1. Introduction
Coastal cities have long been interconnected with the dynamic marine ecosystems surrounding them, as significant hubs of human settlement and economic activity [
1,
2,
3]. Coastal regions rely on marine resources, with residents depending on the ocean for economic activities, commercial transactions, and communal interactions [
4]. However, the unsustainable exploitation of marine resources [
5] and the pollution of manufacturing and living areas in coastal cities present a range of challenges for the effective execution of marine conservation efforts [
6,
7,
8]. In response, implementing marine protected areas has been recognized as a crucial conservation approach to ensure the healthy operation of marine ecosystems and the adjacent terrestrial environments [
9,
10].
Ecosystems in coastal zones exhibit interconnectivity. The environmental implications of a national marine park could exceed its delimited boundaries and exert profound impacts on the ocean and adjacent coastal areas. NMPs are responsible for protecting the sea and land–sea interface zones. These places, where fishing and tourism are allowed sparingly, are classified as limited development zones, so tourism within national marine parks is extraordinarily cautious. In addition, NMPs can act as “ecological corridors”, facilitating the exchange of organisms, nutrients, and energy among the designated protected areas and the neighboring coastal land [
11]. These intricate relationships profoundly affect the ecological variation of coastal cities [
12], thereby necessitating the integration of the ecological benefits of NMPs and coastal cities into a unified research framework.
The eastern coastline cities of China are the most developed regions, characterized by numerous mega-cities, and their concentrated economic operations have caused a decline in China’s offshore ecosystem [
13]. The main purpose of establishing NMPs is to safeguard the health of the littoral, island, and marine ecosystems, and to develop a healthy ecological network in coastal regions. According to related governmental documents in China, namely the National Marine Main Functional Area Plan, Management Measures for Marine Special Protected Areas, National Marine Functional Zoning (2011–2020), and the General Programme for the Establishment of National Park Systems, it is evident that NMPs are categorized as a portion of Marine Specially Protected Areas (MSPAs). China’s NMPs can be divided into two categories: ecological conservation areas and rational use areas. The ecological conservation area is a core region with very stringent limits on social activities, and even scientific study requires prior application; the rational use area can retain an operational ecological fishing area, which also serves as a destination for eco-tourism. Overall, the ecological attributes of marine parks are more prominent in China.
National marine parks have a variety of positive and negative impacts on coastal areas. Much of the previous research on NMPs has focused on two areas: the effects of NMPs on marine ecology, and the effects of NMPs on coastal residents. Cockerell, L. et al. [
14] highlighted the importance of national marine parks as crucial protections for coastal ecological security, emphasizing their capacity for improving the resilience and well-being of coastal urban ecosystems. Campbell, S. and Edgar, G. [
15] noted that NMPs can maintain the abundance and diversity of previously targeted fish populations. McKenna, M. et al. [
16] had similar conclusions, and they pointed out that the NMP can halt and possibly reverse declines in local fish stock abundance and productivity. According to Denny, C. and Babcock, R. [
17], there should be stringent limitations on productive endeavors within the NMP region, including a complete ban on all fishing activities. However, the closure of the core area of the NMP may cause partial transfer of marine exploitation activities to adjacent sea areas [
18], and may also generate conflicts among various stakeholders [
19].
Studies on the economic impacts of NMPs are divergent. Tadjuddah, M. et al. [
20] argued that NMPs can deliver ecologically valuable services and simultaneously offer sustainable livelihood opportunities for coastal communities. Similarly, Davis, G et al. [
21] maintained that NMPs contribute to the economic well-being of local communities through tourism. However, sprawling development plans and inadequate enforcement of environmental regulations in NMPs may marginalize local citizens in terms of decision-making processes and compromise their rights [
22]. Elliott, G. et al. [
23] revealed that the management plan implemented for Wakatobi Marine Park in eastern Indonesia failed to consider the needs and interests of the local population adequately. In any case, the construction of national marine parks must be in the general public’s best interests [
24,
25], focusing preserving vital marine ecosystems while considering the appropriate utilization of resources for recreational activities, scientific investigations, and educational purposes [
26].
Urban eco-efficiency seeks to integrate sustainable development concepts into all facets of city planning, with the objective of minimizing adverse environmental effects while simultaneously promoting economic advancement and societal welfare. The notion is increasingly recognized as a significant paradigm for evaluating urban ecosystems. Marzluff, J. et al. [
27] examined the correlation amongst urban ecosystems, with a particular emphasis on the necessity of incorporating natural elements into urban areas to enhance ecological efficiency and facilitate sustainable urban development. Lin, J. et al. [
28] proposed a new method to measure sustainability based on urban eco-efficiency, which compares the relative sustainability state of different regions at different times. The factors impacting urban eco-efficiency have also been extensively discussed in several studies: Walker, R. et al. [
29] conducted a comprehensive review of nutrients, water, and energy in urban systems to determine the factors contributing to enhanced eco-efficiency. Bleischwitz, R. and Hennicke, P. [
30] investigated the regulatory policies for eco-efficiency. Other concerning factors include technological innovation [
31], urban planning and design [
32], urban greening [
33], and environmental policy [
34].
Although several studies have examined the ecological benefits of NMPs, there is currently a lack of investigation into the impact of NMPs on the ecology of coastal cities. This casts doubt on whether NMPs can be used as a comprehensive tool that benefits coastal areas. The quantitative evaluation of environmental effects in national maritime parks is constrained by the complexity of the marine ecosystem and the diversity of human activities. Furthermore, there is a significant gap in the research on China’s NMPs. In China’s practice, implementing NMPs has encountered multiple challenges, including the fact that certain coastal regions prioritize their demands over the protection of the ocean, have inadequate supervision, and consider canceling established protected areas in favor of economic development. These factors demonstrate that it is more practical to scientifically identify the impact of China’s NMPs on eco-efficiency. This study aims to investigate the impact of NMPs on the eco-efficiency of Chinese coastal towns by treating the establishment of NMPs as a “quasi-natural experiment”. The DID model was employed to evaluate the ecological effects of NMPs based on panel data from 53 coastal cities in China from 2003 to 2020.
The potential contributions of this study are as follows: (1) This study is the first to use econometric analysis to investigate how NMPs affect urban eco-efficiency, which provides a supplementary examination of the effects of NMPs. (2) The calculation of urban eco-efficiency was conducted using the SBM model. Afterwards, a rigorous empirical analysis was performed using the multi-period DID model to investigate the relationship between NMPs and urban eco-efficiency. To ensure the findings’ reliability, a series of robustness and heterogeneity tests were performed to identify the causal relationship more rigorously. (3) The mechanism of NMP’s effect on coastal cities’ eco-efficiency is explored from three perspectives, thereby offering valuable theoretical and empirical insights that contribute to the advancement of NMPs and marine eco-protected areas.
2. Mechanism Analysis and Research Hypotheses
The establishment of NMPs necessitates the procurement of a portion of the fisheries’ land and the prohibition of other associated fishery production activities. Thus, fishermen will experience a loss of traditional fishing areas, leading to increased expenses associated with fishing operations and diminished quantities of catches. If the fishermen encounter inadequate alternative sources of income, a portion of them may be compelled to transition from their customary fishing areas to engage in activities within the secondary or tertiary sectors [
35,
36]. This will significantly influence the ratio of production factors, such as labor force and capital, among sectors.
Furthermore, NMPs could offer specifically allocated areas for tourism-related endeavors. Tourism could provide substantial job opportunities for fishermen, enabling them to remain involved in various fishery-related occupations. A majority of NMPs have been shown to create significant visitor attraction and sustained economic advantages that well surpass their benefits in terms of fisheries [
37,
38]. The tourism industry plays a substantial role in promoting the diversification and advanced development of coastal industries, facilitating the positive expansion of all three sectors and enhancing the ecological efficiency of coastal cities.
Hypothesis 1. NMPs could improve eco-efficiency in coastal cities through the mediating effect of industrial structure optimization.
The living conditions of indigenous populations residing in protected marine areas are typically in a poor general state. This may be attributed to several constraints, including inadequate transportation infrastructure, restricted access to information, and restricted availability of fishing grounds. NMPs have the potential to enhance the economic well-being of people in the local community by leveraging sustainable fisheries and the tertiary sector, while concurrently fostering the generation of job possibilities [
39]. Through the strategic utilization of various resource elements across NMPs, it becomes feasible to unleash the economic potential of the area, leading to a more substantial enhancement of the livelihoods of coastal fishermen and fostering economic prosperity around the coastal area [
40].
Furthermore, implementing NMPs requires an adequate allocation of dedicated financial resources. It is necessary to compensate indigenous people for the opportunity costs they face when they give up their traditional mode of production. Additionally, creating ecological tourism programs in NMPs requires the mobilization of increased social capital to effectively complement the park’s development. This could potentially promote a novel form of collaboration between the government and social capital, expand avenues for financial support, and invigorate the local economy.
Hypothesis 2. NMPs could improve eco-efficiency in coastal cities through the mediating effect of activating the local economy.
In contrast to land-based national parks, marine environments exhibit greater complexity and fluidity. Further, the boundary between land and sea ecosystems lacks distinct and rigid limits. In the context of China, there is an ongoing issue of excessive discharges originating from direct sources of sea pollution. These discharges, stemming from industrial, residential, and combination sources, have a detrimental effect on the overall health of the seas. China has been grappling with persistent issues of direct sea discharge pollution and excessive sewage discharge into the sea, and industrial, domestic, and combined discharges pose great challenges to the health of marine life along the coastal zone. China’s 457 direct sources of marine pollution released 750,199,000 tons of sewage in 2022 [
10]. Only 30.4% of the 230 state-controlled river sections that discharged into the ocean and were examined had water quality within categories Ⅰ to Ⅱ, which is not encouraging.
NMPs can function as a barrier, effectively inhibiting the transmission of inland wastewater contaminants into the marine ecosystem. Firstly, NMPs have the potential to effectively mitigate the concentration of detrimental elements within the seawater. This is achieved via the imposition of regulations that strictly limit sewage and wastewater discharge to NMPs in coastal areas, as well as the imposition of limitations on industrial and agricultural operations in proximity to NMPs. Furthermore, most NMPs function as crucial habitats that should ideally be safeguarded from human activity in order to preserve the stability of water quality and ecological health. Additionally, stable ecosystems in the NMPs could help to uphold the integrity of the food chain, which contributes to a reduction in organism mortality and the accumulation of metabolites, thereby reducing the risk of eutrophication in water bodies. In summary, NMPs can enhance eco-efficiency by curbing discharge behaviors and alleviating environmental stress in coastal areas.
Hypothesis 3. NMPs could improve eco-efficiency in coastal cities through the mediating effect of the suppression of pollution behaviors.
5. Conclusions
NMPs assume a significant role in facilitating the sustainable development of coastal regions by serving as an effective tool for ecological preservation. Nevertheless, previous research has neglected to examine the ecological impacts of NMPs on their locations using econometric approaches. This study considers the establishment of NMPs a “quasi-natural experiment”. The initial step involves conducting a qualitative analysis of the interaction and influencing mechanism between NMPs and the eco-efficiency of coastal cities. Subsequently, this study uses the multi-period DID model to examine the effectiveness of NMPs in enhancing the eco-efficiency of coastal cities, utilizing panel data from 53 Chinese coastal cities from 2003 to 2020. Additionally, this study conducts various robustness and heterogeneity tests to ensure the reliability and validity of the findings. Based on the research above, this study draws the following main findings. (1) It has been discovered that the implementation of NMPs resulted in a significant rise of 3.87% in the eco-efficiency of coastal cities in China. This result remains robust after the parallel trend test, placebo test, PSM-DID, and exclusion of other policy interferences and outliers. (2) The examination of mechanisms reveals that NMPs contribute to improving eco-efficiency in coastal cities by energizing the local economy, optimizing the industrial structure, and suppressing sewage pollution. (3) There is heterogeneity in the impact of NMPs on the eco-efficiency of coastal cities, and its promoting effect is more significant in the East China Sea, the South China Sea, and cities with large areas of NMPs. The South China Sea coastal regions had a significant 13.5% increase in ecological efficiency following the establishment of NMPs, which had the most significant effect. Coastal cities’ eco-efficiency increased by 5.18% as a result of implementing large-area NMPs. However, NMPs with an area of less than 100 km2 failed the significance test.
This study’s conclusions have the following implications for policies for advancing NMP implementation and enhancing the eco-efficiency of coastal cities. The construction of NMPs must be undertaken carefully, considering the specific requirements and priorities of local development. The effective carrying out of the ecological function of NMPs depends on the scientific design, efficient administration, and collaboration among diverse stakeholders. It is recommended that the government assumes a more proactive role in the design, management, implementation, cooperation, education, monitoring, and assessment of NMPs. This way, optimizing human welfare in neighboring regions using NMPs can be achieved while preserving the marine ecosystem’s equilibrium. The tourism development strategy for NMPs should embrace a small-scale, multi-restricted, and sustainable approach. It is imperative to prioritize ecosystem protection while providing the public with limited opportunities for leisure and recreational activities. It is crucial to emphasize that the entire touring process must be subject to stringent regulations. In most instances, large-area NMPs demonstrate a greater capacity to maintain the integrity of marine ecosystems. Nevertheless, there are fewer large-area NMPs in China, and a dispersed and blocky distribution characterizes the existing NMPs. Hence, expanding the area of specific adjacent NMPs is feasible to establish linkages among smaller-area NMPs. Alternatively, further NMPs with large-area and more comprehensive biological networks may be necessary.
Some of the restrictions of this study may serve as motivation for further investigation. In future research endeavors, it is necessary to incorporate a broader range of indicators that relate to oceans and fisheries and give more significant thought to how NMPs may affect marine eco-efficiency. Furthermore, in future investigations, we intend to examine the impacts of NMPs on coastal communities and fishermen, using more thorough approaches to enhance the depth and breadth of NMP-related research.