presented in

Table 4. Comparison of model-based predicted values with experiment results.

Item	Effective Fixation Rate (E, %)	Damage Rate (D, %)
Average experimental value	92.08	4.71
Model-based predicted value	93.22	4.89
Relative error	1.22	3.68

.

As shown in Table 4, the relative error between the experimental value and the model-optimized value for the effective fixation rate (E) is 1.22%, while the relative error for the damage rate (D) is 3.68%. Both relative errors are less than 5%, indicating that the parameter optimization model is reliable. Therefore, the combination of a fixation mechanism inclination of 43°, a negative pressure suction cup span of 27 mm, and a horizontal distance of 179 mm between the turnover and fixation mechanisms can be considered an optimal operational parameter set for the oyster spat insertion device, ensuring effective and efficient spat insertion.

4. Conclusions

In this study, an automated longline-suspended oyster spat insertion device was developed, integrating functions such as spat separation, turnover, fixation, conveying, and storage. An automatic control system was also designed to manage the actions of each actuator. Performance testing of the spat insertion was conducted, focusing on three key operational parameters: the fixation mechanism inclination, the negative pressure suction cup span, and the horizontal distance between turnover and fixation mechanisms. Quadratic polynomial regression models were established to analyze the effects of these parameters on the effective fixation rate and damage rate.

The performance tests revealed that the significance of the experimental factors affecting the effective fixation rate ranked as follows: fixation mechanism inclination, horizontal distance between turnover and fixation mechanisms, negative pressure suction cup span. For the damage rate, the significance of the factors was: fixation mechanism inclination > negative pressure suction cup span < horizontal distance between turnover and fixation mechanisms. The interactions between the fixation mechanism inclination and the negative pressure suction cup span, as well as between the inclination and the horizontal distance, had a significant impact on both performance indicators, whereas the interaction between the negative pressure suction cup span and the horizontal distance was not significant.

The parameter optimization and verification tests showed that the optimal combination of parameters for the spat insertion device was a fixation mechanism inclination of 43°, a negative pressure suction cup span of 27 mm, and a horizontal distance of 179 mm between turnover and fixation mechanisms. Under these conditions, the effective fixation rate was 92.08%, and the damage rate was 4.71%, with the relative errors between the experimental and model-predicted values being less than 5%.

In this study, center-perforated scallop shells were used as spat carriers, however, in longline-suspended oyster aquaculture, materials such as oyster shells are also commonly used as spat carriers. Therefore, in future research, it will be necessary to further optimize the design of the actuators to ensure that the automated oyster spat insertion device can accommodate different spat carrier materials.