Influencing Factors on Oyster Recruitment and Performance Evaluation for Oyster Reef Restoration in Tianjin Coastal Zones

Abstract

Global oyster reefs have suffered severe degradation due to human activities, environmental pollution, and climate change. The construction of artificial reefs offers a promising strategy to enhance oyster recruitment and mitigate population decline. However, the factors influencing oyster recruitment in artificial frameworks remain largely unclear, and it is still challenging to evaluate the effectiveness of different restoration methods. In this study, a series of oyster reef restoration experiments were conducted in the Tianjin coastal zone to identify key factors affecting recruitment success. These factors included restoration methods (shell string, mesh bag, and mesh cage), seeding with juvenile oysters, oyster shell orientation, and cultch hanging height. Our results indicated that the mesh bag method achieved the highest oyster settlement density in the intertidal zone, while the shell string method supported the fastest growth rates of oysters in the subtidal zone. The lower hanging height of cultches in the artificial frames increased oyster settlement density; however, hanging the cultches too close to the sediment negatively impacted oyster growth rates. Additionally, seeding with juvenile oysters and orienting the rough side of the shell upward enhanced recruitment performance. Oyster settlement density was greater in the intertidal zone (Bagua Shoal) compared to the subtidal zone (Dashentang), while oysters in the subtidal zone exhibited faster growth rates. Using redundancy analysis, the influence of environmental factors on the oyster recruitment performance was assessed. Oyster growth in the subtidal zone was positively correlated with dissolved oxygen and pH, whereas oyster settlement density in the intertidal zone was positively associated with water temperature, chlorophyll a concentration, and salinity. Finally, we evaluated the effectiveness of different restoration methods by considering factors including reef construction costs, oyster settlement abundance, average daily shell growth rate, water purification potential, and reef subsidence. Our results demonstrated that the shell string method was the most effective in the Dashentang subtidal zone, while the mesh bag method with oyster seeding was optimal in the Bagua Shoal intertidal zone. Our findings can provide valuable insights and guidance for oyster reef restoration projects.

1. Introduction

Oyster reefs are biological reefs found in estuarine and coastal areas, formed by oysters that attach and grow on hard substrates. As an important component of marine ecosystems, oyster reefs provide critical ecosystem services, such as habitat provision [1,2,3,4], water purification [5,6,7], energy coupling [8], shoreline erosion prevention [9,10], and carbon sequestration [11,12], contributing significant ecological and economic value [13]. They play a vital ecological role in the intertidal and subtidal zones where they reside, benefiting the surrounding environmental and ecological development [14]. However, factors such as coastal development, dredging, overfishing [15], climate change, pollution, disease, aquaculture, and the invasion of non-native species [16,17,18,19] have exacerbated the global degradation of oyster reefs [13]. It is estimated that 85% of global oyster habitats have been lost, and the majority of the remaining natural oyster populations are in poor condition [13]. The health of oyster reefs directly impacts marine biodiversity and the living environment of humans. The degradation of oyster reef ecosystems has led to phenomena such as the occurrence of red tides, declines in fishery resources, the destruction of intertidal ecosystems, and a shortage of intertidal species [20,21,22]. Therefore, the restoration of oyster reefs is an immediate necessity.

The construction of artificial frameworks in marine ecosystems to restore oyster reefs is a common approach. These structures exhibit performance comparable to that of natural oyster reefs. Deploying artificial frameworks in areas with declining oyster populations can help ensure the restoration of these valuable ecosystems [23]. During the process of using artificial frameworks to rebuild oyster reefs, factors such as height, spacing, and structural design are key elements in reef formation. The height of the framework plays a critical role in determining the long-term sustainability and adaptability of the oyster population [24,25]. The evaluation of the restoration effectiveness of oyster reefs primarily focuses on their economic benefits and ecological functions, including environmental physicochemical factors and biological indicators [26]. There is still considerable debate internationally regarding the criteria and standards for evaluating the success of oyster reef restoration. Commonly used evaluation indicators include oyster density, individual size, reef area, reef height, and reef-associated fauna [17,27].

China’s oyster reefs, especially those in Tianjin, have been severely degraded due to factors such as coastal development, overfishing, and environmental pollution [28,29]. The Dashentang oyster reef in Tianjin is the largest extant modern living oyster reef cluster in China [28,29], playing a crucial role in maintaining the ecological balance of the Bohai Bay coastal marine ecosystem. Therefore, it is urgent to implement appropriate measures for oyster reef restoration. Interest in oyster restoration is increasing in many coastal areas, including Dashentang reef area in Tianjin [30]. Yin et al. [31] proposed recommendations for the rational development and use of natural resources, as well as for implementing rotational harvesting, to optimize the construction of the Dashentang oyster reef area. Xu et al. [32] investigated an analysis of the subsidence of artificial fish reefs in the Dashentang area and explored the influencing factors. Fang et al. [33] used acoustic methods to survey the artificial fish reef area in Tianjin’s Dashentang, for the first time obtaining the detailed spatial distribution information of artificial fish reefs in this region, which aids in the assessment of restoration effectiveness. Sha et al. [34] discovered an oyster reef with a total area of approximately 19.71 hm² in the Machengkou area on the west coast of Bohai Bay and studied the biological characteristics of the intertidal Spartina alterniflora–oyster symbiotic community in this region.

Methods for oyster reef restoration worldwide typically involve establishing oyster reef protection zones, while simultaneously analyzing factors such as oyster replenishment in the restoration area, sediment type, and environmental conditions to develop region-specific restoration strategies [26,35]. However, many restoration efforts fail, and significant uncertainty remains regarding the best practices for designing and implementing these initiatives across diverse ecological niches. Furthermore, studies evaluating the effectiveness of artificial framework oyster reefs for ecological restoration are relatively limited. Therefore, it is crucial to reduce this uncertainty through restoration experiments before the large-scale deployment of artificial frameworks.

In this study, a series of oyster reef restoration experiments were conducted in the coastal areas of Dashentang and Bagua Shoal in Tianjin. Key factors influencing oyster recruitment were investigated. These factors included restoration methods (shell string, mesh bag, and mesh cage), seeding with oysters, oyster shell orientation, and the hanging height of the cultches. This study can provide theoretical and technical support for oyster reef restoration projects.

2. Methods

2.1. Site Description

The subtidal experimental site (Site A) is located near Dashentang in Tianjin coastal area of Bohai Bay (117°55′ E–118°10′ E, 39°15′ N–39°23′ N). This area is a semi-enclosed shallow sea with a water depth of 3–6 m. Due to coastal development, trawl fishing, and industrial pollution, the natural oyster reefs in this area have been significantly degraded. Currently, an artificial oyster reef restoration project has been implemented in the region, with a restored reef area of approximately 8.465 square kilometers [33,36], providing a suitable site for this artificial restoration experiments. The intertidal experimental site (Site B) is located in the Bagua Shoal Wetland Reserve in Tianjin (39°08.1780′ N, 117°48.1381′ E). The intertidal flats of Bagua Shoal are primarily muddy, with a semi-diurnal tidal pattern, exhibiting unequal tidal fluctuations and pronounced low tides [37]. Figure 1a–c shows the locations and digital photos of the study sites. The dominant oyster species at both sites are Crassostrea gigas, and the dominant large benthic fauna consists of mollusks and annelids. However, the abundance, evenness, and diversity indices of the benthic organisms are relatively low.

2.2. Oyster Recruitment Experiments

2.2.1. Effects of Three Restoration Methods on Oyster Recruitment in Intertidal and Subtidal Zones

From July 2022 to November 2022, artificial framework experiments were conducted at Sites A and B to assess the impact of different restoration methods on oyster recruitment. Figure 2 shows the digital photo and schematic diagram of the following three types of artificial framework restoration methods: shell string, mesh bag, and mesh cage. After the experiment, the juvenile oyster abundance (individuals/shell, or ind/shell for short) and juvenile shell height (mm) for each restoration method were recorded and analyzed.

The artificial frameworks were welded steel frames with dimensions of 1 m (length) × 1 m (width) × 1.2 m (height), and the bottom was designed with a 20 cm high frame leg to prevent settling. Eight such small frames were welded together to form a large frame (Figure 2). Three large frames were deployed at the intertidal and subtidal zones, respectively. On each large frame, two small frames were hung with shell strings, four small frames were hung with mesh bags, and two small frames were placed with nylon mesh cages. The three different restoration methods are described, as follows:

Mesh bag method: Each small frame was hung with two mesh bags containing oyster shells (in total, eight mesh bags in a large frame). The mesh bag was 30 cm in diameter and 1 m in length. There were five layers with different top-to-bottom heights (0.3 m, 0.5 m, 0.7 m, 0.9 m, and 1.1 m). Two oyster shells were placed in each layer.

Shell string method: Each small frame was hung with twelve shell strings (in total, 24 shell strings in a large frame). Oyster shells with a shell height of 5–10 cm were selected to make the shell strings. To fix the oyster shells, we drilled a hole in the center of the oyster shells and tied knots of nylon rope at heights of 0.3 m, 0.5 m, 0.7 m, 0.9 m, and 1.1 m.

Mesh cage method: One nylon mesh cage containing oyster shells was placed in each small frame (in total, two mesh cages in a large frame). Ten oyster shells were placed at the bottom of each mesh cage.

Table 1. The experimental design of this study.

Group No.	Zone Type		Seeding with Juvenile Oyster		Restoration Method
	Intertidal Zone	Subtidal Zone	With Seeding	Without Seeding	Mesh Bag	Shell String	Mesh Cage
M1		◯		◯	◯
M2		◯		◯		◯
M3		◯		◯			◯
N1	◯			◯	◯
N2	◯			◯		◯
N3	◯			◯			◯
N4	◯		◯		◯
N5	◯		◯			◯
N6	◯		◯				◯

shows the experimental design of this study. There were three sets of experiments in the subtidal zone and intertidal zone, respectively (M1, M2, and M3 were for the subtidal zone experiments, while N1, N2, and N3 were for the intertidal zone experiments).

2.2.2. Effects of Seeding with Juvenile Oysters, Shell Orientation, and Hanging Height of Shell Cultches on Oyster Recruitment

In intertidal zone experiments, additional experiments were conducted to investigate the effects of seeding with juvenile oysters (for all the three restoration methods), shell orientation, and hanging height of shell cultches (only for shell string method) on oyster recruitment.

(1) Effects of seeding with juvenile oysters

The shells attached with juvenile oysters (5~7 oysters/shell with shell height of 8–13 mm) were purchased from a local oyster hatchery factory. The experimental group consisted of 2 mesh bags (N4), 5 shell strings (N5), and 1 mesh cage (N6) (Table 1). Different restoration methods were designed in the same way, as described in Section 2.2.1. The abundance and average shell height of oyster larvae on the shells were recorded before the experiments. During the experiment, changes in oyster abundance (ind/shell) and average shell height (mm) were determined, and the average daily growth rate of oysters (mm/d) was calculated.

(2) Effects of oyster shell orientation and hanging height of shell cultches

We prepared oyster shell strings according to the instructions in Section 2.2.1 and arranged the perforated oyster shells in the desired orientation. The experiment included the following two groups: one with the rough side of the oyster shells facing upwards and the other with the smooth side facing upwards.

2.2.3. Effects of Environmental Variables at Different Restoration Sites on Oyster Recruitment

Environmental variables were measured from July 2022 to November 2022 in triplicate at the experiment sites about once a month. Water samples were collected and analyzed according to the methods specified in “The specification for marine monitoring—Part 4: Seawater analysis” (GB 17378.4-2007) [38]. Water temperature, pH, and salinity were measured using a portable YSI professional plus multiple parameter water quality meter (YSI, Yellow Springs, OH, USA). The chlorophyll a concentration was determined spectrophotometrically on a UV-3000 spectrophotometer (Mapada, Shanghai, China).

Redundancy analysis (RDA) was used to explore the relationships between oyster settlement density, shell length, shell height, shell width, and various environmental variables at artificial frameworks in both field sites.

2.3. Statistical Methods

After the experiment, the changes in oyster abundance (ind/shell), average oyster shell height (mm), and average daily oyster growth rate (mm/d) were calculated for the oyster shells in the artificial frames.

Oyster Abundance: The number of newly attached oysters on each shell was counted after sampling.

$$\mathrm{A}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{O}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{S}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{H}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\overline{H}={\displaystyle \frac{\mathrm{S}\mathrm{u}\mathrm{m}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{s}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{s}}}$$

(1)

$$\mathrm{D}\mathrm{a}\mathrm{i}\mathrm{l}\mathrm{y}\mathrm{O}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{S}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{G}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{t}\mathrm{h}={\displaystyle \frac{\overline{H_1}-\overline{H_0}}{t_1-t_0}}$$

(2)

In Equation (2), $\overline{H_1}$ is the average shell height of oysters at the end of the experiment (mm), $\overline{H_0}$ is the average shell height of oysters at the beginning of the experiment (mm), t₁ is the end date of the experiment, and t₀ is the start date of the experiment.

Experimental data were subjected to a log transformation to validate normality and homogeneity of variance. A two-way ANOVA was then implemented to assess the effects of restoration site, artificial framework type, juvenile oysters attaching, shell orientation, and hanging height of shell cultches on oyster abundance and shell height. Tukey’s HSD test at the p < 0.05 level was performed as post hoc statistical analysis for an all-pairwise multiple comparison procedure.

2.4. Comprehensive Performance Evaluation

The evaluation indicators were selected based on the “Technical guideline on coastal ecological rehabilitation for hazard mitigation—Part 6: Oyster reef” (T/CAOE 21.6-2020) [39] and the “Technical directives for investigation and assessment of coastal ecosystem status—Part 7: Oyster reef” (T/CAOE 20.7) [40]. They included artificial framework construction costs, the settlement density of oysters, the daily growth rate of oysters (increase in shell height), water quality, and reef subsidence. Their weights were set to 0.30, 0.30, 0.30, 0.05, and 0.05, respectively, determined by using an expert survey questionnaire. The baseline score for this evaluation system is set at 100 points. Restoration methods with evaluation scores of 100 points or higher are deemed feasible for recommendation. The detail calculation methods are provided in the Supplementary Materials (Method S1). The performance of different restoration methods at Site A (subtidal zone) and Site B (intertidal zone) were evaluated, respectively.

3. Results and Discussion

3.1. Comparison of Three Restoration Methods in the Intertidal Zone and the Subtidal Zone

Figure 3a,b illustrates the effects of different restoration methods on oyster density and daily growth rate, respectively. The mesh bag restoration method resulted in the highest oyster density, with 27.6 individuals per shell in the intertidal zone and 19.3 individuals per shell in the subtidal zone. In contrast, the shell string restoration method achieved the fastest oyster growth rate, measuring 0.29 mm/day in the intertidal zone and 0.38 mm/day in the subtidal zone. The lowest oyster density and growth rate were observed in the mesh cage restoration method, with 20 individuals per shell and a growth rate of 0.25 mm/day in the intertidal zone and 1.85 individuals per shell and a growth rate of 0.26 mm/day in the subtidal zone.

The varying performance of the three restoration methods can be attributed to the different isolated environments they provide. Bivalves, from larvae to adults, are vulnerable to predation throughout all life stages [41,42]. Mesh bags offer partial protection to oyster larvae from certain predators, leading to higher oyster density; however, they also restrict feeding [43]. As a result, oysters in the shell string restoration system exhibited faster growth rates. In contrast, the mesh cages have a smaller mesh size than the mesh bags, providing stronger isolation but further disrupting normal feeding behavior, making them the least effective method.

3.2. Effects of Seeding with Juvenile Oyster, Shell Orientation, and Hanging Height of Cultches on Oyster Recruitment

As shown in Figure 4a,b, the oyster density and growth rate are significantly higher in frames with seeding compared to those without. These results suggest that oysters attached to seeding shells exhibit greater densities and faster growth rates. A possible explanation for this phenomenon is that larval settlement is influenced by specific chemical cues. Evidence indicates that oyster larvae preferentially settle on substrates already colonized by adult oysters, and there was a positive correlation between the abundance of reproductive larvae and the adult oyster population in both natural and restored oyster reefs [44]. Oysters attached to substrates release chemical cues downstream, triggering settlement responses in nearby larvae [45]. Additionally, oyster larvae have been shown to prefer natural oyster shells over PVC substrates for settlement. The presence of resident organisms on oyster reefs further promotes settlement, attracting more larvae. Notably, oyster larvae tend to settle more readily in areas covered by barnacles rather than on clean, artificial surfaces [46,47].

Figure 4c,d shows that the oyster density and growth rate are higher on roughened shells than on smooth shells, indicating that shell surface characteristics directly influence oyster settlement. Among oysters attached to shell strings at different hanging heights, density is highest at lower hanging positions. The maximum growth rate occurs at 0.5 m (0.38 mm/day on rough surfaces and 0.32 mm/day on smooth surfaces). Depth and tidal zone levels significantly affect oyster exposure to air, making them critical factors in oyster reef growth in intertidal estuarine zones [48,49]. Additionally, the composition and concentration of suspended particles in the water play a key role in oyster growth [48]. The spatial distribution of oyster larvae varies across developmental stages, with significant differences observed between species and locations [50]. These variations result in differing growth conditions at different cultch hanging heights, impacting the success of oyster reef restoration. During the larval stage, oysters have limited swimming ability and are generally found in lower water layers. Consequently, oysters at lower hanging heights tend to have higher densities. Proximity to the sediment increases the concentration of suspended particles, providing more food for larvae but potentially inhibiting growth to some extent. As a result, young oysters exhibit the fastest growth at lower heights. Conversely, oysters at higher cultch hanging heights experience prolonged air exposure during low tide, leading to lower densities and slower growth rates.

3.3. Comparison of Intertidal Zone and Subtidal Zone and the Driving Environmental Factors

In terms of habitat location, the oyster settlement density in the intertidal zone (23.75 ind/shell) is higher than that in the subtidal zone (12.58 ind/shell), while the growth rate of oysters in the subtidal zone is faster (0.316 mm/day > 0.273 mm/day) (Figure 5a,b).

The difference in oyster settlement at different habitat locations is mainly determined by environmental factors, as measured during each sampling event. Figure S1 shows the environmental variables monitored in the subtidal and intertidal habitats (air temperature, water temperature, chlorophyll a concentration, dissolved oxygen, pH, and salinity). RCA analysis was conducted to reveal the relationship between environmental variables and oyster characteristics (Figure 5c). The graph divides the elements into two areas, left (I) and right (II), based on the vertical axis. The position of each species in the ordination plot reflects the degree of environmental influence on the species and serves as a comprehensive reflection of the species’ ecological niche. Species that cluster together in the ordination plot have similar habitat requirements. Samples in Area I are entirely from the intertidal zone, and the attachment density of these samples is positively correlated with temperature, salinity, and chlorophyll concentration. Samples in Area II are all from the subtidal zone, and the shell length, shell height, and shell width of these samples are positively correlated with dissolved oxygen and pH. The distinct separation of the two sample groups indicates that environmental variables in these two zones have a significant impact on oyster attachment and growth.

The RDA analysis results suggest that oysters at Site A (S2) exhibit better growth, with shell length, shell height, and shell width being positively correlated with dissolved oxygen and pH. In contrast, oysters at Site B (S1) have a higher attachment density, which is positively correlated with water temperature, chlorophyll a concentration, and salinity. The results indicate that higher dissolved oxygen [51] and pH [52] at Site A are more conducive to oyster growth. Dissolved oxygen in seawater, especially in the subtidal zone, has a significant effect on oyster growth. During the rainy season, the low levels of DO and hypoxic conditions can adversely impact the growth and survival of oysters [53,54]. Lower seawater pH can affect the growth and metabolism of oysters, which are calcifying organisms, leading to a reduction in growth rate. The pH at Site A remains above 8, so it does not adversely affect oyster growth. At Site B, the temperature [55], salinity [56,57], and chlorophyll a concentration are more favorable for oyster reproduction and larval development. The suitable temperature range for oyster reproduction in northern China is 20–26 °C [55], and Site B maintains this temperature range for a longer period, which is beneficial for oyster reproduction. Salinity can affect oyster growth and mortality and, to a lesser extent, reproduction [56]. The salinity at both sites is within the optimal range for oyster growth, and Site B has a slightly higher salinity, which may benefit oyster reproduction and larval survival.

3.4. Evaluation of Different Restoration Methods in Intertidal and Subtidal Zones

Based on experimental results from July to November 2022, the recruitment effectiveness of different methods in the subtidal zone (Site A) was evaluated using the five indicators. The scoring results for the three artificial framework types are presented in

Table 2. Scores for each parameter across different restoration methods in the subtidal and intertidal zones.

Restoration Method	Score A (Cost)	Score B (Settlement Abundance)	Score C (Average Daily Growth)	Score D (Marine Water Quality)	Score E (Reef Sedimentation)	Total Score
M1	37.21	10.92	37.74	5.08	5	95.95
M2	26.49	98.98	39.34	5.03	5	174.84
M3	18.80	0.00	0.00	4.39	5	28.19
N1	30.45	31.47	28.58	5	5	100.50
N2	31.79	26.97	27.27	5	5	96.03
N3	31.36	22.80	27.73	5	5	91.89
N4	27.69	37.32	32.82	5	5	107.83
N5	29.73	31.70	31.52	5	5	102.95
N6	29.35	29.74	32.08	5	5	101.17

, with rankings as follows: M2 > M1 > M3. As shown in Figure 6, both M1 and M2 achieved scores exceeding 100. This indicates that, in the subtidal zone, the shell string method demonstrates the highest recruitment performance and meets the requirements for future oyster reef restoration.

The effectiveness of different methods in the intertidal zone of Bagua Shoal were also evaluated using the five indicators. The scores for the five indicators are shown in Table 2. Due to the small study area, the marine water quality at each experimental site was similar, and no reef settlement was observed for any of the restoration methods. As shown in Figure 6, four methods (N1, N4, N5, N6) exceeded the baseline score of 100. The results indicate that the seeding to oyster shells improved restoration outcomes. The methods that exceeded the baseline score ranked, as follows: N4 > N5 > N6 > N1. Therefore, the mesh bag method with seeding showed the best performance and may be considered for future oyster reef restoration.

Based on oyster spat attachment in both the intertidal and subtidal zones, as well as the comprehensive scores from the technical comparison, the shell string method demonstrated the best recruitment performance in the subtidal zone. The shell string method supported oyster attachment throughout the entire restoration period, whereas the mesh bag method was more beneficial for the early placement of oysters, promoting their survival and growth.

Additionally, seeding with juvenile oysters enhanced recruitment performance. In the intertidal zone experiments, oyster attachment was notably higher when seeding was employed. Furthermore, the mesh bag method proved more effective than the shell string method in the intertidal zone, likely because the mesh cages provided protection against predators.

Based on the overall survey results and technical comparison scores, future oyster reef restoration efforts in Tianjin coastal area should integrate the shell string method with the mesh bag method to cope with different environmental conditions. When deploying oyster seeding in artificial frameworks, it is essential to balance cost and recruitment effectiveness, while ensuring the proper coordination between reef construction and deployment to prevent the prolonged air exposure of live oysters. In addition to structural and environmental factors influencing oyster recruitment, the pre-conditioning of settlement substrates is a widely used practice in commercial oyster aquaculture. Typically, settlement devices are placed above existing oyster beds for periods ranging from several months to years to allow biofilm development and the absorption of chemical cues that attract oyster larvae. Although our study did not specifically incorporate pre-conditioned materials, future restoration projects might benefit from integrating this technique to potentially enhance natural spat settlement and improve overall reef establishment success.

4. Conclusions

In this study, different influencing factors on oyster recruitment in artificial frameworks were investigated in both the intertidal and subtidal zones of Tianjin coastal area. Among the three restoration methods, the mesh bag method and shell string method yielded better recruitment outcomes. Specifically, the mesh bag method can partially prevent predation, which favors the attachment of oyster larvae, while the shell string method allows oysters to feed effectively, promoting their growth. In addition, seeding with juvenile oysters in the artificial frameworks, orienting the rough side of the oyster shells upward, and hanging the cultches at lower heights contributed to improving the effectiveness of oyster recruitment. The different environmental conditions in the intertidal and subtidal zones also had critical impacts on oyster recruitment. To increase the restoration effect, it is recommended to increase oyster density in the subtidal zone and increase feed in intertidal zone restoration. Finally, a comprehensive evaluation of the restoration methods was conducted. In the subtidal zone, the shell string method was identified as the best approach, while in the intertidal zone, the mesh cage method with oyster seeding proved to be the most effective. This study provides a valuable guidance for the implementation of large-scale artificial framework oyster reef restoration projects.