1. Introduction
Inshore water areas provide a wide range of essential ecological services to humanity, including the provision of food and raw materials, environmental purification, climate regulation, and cultural enrichment [
1,
2]. Although they cover only 18% of the Earth's surface, these areas contribute to 25% of the planet's primary productivity and account for 90% of global fishery yields [
3]. In particular, inshore waters are of vital importance to China, playing a critical role in ensuring food security and maintaining ecological balance [
4,
5]. However, in recent years, human activities such as industrialization, urbanization, agricultural expansion, and climate change have severely degraded China's inshore habitats and fishery resources, resulting in reduced fishery yields, diminished biodiversity, and the simplification of food webs [
6].
In response to the challenges posed by the degradation of inshore ecosystems, China has proposed the establishment of a marine ecological civilization and implemented a range of measures aimed at protecting and restoring marine ecosystems. These initiatives include the restoration of nearshore environments, the development of marine ranching projects, the enforcement of seasonal fishing moratoriums, and the regulation of fishing vessels—efforts designed to enhance the stability, diversity, and sustainability of marine ecosystems [
7,
8,
9]. Among these, the development of marine ranching projects is viewed as a key strategy for transforming and upgrading China’s marine fisheries while safeguarding the marine environment [
10].
Unlike many other countries, China places greater emphasis on the development of artificial reefs, hatchery releases, and the establishment of online monitoring systems. The goal is to create a comprehensive industrial development framework for marine ranching that encompasses site selection, planning and layout, habitat restoration, resource conservation, and safety assurance [
11]. Effective assessment of the structural and functional characteristics, as well as the carrying capacity of marine ranching ecosystems, is a fundamental prerequisite for the successful construction and management of marine ranching in China [
6]. This is not only crucial for holistic planning but also for ensuring the scientific construction and management of these ecosystems [
12,
13,
14]. Specifically, these assessments provide essential data to guide decisions on the construction area, type, and scale of marine ranching projects. For example, they help in selecting appropriate hatchery release species, estimating optimal stocking densities, and determining the appropriate scale for artificial reef construction [
15,
16,
17,
18,
19]. Moreover, these assessments are critical for ensuring the long-term sustainability of marine ranching through effective management, which includes determining strategies for sustainable harvest based on carrying capacity [
20,
21,
22].
Due to limitations in carrying capacity assessment technology, most marine ranching projects in China have not conducted comprehensive evaluations of carrying capacity [
23]. Existing assessments have largely focused on biomass community structure, water quality, and the enhancement effects of individual species [
18,
24,
25,
26,
27,
28], with little attention to the overall performance of marine ranching systems [
8,
14,
29]. Even system-level evaluations generally compare marine ranching with small adjacent areas, lacking a robust evaluation system and clear grading standards [
28,
30,
31,
32,
33,
34], which undermines the credibility of the results. As a result, most marine ranching projects rely heavily on empirical knowledge, with insufficient scientific foundations for selecting hatchery release species, estimating stocking densities, and designing habitat construction strategies. This leads to significant ecological and economic risks [
6,
14,
35,
36,
37,
38,
39]. Therefore, there is an urgent need for the development of systematic evaluation and simulation methods for marine ranching ecosystems to enhance the scientific rigor and sustainability of marine ranching projects in China.
Ecological Network Analysis (ENA) is a systems-based methodology that quantifies the structure and function of marine ecosystems by analyzing the material and energy flow relationships among all components within the ecosystem food web [
40,
41,
42,
43,
44,
45,
46,
47,
48]. This approach has proven to be a valuable tool for assessing ecosystem health and for linking ecological and socio-economic systems. It is widely applied in marine ecosystem management and ecological restoration, providing critical insights for decision-making and policy development [
49,
50,
51].
The Ecopath with Ecosim (EwE) model is widely used to calculate ENA indices for marine ecosystems. It consists of three main components: Ecopath, Ecosim, and Ecospace. Ecopath is primarily employed to analyze the material and energy flows between various ecosystem components, while Ecosim simulates the dynamics of ecosystem food webs under different pressures. The EwE model has been extensively applied in recent studies to assess and simulate marine ranching ecosystems [
52,
53].
In this study, a marine ranching ecosystem and its control counterpart in the Beibu Gulf of China were selected as research subjects. Ecopath models were constructed for both ecosystems to evaluate their energy flow, trophic structure, and ecological carrying capacity. An index system based on ENA and a fuzzy comprehensive evaluation model was developed to assess the status of these ecosystems. SuiTable Atock enhancement groups for the marine ranching ecosystem were identified, and ecosystem dynamics were simulated under different stock enhancement strategies. This research aims to provide essential technical and theoretical support for the comprehensive evaluation and ecosystem-based stock enhancement of marine ranching. Furthermore, it seeks to contribute to the sustainable and high-quality development of marine ranching in the Beibu Gulf and across China.
4. Discussion
In China, the primary objective behind constructing marine ranching was to restore the marine environment and ensure the sustainable yield of fishery resources [
76]. Following almost four years of development, the Jinggong marine ranching has experienced significant transformations in both biotic community composition and system functionality compared to the control ecosystem. The biomasses of fish, crustaceans, and mollusks within the marine ranching are 1.82, 16.99, and 33.51 times greater, respectively, than those observed in the control ecosystem. The substantial increase in mollusk biomass can be primarily attributed to the deployment of artificial reefs, which serve as crucial substrates for attached organisms like mussels and oysters [
77,
78,
79]. The robust filter-feeding and biodeposition activities of mussels and oysters contribute significantly to nutrient enrichment in the benthic environment surrounding the artificial reefs [
80]. Furthermore, the pronounced spatial heterogeneity offered by artificial shellfish reefs provides effective refuge and foraging grounds for marine organisms. This dynamic supports the growth and reproductive success of various organisms, including echinoderms, crustaceans, and fishes inhabiting the reef area [
81,
82].
SOI and CI serve as crucial indicators of ecosystem food web complexity. In the marine ranching and control ecosystems, CI values were 0.28 and 0.32, respectively, while SOI values were 0.13 and 0.21, respectively. Comparatively, the CI in the marine ranching system was rated at a “medium” level in relation to coastal marine ecosystems collected in this study, while the SOI was rated as “relatively poor”. Moreover, both CI and SOI values were notably lower compared to small marine ecosystems (1−10 km2) investigated by Heymans et al. (2014) [
47]. CI reflects the ratio of actual links to potential links within an ecosystem’s food web, whereas SOI delineates the distribution of feeding interactions across TLs (Christensen et al., 2005; Libralato, 2008) [
45,
56]. SOI compensates for the limitations of CI in delineating food web complexity, especially given that CI remains constant despite variations in prey proportions within predator diets [
56,
83]. The low SOI observed in the marine ranching ecosystem suggests weak connections among functional groups, potentially resulting in lower energy flux and a simplified food web structure. The lower CI and SOI in marine ranching compared to the control ecosystem may stem from increased biomasses of mollusk, echinoderm, and barnacles. The simpler diets of these species contribute to a less complex food web structure. The increased biomass of these groups also contributed to increased EE values for phytoplankton and detritus within the marine ranching ecosystem, resulting in a notable enhancement of energy utilization efficiency between TLs I and II. However, despite these improvements, the TE values among TLs II to V remained notably low, averaging 5.84%. This level was classified as “relatively poor” according to the index system, falling far below the natural ecosystem average of 10% [
84].
A/C serves as an indicator of ecosystem organization and efficiency. Both ecosystems in this study were classified at the “relatively poor” level, suggesting diminished organization and system efficiency. The observed low A/C value in the marine ranching ecosystem may be attributed to the fact that increasing system organization levels hinge on the succession of community structure, a process that typically unfolds over an extended period [
85,
86]. Additionally, external disturbances can also disrupt the self-organization process of the ecosystem [
87], thereby impeding improvements in its organizational levels.
The low complexity of the food web, TE, and A/C values in marine ranching suggest that this system may be subject to significant external pressures [
47,
88,
89,
90], resulting in the majority of TST occurring at low TLs. Overfishing emerges as a prominent source of pressure among various external stressors. Indeed, overfishing has led to a severe decline, and in some cases depletion, of large carnivorous fish in China's coastal ecosystems [
6,
91], echoing the phenomenon of “fishing down the marine food web” observed in diverse contexts [
92,
93,
94]. Modeled results from Ecosim in this study indicate that large crabs in the marine ranching are subject to overfishing. Chen et al. (2008) similarly observed overfishing in the Beibu Gulf [
95]. Fishing activities in areas adjacent to marine ranching may adversely affect marine ranching. The migration patterns of many marine creatures within and around these ranches, particularly for type II and III fish species that only spend part of their lifecycle in the ranches, expose them to significant fishing risks during their movement between regions [
14,
96,
97,
98]. Considering the diverse habitat requirements of marine creatures at different life stages, proposals such as those by Yang and Ding (2022) are noteworthy [
99]. They suggest constructing global aquatic ecological ranching systems that designate entire estuaries or bays as ranching areas. Similarly, Liang et al. (2020) propose developing marine ranching facilities designed to meet all habitat needs across different life stages of fish [
98]. In the long term, these proposals hold the potential to mitigate challenges faced by marine ranching, including strong external disturbances and difficulties in maintaining a complex food web structure and high system organization levels.
The TPP/TR and TPP/TB ratios in the marine ranching and control ecosystems were 1.82 and 8.93, and 17.27 and 85.22, respectively. These values in the marine ranching were rated as “good”, whereas those in the control ecosystem were deemed “relatively poor”. Additionally, the D/H ratio in marine ranching exceeded that in the control ecosystem. These findings suggest that the establishment of marine ranching has significantly enhanced ecosystem maturity and stability [
100,
101]. Despite comparable total primary production in both ecosystems, the decline in TPP/TR and TPP/TB ratios can be attributed to increases in total biomass and total respiration. Notably, in marine ranching, filter-feeding bivalves accounted for the largest portion of both total biomass and total respiration. These bivalves exhibit ecological characteristics similar to those of zooplankton, but with lower turnover rates. While zooplankton can quickly respond to ecosystem disturbances, bivalves exhibit slower responses, thus creating a pathway for slow energy flow. The asynchronous dynamics of bivalves and zooplankton likely play a pivotal role in sustaining ecosystem stability [
102].
The evaluation based on the index system revealed that the ecosystem status in marine ranching was “relatively good”, contrasting with the “relatively poor” status of the control ecosystem. Notably, marine ranching's maturity indices, such as TPP/TR, TPP/TB, and TB/TST, significantly outperformed those of the control ecosystem. Furthermore, indices like FCI and FML demonstrated marked improvements, indicating that marine ranching enhances system maturity, energy recycling efficiency, and food chain length. Moreover, metrics such as TST, total production, and total biomass in marine ranching were 2.75, 1.40, and 5.56 times higher, respectively, than those in the control ecosystems. The carrying capacities for fish, crustaceans, and shellfish have also significantly increased in marine ranching areas, underscoring the positive impacts of marine ranching construction efforts. However, despite these achievements, the construction efforts have yet to effectively improve metrics such as TE, the level of system organization, and food web complexity.
Stock enhancement serves as a pivotal approach to improving marine environments and achieving sustainable utilization of fishery resources [
103]. However, the efficacy of stock enhancement in restoring marine resources varies across different initiatives [
18,
104,
105,
106,
107]. Some studies even contend that this method is entirely ineffective [
108]. Simulation scenarios conducted in this study revealed that stocking single fish or crab species did not notably optimize the ecosystem. However, the stock enhancement of the bivalve-dominated MOB group enhanced the ecosystem status from “relatively good” to “good”. This may be attributed to the higher enhancing capacity of MOB compared to crab and fish groups. Additionally, the augmentation in biomasses of mussels, barnacles, and oysters expanded shellfish reefs, providing increased refuge and food resources for crustaceans, cephalopods, fish, and other organisms [
79]. This indirect effect also contributed positively to optimizing the ecosystem status.
Yang et al. (2023) demonstrated that stock enhancement of multiple species may have better ecological effects than single-species stock enhancement [
109]. This finding aligns with the results of our study. For instance, in scenarios involving single-group stock enhancement of crab, fish, and the MOB group alongside fishing activities, the populations of large crabs or large and medium demersal fishes rapidly collapsed. Conversely, in scenarios combining MOB with crab and fishing, or MOB with crab, fish, and fishing, all functional groups maintained relatively stable biomasses, leading to an increase in ecosystem status to a higher level (from “relatively good” to “good”). These findings suggest that stock enhancement involving multiple species from different TLs represents a more effective strategy for optimizing ecosystem structure and function. However, it is important to note that maintaining the biomass of stock enhancement groups at the group’s carrying capacity in simulation scenarios may not fully reflect real-world conditions. This is because the carrying capacity of each functional group is dynamically influenced by external factors, such as climate change and human activities [
20]. To improve ecological outcomes, it is essential to strengthen research on the dynamics of ecological carrying capacity in marine ranching and adjust stock enhancement strategies accordingly [
19,
110,
111]. Moreover, enhancing activities aimed at boosting the biomass of stock enhancement groups to carrying capacity levels may pose ecological risks, potentially pushing the ecosystem toward a critical state of energy imbalance. Integrating the ecological roles of different organisms and implementing multi-species stock enhancement [
107], or reducing the stocking density of target organisms, can help mitigate these risks.
In scenarios focused on MOB (excluding MOB + fishing and MOB + fish + fishing), the ecosystem status was elevated to a “good” level, indicating that ecosystem structure and function were effectively optimized compared to the initial state. However, this does not imply that the ecosystem has reached an ideal or perfect state. The index system we constructed in this study provides a general overview of coastal ecosystems, and achieving a 'good' level does not necessarily align with the standards of mature ecosystems as described by Odum (1969) [
100]. For example, values of TE, A/C, CI, and SOI showed only slight changes in these scenarios, remaining at “relatively poor” or “medium” levels. This indicates that the simulated stock enhancement strategies had a limited impact on optimizing the food web structure, system organization level, and energy transfer efficiency. This finding aligns with the conclusions of Yang et al. (2023) [
112], who reported that stock enhancement did not significantly increase ecosystem species diversity, and its restoration effects were limited. It can be inferred that stock enhancement of only a few species may not be sufficient to fully optimize system structure and function. In line with this, Deevesh et al. (2024) emphasized the importance of prioritizing protection and natural recovery to enhance ecosystem structure and function [
113]. To truly optimize the structure and function of marine ranching ecosystems, it is crucial to adopt a more integrated approach that includes strengthening ecological connectivity between marine ranching and adjacent areas, improving fishing management strategies, and advancing the design and deployment of artificial reefs [
22,
114,
115]. When implemented together, these measures could provide a more sustainable and holistic framework for ecosystem restoration and management in marine ranching [
22,
114,
115].