# Non-Indigenous Species of Macroalgae in French Mediterranean Marine Protected Areas: Distribution and Trends

Marie Borriglione, Sandrine Ruitton, Aurélie Blanfuné, Michèle Perret-Boudouresque, Bastien Thouroude, Marc Verlaque, Charles-François Boudouresque, Thierry Thibaut

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Marie Borriglione, Sandrine Ruitton, Aurélie Blanfuné, Michèle Perret-Boudouresque, Bastien Thouroude, et al.. Non-Indigenous Species of Macroalgae in French Mediterranean Marine Protected Areas: Distribution and Trends. Journal of Marine Science and Engineering, 2025, 13, 10.3390/jmse13020374. [https://doi.org/10.3390/jmse13020378]

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Review

# Non-Indigenous Species of Macroalgae in French Mediterranean Marine Protected Areas: Distribution and Trends

Marie Borriglione ID, Sandrine Ruitton ID, Aurélie Blanfuné *ID, Michèle Perret-Boudouresque, Bastien Thouroude ID,
Marc Verlaque, Charles-François Boudouresque and Thierry Thibaut ID

Mediterranean Institute of Oceanography (MIO), CNRS, IRD, Campus de Luminy, Aix Marseille University and University of Toulon, 13009 Marseille, France; marie.borriglione@univ-amu.fr (M.B.); sandrine.ruitton@mio.osupytheas.fr (S.R.); charles.boudouresque@univ-amu.fr (M.P.-B.); bastien.thouroude@univ-amu.fr (B.T.); marclouis.verlaque@gmail.com (M.V.); charles.boudouresque@mio.osupytheas.fr (C.-F.B.); thierry.thibaut@univ-amu.fr (T.T.)

* Correspondence: aurelie.blanfuné-thibaut@univ-amu.fr

**Abstract:** The Mediterranean Sea, a biodiversity hotspot, faces significant threats from non-indigenous species (NIS), which drive biodiversity changes. Over the past century, the introduction of NIS has accelerated due to maritime traffic, aquaculture, and interoceanic canals, fostering biological invasions. Marine protected areas (MPAs), established to preserve biodiversity, are increasingly impacted. This review quantified and characterized French Mediterranean MPAs, analyzing non-indigenous macroalgae distribution based on the existing literature and the authors' observations. Results revealed widespread occurrence, with the highest NIS richness in strictly regulated MPAs; their proximity to large harbors highlights the paramount importance of the introduction pathways. In addition, there is a significant knowledge gap regarding the distribution of NIS within MPAs, complicating efforts to monitor and study these species effectively. These findings highlight the challenges in monitoring and managing invasions and the urgent need for controlling primary and secondary invasion pathways, within and outside the MPAs, international collaboration to control them, and enhanced funding for NIS monitoring. Without adaptive management, even strictly protected MPAs are vulnerable to the escalating impacts of invasive species.

**Keywords:** marine protected areas; non-indigenous macroalgae; Mediterranean Sea; ecosystem management

## 1. Introduction

The rapid development of the crisis of biodiversity, exacerbated by climate change, stands as one of the defining environmental challenges of this era [1]. In response to this crisis, there has been a significant increase worldwide in the development of terrestrial and marine protected areas (MPAs) in recent decades. These protected areas are crucial for biodiversity conservation [2,3] and to achieve the Aichi target, particularly target no. 11, which aims to conserve at least 10% of coastal and marine areas of particular importance for biodiversity through effective and equitable management by 2020 [4].

The majority (in number not in area) of MPAs combine protection objectives with the sustainable development of activities, but the largest MPAs (European Union Natura 2000 sites) are the least restrictive regarding human activities. Their governance generally involves users, elected representatives, experts, and other stakeholders in the management of these marine areas [5].

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Academic Editor: César Peteiro

Received: 6 January 2025

Revised: 7 February 2025

Accepted: 13 February 2025

Published: 18 February 2025

**Citation:** Borriglione, M.; Ruitton, S.; Blanfuné, A.; Perret-Boudouresque, M.; Thouroude, B.; Verlaque, M.; Boudouresque, C.-F.; Thibaut, T. Non-Indigenous Species of Macroalgae in French Mediterranean Marine Protected Areas: Distribution and Trends. *J. Mar. Sci. Eng.* **2025**, *13*, 374. https://doi.org/10.3390/jmse13020374

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
MPAs should confer numerous benefits in terms of biodiversity including the pro-
tection of habitats from destruction, the maintenance of genetic and functional diversity
and food webs and ensure that ecosystems function properly and are resilient to distur-
bance [6,7]. MPAs are recognized for their “reserve effect”, which manifests itself by a
return to a functioning of the ecosystem and to a balance between populations close to
the supposedly natural state; for many iconic and commercially valuable species, this
results in a greater abundance and size diversity of individuals as well as a cascade effect
linked to the presence of top predators within MPAs, this effect being especially due to
the prohibition or regulation of fishing [8,9]. This effect is particularly noticeable in the
most common commercial species [10]. These benefits contribute to the regeneration of
fish stocks and the protection of key species, thereby ensuring the long-term sustainability
of marine resources [11,12]. Marine protected areas should be subject to strict regulation
aimed at limiting human pressures such as intensive fishing, habitat destruction, and boat
anchoring, in order to preserve the marine biodiversity that they harbor [13,14]. However,
despite the numerous regulations aimed at limiting the impact of anthropogenic pressure
in the various MPAs, there are generally no regulations specifically concerning the introduc-
tion of non-native species (NIS). An NIS is defined as a species accidentally or deliberately
transported by humans into a new geographical area. These species can become invasive
and cause ecological and socio-economic problems [15].

In marine environments, the NIS are considered to be one of the main factors causing alterations in ecosystem functioning, along with coastal development and overfishing, and the consequences of these impacts are typically irreversible. It is widely acknowledged that introduced species are one of the major factors in the biodiversity crisis [1,16,17]. A significant concern is the impact of invasive species in MPAs. These areas are established with the primary objective of preserving marine ecosystems, maintaining biodiversity, and ensuring the continued provision of essential services to numerous communities [18].

The impact of invasive species on marine communities is well-documented. One notable example is that of the ctenophore *Mnemiopsis leidyi* (A. Agassiz, 1865), whose proliferation in the late 1980s and early 1990s in the Black Sea and Adriatic Sea had a considerable effect on fish stocks. This occurred through direct predation on ichthyoplankton, mesoplankton, and fish resources, leading to a reduction in ichthyofauna abundance [19–21]. The presence of *M. leidyi* in the Black Sea is estimated to have caused a potential annual loss of 16.7 million USD for all countries engaged in fishing in the region [22].

Another example is the case of the lionfish *Pterois volitans* (Linnaeus, 1758), which invaded the Atlantic Ocean, including areas such as the Caribbean and the Gulf of Mexico, significantly impacting local ecosystems by preying on native species and competing for resources. In MPAs, such as coral reefs, lionfish reduce the abundance of native fish species and alter community structures, threatening biodiversity and the health of these ecosystems. They also generate significant economic costs, with losses reaching up to 10 million USD in the French West Indies [23–25].

The Mediterranean Sea is a biodiversity hotspot but is also heavily impacted by non-native species. Currently, more than 5% of its marine species, approximately 1000 species, are considered to be non-indigenous species (NIS), meaning that they have been introduced beyond their natural geographical range, of which 751 are definitely established, making the region one of the most affected worldwide by biological invasions [26–28].

Concerning macroalgae, more than 10% of seaweed flora are considered to be non-
indigenous in the Mediterranean Sea, with 107 species identified as introduced specifically
in the northwestern part, making them the group with the highest number of introductions
in this region. Among them, 12 are classified as invasive due to their rapid proliferation,
significant disruptions to native ecosystems, and socio-economic impacts [29,30].
These dynamics present unique challenges for MPA managers, who must balance conservation with the management of harmful algae species. The aim of this review is to provide a synopsis of the current knowledge on the presence of non-indigenous macroalgae in French Mediterranean MPAs, identify gaps in current research, and propose directions for future studies and the management of NIS in these protected areas.

**2. Material and Methods**

Initially, a map of the various MPAs along the coast of mainland France was created using QGIS software (version 3.30.3). Only open sea MPAs were considered; MPAs in lagoons and ponds were not included in the data analyses. For the mapping of marine protected areas (MPAs) in the Mediterranean, only those with a substantial surface area and a coastal component were represented including national parks, nature reserves, Natura 2000 sites, and marine nature parks. Biotope decrees ("arrêtés de biotope") were not considered due to their limited area compared with the overall scale of the MPAs. Most of the information was extracted from the government website on MPAs [31] and the MedAMP website [32] as well as from the websites of the MPAs themselves.

The dataset and non-indigenous macroalgae distribution were mainly based on the CIESM Atlas [33] and on articles by searching for "introduced species", "Mediterranean", and "French marine protected areas" on various browsers such as PubMed, Scopus, or ResearchGate. The ALIEN Corse network sightings were also considered, except for *Codium fragile* and *Melanothamnus harveyi*, which are difficult to identify in situ by divers and require microscopic examination and taxonomic expertise. In addition, a thorough analysis of the documents held at the Macrophytes Platform of the Mediterranean Institute of Oceanography (MIO) at Aix-Marseille University was conducted. This platform houses hundreds of documents of scientific grey literature, unpublished field notes, herbarium vouchers—HCOM herbarium (Marseille)—concerning NIS along the French Mediterranean coast since the 19th century. Within each MPA, we distinguished species that were actually reported (articles, herbarium specimens, unpublished observations, etc.) using the symbol "1" from species potentially present in the area, according to the CIESM Atlas of Verlaque et al. [34], which were marked with "?". When a species was absent from all reviews, it was indicated by "0". This bibliographical research enabled us to include articles published between 1907 and 2024.

We reviewed all possible documents from the published articles, grey literature, herbarium vouchers, and field notes, in order to analyze the trends and abundance of non-indigenous macroalgae within French Mediterranean MPAs (i.e., stable, in expansion, or in decline). NIS were classified using a qualitative scale, with "high" indicating invasive species that proliferate and form dense populations, "common" referring to species frequently observed across various geographical areas, and "confined" for those reported at only a very limited number of sites. In addition, we considered the first and last recorded years of observation to refine our assessment.

To identify the different non-indigenous algae in various MPAs, only national parks, nature reserves, and Natura 2000 areas were included. We did not include marine nature parks, which are vast areas mainly off the coast.

In order to enhance the efficiency of the mapping process and facilitate greater clarity in the quantification and distribution of species within MPAs, specific adjustments were made to the selected MPAs. Areas of the same type that were adjacent to one another were grouped together such as MPA13, MPA19, and MPA21. Furthermore, the Natura 2000 Camargue MPA was subdivided into two distinct zones, separating the Camargue and Fos-sur-Mer areas (MPA8 and MPA9, respectively). The latter encompasses a major commercial port, which could potentially influence the prevalence of invasive species.
In cases where an MPA was wholly or predominantly encompassed by a more strictly regulated MPA, it was removed from the dataset in favor of the more regulated area (e.g., MPA11, MPA14, MPA25). Finally, the "Îles du Cap Corse" nature reserve was excluded due to its very small surface area (38.40 ha). All of the MPAs had their own identifier (ID), and all the information relating to them is presented in Supplementary Table S1.

## 3. Results and Discussion

### 3.1. Characteristics and Regulations of Different Categories of Marine Protected Areas in the French Mediterranean

There are various categories of MPA, each with its own management plan, highlighting the diversity and heterogeneity of the regulations in force. The strictest restrictions apply to national parks and nature reserves, with no-take zones (NTZs) and reinforced protection zones (RPZs) recognized for promoting the recovery of fish stock biomass through stringent regulations that strictly limit or prohibit fishing activities. Multi-use management (MUM) areas, with artisanal fishery authorized but subject to strict and properly implemented regulations (mesh size, net length, soaking time, etc.), associated with, for example, a ban on angling, spear fishing, and trawling such as French national parks, belong to this category [34–37]. In contrast, Natura 2000 areas, biotope decrees, and natural marine parks are subject to fewer or no regulations (with the exception of regulations common to all French marine waters) (Table 1). Regardless of the type of MPA and their protection status, none of them currently benefit from measures specifically intended to limit the introduction of marine NIS, except for public information campaigns.

<table><caption>**Table 1.** Different regulations applying to the various categories of MPAs.</caption><thead><tr><td>MPAs Type</td><td>Type of Regulation by MPA</td><td>Governance</td><td>Presence of No-Take Zone (NTZ)/Reinforced Protection Zone (RPZ)</td></tr></thead><tbody><tr><td>Nature reserve</td><td>More or less regulated and monitored</td><td>Management bodies</td><td>Yes/Yes</td></tr><tr><td>National Park</td><td>More or less regulated and monitored</td><td>National public establishment</td><td>Yes/Yes</td></tr><tr><td>Marine nature park</td><td>More or less regulated and monitored</td><td>Local authorities, socio-professionals</td><td>No</td></tr><tr><td>Natura 2000 areas</td><td>More or less unregulated and monitored</td><td>Steering committee</td><td>No</td></tr></tbody></table>

There are two marine national parks in the French Mediterranean: the Calanques National Park (CNP) created in 2012 and the Port-Cros National Park (PCNP) created in 1963, making it the oldest MPA in France. These parks were created to protect and manage nature in such a way as to guarantee the long-term survival of the exceptional heritage they contain. Managed by a national public establishment under the supervision of the Ministry of the Environment, they have scientific councils that apply strict regulations in the core areas of the park, and more flexible regulations in the zones of adhesion. In both national parks, in the framework of the MUM strategy, no-take zones have been established, prohibiting all forms of fishing as well as reinforced protection zones that restrict fishing activities only to specific types. In the CNP, seven NTZs have been implemented. At PCNP, certain areas prohibit anchoring entirely, regardless of activity, and most of the Port-Cros archipelago bans recreational fishing within 600 m off the coast. Only a small area permits trolling, subject to strict quotas and conditions [11,38].
There are four nature reserves in the French Mediterranean, one in the Pyrénées-Orientales (Cerbère-Banyuls) and three in Corsica (Scandola, Bouches de Bonifacio, and Îles du Cap Corse). These areas were established to protect habitats and support the restoration of animal and plant populations. These MPAs are run by management bodies and have scientific committees. In the Scandola Nature Reserve, for example, all forms of spear fishing, trawling, and hunting are banned, as are the anchoring of boats and fishing in the core area. In the Cerbère-Banyuls Nature Reserve, all forms of fishing are banned from a core area, certain enhanced protection zones prohibit recreational fishing, while the majority of the reserve allows regulated fishing requiring specific authorization [39].

There are two marine nature parks in the French Mediterranean, the "Parc Naturel du Golfe du Lion" and the "Parc Naturel Marin du Cap Corse et de l'Agriate", created with the aim of "protecting nature and managing resources sustainably". Managed mainly by the local authorities, stakeholder, and users, with the state playing a minor role, these parks are involved in raising awareness among schoolchildren and the general public. Marine natural parks do not have restricted protection zones (RPZs); however, they do have wardens who can issue reports, enabling them to address issues such as marine pollution, fishing, and navigation rules to protect the area [40].

Natura 2000 was created to implement two European Union (UE) directives: the "Habitats, Fauna and Flora" directive and the "Birds" directive. Their aim is to ensure the long-term survival of species and habitats of European community interests and that are particularly threatened in Europe. Managed by a steering committee that draws up the documents defining the objectives, these areas have coordinators who raise awareness among professionals and other stakeholders. These areas are subject to recommendations or incentives from the steering committee and not to proper regulations [41]. Most of the Natura 2000 sites included in this study were established between 2006 and 2015.

This analysis highlights the diversity of regulations depending on the category of MPA as well as the specific variability of each MPA, even within the same category. Regulations can range from very strict protection (as in the Scandola Nature Reserve and the PCNP) to lighter rules (nature marine park and Natura 2000 areas) (Table 1).

In terms of surface area, Natura 2000 areas make up the largest network with 2,062,279 hectares, representing 59% of the total surface area of French MPAs (Table 2 and Figure 1). Marine nature parks came second with 32% of the total surface area, followed by national parks (7%) and nature reserves (2%) (Table 2, Figure 1).

In fact, MPAs with strict regulations (i.e., national parks and nature reserves) accounted for only 9% of the surface area of French MPAs in the Mediterranean (Table 2 and Figure 1). It is particularly noteworthy that many of these areas often overlap marine protected areas with stricter legislation. For example, of the 36 Natura 2000 sites, 26 overlap MPAs with a higher level of regulation, and over 43% of these areas are included in marine protected areas with a higher level of protection (Table 2).

<table><caption>**Table 2.** Surface area in (ha) of each MPA in the French Mediterranean according to category. Data extracted from MedAMP [32].</caption><thead><tr><td>Category of MPA</td><td>Total Surface (ha)</td></tr></thead><tbody><tr><td>Natura 2000</td><td>2,062,279</td></tr><tr><td>Natura 2000 outside MPA better protected sites</td><td>1,178,044</td></tr><tr><td>Marine nature park</td><td>1,091,427</td></tr><tr><td>National Park</td><td>244,884</td></tr><tr><td>Nature reserve</td><td>79,265</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Percentage of the area covered by each MPA category. The orange outline highlights MPAs with a higher level of protection (data from MedAMP [32]).</figcaption></figure>

The biocenoses of all these MPAs are similar, corresponding to a mosaic of habitats including *Posidonia oceanica* beds, *P. oceanica* dead matte, coralligenous formations, rocky reefs, sand, and coastal detrital sediments.

## 3.2. *Trends and Distribution of Non-Indigenous Algae Species Within Marine Protected Areas*

According to the maps drawn up based on the literature, no MPA is spared from the presence of non-indigenous macroalgae, a fact that has been widely highlighted worldwide [42,43]. Non-indigenous macroalgae may even be more numerous within than outside MPAs [44–48]. Based on confirmed in situ observations, the Occitanie region hosts an average of 3.4 ± 4.0 NIS (mean ± SD) (Figure 2), while the PACA region has 7.5 ± 6.4 (Figure 3), and Corsica 4.0 ± 3.1 (Figure 4).

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Survey and distribution of reported non-indigenous macroalgal species in marine protected areas of the Occitania region: cumulative numbers (present or absent) and protection status.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Survey and distribution of reported non-indigenous macroalgal species in marine protected areas of the Sud (PACA) region: cumulative numbers (present or absent) and protection status.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Survey and distribution of reported non-indigenous macroalgal species in marine protected areas of the Corse region (Corsica): cumulative numbers (present or absent) and protection status.</figcaption></figure>
When considering averages by type of MPA, national parks hosted the highest average number of invasive species, with 18.0 ± 1.4 NIS, followed by nature reserves (8.3 ± 4.7) and Natura 2000 sites (3.7 ± 3.3).

Conversely, when including both confirmed NIS within MPAs and those likely present within the geographical area, the Sud region remained in the lead, with 18.0 ± 3.1 NIS, followed by Corsica with 8.9 ± 1.1, and Occitania with 13.0 ± 1.1. Regarding averages by type of MPA in this broader scenario, national parks still hosted the highest average number of species (23.0 ± 2.6 NIS), followed by nature reserves (11.0 ± 3.0) and Natura 2000 sites (13.0 ± 3.6).

That the highest number of non-indigenous seaweeds were found in the Sud region can be partly explained by the presence of 122 ports including 109 marinas, 10 commercial ports, with the most important being Marseille-Fos, and 3 naval ports including the largest French military base at Toulon. Ports are recognized as major introduction beachheads and may facilitate the establishment of NIS in these areas [49]. These ports facilitate introductions through multiple pathways such as ballast water discharge, hull fouling on vessels, and dispersal via fishing activities and yachting. A well-known example of the importance of ports in the transport of NIS is that of the Californian crab *Pyromaia tuberculata* (Lockington, 1877). After becoming established in San Francisco, this crab was subsequently transported to Japan, Korea, Asia, and then New Zealand. At each stage of its spread, the species had previously established itself in ports before being transported to other ports by ships [50–52]. In the Mediterranean, ports such as Gibraltar, Suez, and Istanbul maintained their high-risk status for the introduction of NIS between 2012 and 2018, connecting different regions and thereby facilitating the spread of various NIS in the Mediterranean [53]. Regarding the invasion of *Caulerpa taxifolia*, initially introduced through the Monaco Oceanographic Museum, and *Caulerpa cylindracea*, their spread is primarily attributed to fishing activities, particularly through fishing nets and yachting as well as recreational boats via anchoring. Thus, ports, through the concentration of maritime vessels and traffic, increase the risk of the spread of NIS [5]. Therefore, the strategic location and intense maritime traffic of these ports play a significant role in facilitating the establishment and spread of non-indigenous seaweeds in Mediterranean MPAs. In the Sud region, oyster farms are located inside the perimeters of the large commercial ports of Fos-sur-Mer and Toulon [54], close to one or several MPAs. Oyster farming is also one of the main vectors of NIS introduction in the Mediterranean Sea [55].

The highest number of NIS observed in an MPA were in the two national parks, Port-Cros National Park and Calanques National Park, with 17 and 19 species, respectively (Figure 3). This can be attributed to the fact that more rigorous documentation and monitoring measures are applied in these types of MPAs, enabling the detection of a higher number of non-indigenous macroalgae compared with areas in the Natura 2000 network, where there is less monitoring.

Some MPAs, such as MPA 2, MPA 12, and MPA 15, all classified as Natura 2000 sites, have no confirmed observations of NIS. However, it seems unlikely that no species are actually present in these areas, given their geographical location. Considering the total number of species likely present in the area, their estimated diversity could reach 11, 18, and 14 species, respectively (Figures 2 and 3; Table 3).
<table><caption>**Table 3.** Distribution of non-indigenous macroalgal species across French Mediterranean marine protected areas (MPAs): 1 = the species has been reported in the MPA; ? = the non-indigenous species (NIS) is present in the geographical region but has never been definitely reported from the MPA; 0 = the species has never been reported in the region.</caption><thead><tr><th rowspan="3"></th><th colspan="15">Marine Area Protected Identifier (MPA ID)</th><th rowspan="3">Total Confirmed Presences (1)</th></tr><tr><th colspan="3">MPA ID Occitania</th><th colspan="3">MPA ID Sud (PACA)</th><th colspan="3">MPA ID Corsica</th><th colspan="3">MPA ID Occitania</th><th colspan="3">MPA ID Sud (PACA)</th><th colspan="3">MPA ID Corsica</th></tr><tr><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>11</th><th>12</th><th>13</th><th>14</th><th>15</th><th>16</th><th>17</th><th>18</th><th>19</th><th>20</th><th>21</th><th>22</th><th>23</th><th>24</th><th>25</th></tr></thead><tbody><tr><td>Acrothamnion preissii</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>5</td></tr><tr><td>Anotrichium furcellatum</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>10</td></tr><tr><td>Antithamnion amphigeneum</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Antithamnion boergesenii</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Antithamnionella spirographidis</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>1</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>6</td></tr><tr><td>Antithamnionella ternifolia</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>0</td><td>?</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>4</td></tr><tr><td>Asparagopsis armata</td><td>1</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>1</td><td>?</td><td>1</td><td>1</td><td>1</td><td>?</td><td>1</td><td>13</td></tr><tr><td>Asparagopsis taxiformis</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Bonnemaisonia hamifera</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Caulacanthus okamurae</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Caulerpa cylindracea</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>?</td><td>1</td><td>1</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>14</td></tr><tr><td>Caulerpa taxifolia</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>4</td></tr><tr><td>Chondria curvilineata</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Chrysonephos lewisii</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>?</td><td>2</td></tr><tr><td>Codium fragile</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>4</td></tr><tr><td>Colaconema codicola</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>4</td></tr><tr><td>Colpomenia peregrina</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>3</td></tr><tr><td>Cutleria multifida</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td><td>?</td><td>1</td><td>11</td></tr><tr><td>Punctaria tenuissima</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Dictyota cyanoloma</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Goniotrichiopsis sublittoralis</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Hypnea spinella</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Laurencia caduciramulosa</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>1</td><td>1</td><td>?</td><td>1</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>4</td></tr><tr><td>Laurencia okamurae</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Lophocladia trichoclados</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>2</td></tr><tr><td>Melanothamnus harveyi</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>4</td></tr><tr><td>Phrix spatulata</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>1</td></tr><tr><td>Bryocladia atlantica</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td><td>?</td><td>?</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>3</td></tr><tr><td>Rugulopteryx okamurae</td><td>0</td><td>0</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>9</td></tr><tr><td>Sarconema filiforme</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Sargassum muticum</td><td>1</td><td>0</td><td>0</td><td>1</td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>4</td></tr><tr><td>Solieria filiformis</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Spongoclonium caribaeum</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Undaria pinnatifida</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Womersleyella setacea</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>1</td><td>1</td><td>?</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td><td>1</td><td>?</td></tr><tr><td>Total of actual observations of NIS (1)</td><td>12</td><td>0</td><td>1</td><td>3</td><td>4</td><td>3</td><td>1</td><td>1</td><td>8</td><td>9</td><td>19</td><td>0</td><td>3</td><td>17</td><td>0</td><td>9</td><td>10</td><td>6</td><td>4</td><td>1</td><td>5</td><td>10</td><td>4</td><td>1</td><td>3</td></tr><tr><td>Total actual and possible number of NIS (1 + ?)</td><td>14</td><td>11</td><td>12</td><td>14</td><td>13</td><td>12</td><td>12</td><td>12</td><td>18</td><td>18</td><td>23</td><td>18</td><td>18</td><td>22</td><td>14</td><td>18</td><td>16</td><td>16</td><td>9</td><td>8</td><td>9</td><td>11</td><td>9</td><td>8</td><td>8</td></tr></tbody></table>

The variation in NIS diversity within MPAs is likely influenced by several factors such the proximity of donor sites (ports, aquacultures facilities), the frequency of vector
activity (fishing activities, recreational boating), and settlement dynamics, which depend on habitat type and their area, depth range, and thermal regime (wind-induced daily temperature variation). For instance, MPA 15 (Figure 3) is predominantly composed of sandy bottoms, which limits the presence of species associated with photophilic rocky substrates or coralligenous habitats. The same applies to MPAs 2, 3, and 4 (Figure 2). In contrast, MPAs 9 and 10 (Calanques National Park) feature a more diverse range of habitats including *Posidonia* seagrass meadows, coralligenous reefs, sandy bottoms, and infralittoral rocky areas, which may support a greater variety of NIS [56].

The notable difference between the number of species with confirmed presence ("1") and the total number of species potentially present in the MPAs ("1" + ")" underscores the insufficient in situ monitoring of NIS. The number based solely on confirmed field observations was considerably lower than those that included species potentially present in the geographical area. For instance, while certain species such as *Bonnemaissonnia hamifera* are assumed to be widely distributed and invasive, their actual observation remains limited to a very few sites such as Banyuls-sur-Mer (Table 3). However, all Mediterranean reports concern the filamentous sporophyte, *Trailliella intricata* phase, which is very small, very discreet in the environment, and recognizable only by a specialist. A similar observation applies to *Codium fragile*, a species known for its widespread presence but only recorded within four marine protected areas (MPAs). This includes zones with stricter regulations and enhanced monitoring such as CNP, PCNP, the Scandola Nature Reserve, and the Cerbère-Banyuls Natural Reserve. The concentration of *Codium fragile* in these areas underscores the effectiveness of higher protection levels.

A total of 35 non-indigenous macroalgae have been recorded in various French Mediterranean MPAs (Table 3). The most widespread species, such as *Rugulopteryx okamurae*, *Asparagopsis armata*, *Womersleyella setacea*, and *Caulerpa cylindracea*, are considered invasive and have a significant impact on flora and fauna, particularly through their rapid proliferation, interspecific competition for substrates, and resource acquisition [57,58]. In particular, *R. okamurae* is known to cause significant damage to benthic communities in the CNP [59]. This is also the case for *W. setacea*, which impacts populations of the gorgonian species *Paramuricea clavata* (Risso, 1826) and increases the risk of necrosis [60] (Tables 3 and 4). The colonization by *Caulerpa cylindracea* of sandy bottoms has led to the decline in certain bivalve species associated with these habitats [61]. By altering the composition of invertebrate species, *Caulerpa cylindracea* also appears to reduce beta diversity across the invaded sites, resulting in the homogenization of ecosystems [62].

*Asparagopsis armata* is known to impact benthic communities, particularly the calcifying alga *Ellisolandia elongata*, whose biomass significantly decreases when in contact with *A. armata*, especially during its biomass peak in the spring [63].

*Sargassum muticum*, which was mainly recorded in the Pyrénées-Orientales region (Occitania), has become locally extinct in the open sea since the late 1980s [64,65].

Finally, two species have recently been reported, *Lophocladia trichoclados* in 2021 at the PCNP [66], and *Laurencia okamurae* in 2023 at Fos-sur-Mer (Région Sud) (Thibaut, T., unpublished observation) (Table 4).

There is currently little monitoring and recording of NIS in the French Mediterranean MPAs, and significant uncertainty persists regarding the current presence of these algae. Among all records of non-indigenous macroalgae, 34% were noted as new but have not been seen again (Table 4). As a result, the presented list (Table 3) combines non-indigenous macroalgae including non-indigenous macroalgae that may have become locally extinct, recently observed species that might fail to establish, and well-established long-term residents, which could distort the perceived importance of non-indigenous macroalgae at a site. For example, *Codium fragile*, which was widely present in the area of the future Calanques
National Park in the 1960s and large quantities that were stranded were removed by bull-dozers [67], is currently rarely observed in the same area. This decline may be attributed to an increase in the discharge into the sea in the 1980s and its extremely low genetic diversity. Indeed, the Mediterranean population of *C. fragile* is represented by a single haplotype found in Japan [68] and consists exclusively of parthenogenetic females [69], a factor that likely contributes to its long-term vulnerability.

It is also interesting to note that certain NIS including some considered invasive, such as *Caulerpa cylindracea* and *Womersleyella setacea*, which are known for their wide distribution, are nevertheless rarely mentioned in published articles and reports, despite their confirmed presence.

<table><caption>**Table 4.** Non-indigenous macroalgae dynamics occurring in French Mediterranean MPAs. The darker the orange coloring, the more abundant the species is in the MPA; ? = absence of information.</caption><thead><tr><th>Non-Indigenous Species</th><th>Abundance</th><th>Dynamics</th><th>First–Last Observation</th><th>References of First–Last Observation</th></tr></thead><tbody><tr><td>Acrothamnion preissii (Sonder) E.M. Wollaston</td><td>High</td><td>?</td><td>1981–2015</td><td>[38–70]</td></tr><tr><td>Anotrichium furcellatum (J. Agardh) Baldock</td><td>?</td><td>?</td><td>1926–1974</td><td>[71,72]</td></tr><tr><td>Antithamnion amphigeneum A. Millar</td><td>Confined</td><td>?</td><td>2002–2008</td><td>[73,74]</td></tr><tr><td>Antithamnionella boergesenii (Cormaci & G. Furnari) Athanasiadis</td><td>?</td><td>?</td><td>1974–2005</td><td>[72–74]</td></tr><tr><td>Antithamnionella spirographidis (Schiffner) E.M. Wollaston</td><td>?</td><td>?</td><td>1927–2008</td><td>[71–74]</td></tr><tr><td>Antithamnionella ternifolia (Hooker f. & Harvey) Lyle</td><td>Confined</td><td>?</td><td>1926–2005</td><td>[71–74]</td></tr><tr><td>Asparagopsis armata Harvey</td><td>High</td><td>Stabilization</td><td>1930–2024</td><td>[75]–Authors' observations</td></tr><tr><td>Asparagopsis taxiformis (Delile) Trevisan</td><td>Confined</td><td>?</td><td>2005–2014</td><td>S. Ruitton unpublished data–[76]</td></tr><tr><td>Bonnemaisonia hamifera Hariot</td><td>Common</td><td>?</td><td>1968 Single observation</td><td>[77]</td></tr><tr><td>Caulacanthus okamurae Yamada</td><td>Confined</td><td>?</td><td>2004 Single observation</td><td>M. Verlaque unpublished data</td></tr><tr><td>Caulerpa cylindracea Sonder</td><td>High</td><td>Decline</td><td>1997–2024</td><td>[78]–Authors' observations</td></tr><tr><td>Caulerpa taxifolia (M. Vahl) C. Agardh</td><td>Common</td><td>Decline</td><td>1990–2024</td><td>[79]–Authors' observations</td></tr><tr><td>Chondria curvilineata Collins & Hervey</td><td>Confined</td><td>?</td><td>1982–2002</td><td>[73–80]</td></tr><tr><td>Chrysonephos lewisii (W.R. Taylor) W.R. Taylor</td><td>High</td><td>?</td><td>1988 Single observation</td><td>[81]</td></tr><tr><td>Codium fragile (Suringar) Hariot</td><td>Common</td><td>Decline</td><td>1946 Single observation</td><td>[77]</td></tr><tr><td>Colaconema codicola (Børgesen) Stegenga, J.J. Bolton & R.J. Anderson</td><td>Common</td><td>?</td><td>1950–1989</td><td>[82,83]</td></tr><tr><td>Colpomenia peregrina Sauvageau</td><td>Confined</td><td>Decline</td><td>1956–2009</td><td>[84,85]</td></tr><tr><td>Cutleria multifida (Turner) Greville</td><td>?</td><td>?</td><td>1905–2002</td><td>[73–86]</td></tr><tr><td>Desmotrichum tenuissimum (C. Agardh) Athanasiadis</td><td>?</td><td>?</td><td>1975 Single observation</td><td>[87]</td></tr><tr><td>Dictyota cyanoloma Tronholm, De Clerck, A. Gómez-Garreta & Rull Lluch</td><td>Confined</td><td>Increase</td><td>1999–2024</td><td>[88]–Authors' observations</td></tr><tr><td>Goniotrichopsis sublittoralis G.M. Smith</td><td>?</td><td>?</td><td>2002 Single observation</td><td>[73]</td></tr><tr><td>Hypnea spinella (C. Agardh) Kützing</td><td>Confined</td><td>?</td><td>1989–2002</td><td>[73–83]</td></tr><tr><td>Laurencia caduciramulosa Masuda & S. Kawaguchi</td><td>Confined</td><td>?</td><td>2002 Single observation</td><td>[73]</td></tr><tr><td>Laurencia okamurae Yamada</td><td>Confined</td><td>?</td><td>2023–2024</td><td>T. Thibaut unpublished observation</td></tr><tr><td>Lophocladia trichoclados (C. Agardh) F. Schmitz 1893</td><td>Confined</td><td>Stabilization</td><td>2021–2024</td><td>[66]–Authors' observations</td></tr><tr><td>Melanothamnus harveyi (Bailey) Díaz-Tapia & Maggs</td><td>Common</td><td>?</td><td>1958–2002</td><td>[73–89]</td></tr><tr><td>Phrix spatulata (E.Y. Dawson) M.J. Wynne, M. Kamiya & J.A. West</td><td>?</td><td>?</td><td>2002 Single observation</td><td>[73]</td></tr></tbody></table>
<table><caption>**Table 4. Cont.**</caption><thead><tr><th>Non-Indigenous Species</th><th>Abundance</th><th>Dynamics</th><th>First–Last Observation</th><th>References of First–Last Observation</th></tr></thead><tbody><tr><td>Bryocladia atlantica Kapraun & J.N. Norris)<br>Díaz-Tapia 2023 ('2024')</td><td>?</td><td>?</td><td>1984–2002</td><td>[65–73]</td></tr><tr><td>Rugulopteryx okamurae (E.Y. Dawson) I.K.<br>Hwang, W.J. Lee & H.S. Kim</td><td>High</td><td>Increase</td><td>2013–2024</td><td>M. Verlaque unpublished<br>data–Authors' observations</td></tr><tr><td>Sarconema filiforme (Sonder) Kylin</td><td>?</td><td>?</td><td>1984 Single observation</td><td>[65]</td></tr><tr><td>Sargassum muticum (Yendo) Fensholt</td><td>Locally extinct</td><td>Locally extinct</td><td>1986–1989</td><td>[65]</td></tr><tr><td>Solieria filiformis (Kützing) P.W. Gabrielson</td><td>?</td><td>?</td><td>2007 Single observation</td><td>[74]</td></tr><tr><td>Spongoclonium caribaeum (Børgesen) M.J. Wynne</td><td>?</td><td>?</td><td>1974 Single observation</td><td>[90]</td></tr><tr><td>Undaria pinnatifida (Harvey) Suringar</td><td>Locally extinct</td><td>Locally extinct</td><td>1982 Single observation</td><td>[64]</td></tr><tr><td>Womersleyella setacea (Hollenberg) R.E. Norris</td><td>High</td><td>Increase</td><td>1987–2015</td><td>[91,92]</td></tr></tbody></table>

### 3.3. Effectiveness of Marine Protected Areas in Preventing Non-Indigenous Species Establishment

The establishment date of marine protected areas (MPAs) is crucial for the conservation status of their ecosystems [2,5], particularly for species with long life cycles and limited dispersal capacity. For instance, the Port-Cros National Park (PCNP), established in 1963, successfully restricted construction along the island's coastline and protected its flora and fauna for more than six decades [11,38]. This prolonged protection has allowed relatively intact populations of Fucales (*Cystoseira* spp. and *Sargassum* spp.) to persist along the shoreline [93]. Elton's ecological resistance theory (ERT) predicts that high species diversity and complex food webs enhance ecosystem resilience to invasions, whereas disturbances leading to low species diversity and simplified food webs increase vulnerability. This theory also assumes that ecosystems have a species saturation threshold, limiting opportunities for new species to establish [94]. However, the concepts of species saturation and ecosystem resistance to invasions are now widely challenged [95–98]. While MPAs, by promoting diverse marine ecosystems and complex food webs, have been considered more resistant to biological invasions [99–101], several studies suggest otherwise. For example, *Asparagopsis taxiformis* has a higher coverage in MPAs in the Azores than outside [47], while in Ireland, the barnacle *Austrominius modestus* and the brown alga *Sargassum muticum* have been reported in high abundance within the Lough Hyne Marine Reserve [102,103]. In the Eastern Mediterranean, the fish species *Siganus luridus* and *Pterois miles* are observed in higher densities in certain MPAs in Greece and Cyprus compared with surrounding areas [45,46,48,104].

In some cases, fishing bans and regulations can inadvertently promote the establishment and proliferation of introduced species by providing them with refuge areas that enable them to escape fishing pressure. For example, in Cyprus, the density of lionfish (*Pterois miles*) was twice as high in no-fishing zones compared with areas where only commercial fishing was allowed and could reach up to four times this density in protected areas compared with those where both commercial and recreational fishing were permitted [48].

The role of MPAs in controlling invasive species remains poorly understood and lacks consensus, largely due to the scarcity of data and the lack of studies on different taxonomic groups [105,106].

Our results suggest that MPAs are not immune to NIS, regardless of their location or level of protection. In addition, the proximity of certain MPAs to commercial ports and oyster farms or areas of intensive maritime activity may increase their exposure to the introduction of exotic species. Thau Lagoon, where oyster farming relies on the import of Japanese spat, hosts the highest number of non-indigenous seaweed species in the French Mediterranean (58 taxa) [5] and is located near several MPAs, notably MPA 4 and MPA 5 (Figure 2). The CNP is located near a commercial port and major maritime routes, so is
particularly exposed to potential new species arrivals, with over 10 million tons of goods transported through the area in the first half of 2024 [107].

Within marine protected areas with high levels of protection, particularly when maintained over several decades, some introduction vectors, such as fishing and anchoring, could be mitigated. This mitigation would reduce the propagule pressure of NIS. Furthermore, highly protected MPAs may paradoxically attract more recreational boaters, thereby increasing the risk of NIS dispersal. It has been demonstrated that tourism, particularly through yachts frequenting these areas, can facilitate the introduction of non-indigenous species [108]. Despite restrictions aimed at regulating nautical activities, the reserves of Scandola Marine Reserve, Port-Cros National Park, and Calanques National Park are among the most visited sites by recreational boaters in the French Mediterranean, which could intensify propagule pressure and contribute to NIS dispersion. Finally, the significant presence of NIS in some MPAs, despite their protection status, may partly be explained by introductions that occurred before their designation. Most MPAs were established after the 2000s, particularly with the implementation of the Natura 2000 marine network in 2006 and the creation of the Calanques National Park. However, 89% of the NIS recorded in this study had already been observed before 2006 (Table 4), suggesting that their current presence has been the result, at least in part, by historical introductions whose impacts persist to this day.

### 3.4. *How Should Invasive Species Be Managed and Regulated in MPAs?*

In accordance with the Convention on Biological Diversity (CBD) and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), a number of measures can be deployed to restrict the entry pathways of invasive species into these areas [1,13]. Prior to their arrival, prevention can be achieved through the implementation of information, regulation, and quarantine measures. In the initial stages of introduction, it is also possible to detect the species in question at an early stage and implement monitoring, surveillance, and even removal measures before the species proliferates. Finally, once the species has become established, it can be contained by physical removal and controlled. It is of particular importance to direct attention to the prevention and early detection of introduced species, which are the most effective tools for the management of these species in MPAs (Figure 5). Prevention and the dissemination of information on introduced species to the general public, scientists, and local political authorities are also key measures that are already used in many MPAs in France. This is particularly the case with the ALIEN network in Corsica and Occitania, which are awareness-raising and participatory science networks where citizens report introduced species using a form and photo via a website. For the period of 2021–2022, the Corsican ALIEN network reported 127 sightings, twice as many as in 2019–2020 [109].

In addition to mediation efforts, in situ monitoring in MPAs is essential for detecting non-indigenous species (NIS). This monitoring should be simple to implement, cost-effective, and conducted regularly. A proposed protocol involves selecting reference sites representing different habitat types, with the number of sites adjusted based on the size of the MPA and the available resources. It was proposed that diverse surveys using two linear transects are conducted, twice in summer and twice in winter.

When an NIS is detected, it is crucial to photograph and collect the specimen for identification using a microscope or a binocular loupe in the laboratory. If the collected specimen is confirmed as an NIS, its dynamics must be monitored. Therefore, it is imperative that the personnel responsible for monitoring possess taxonomic expertise [110].

When an NIS is detected during these surveys, a detailed assessment is conducted, particularly for non-indigenous macroalgae. To evaluate the coverage of invasive al-
gae and monitor their spatio-temporal dynamics, some protocols recommend using 25 cm × 25 cm quadrats, subdivided into 25 sub-quadrats of 5 cm × 5 cm. These quadrats are randomly placed on the substrate, and for each one, the number of occupied sub-quadrats is recorded [5]. This monitoring method has been particularly used to track the invasion of *Rugulopteryx okamurae* near the Strait of Gibraltar using permanent quadrats. From 2013 to 2020, this monitoring survey not only tracked the expansion of *R. okamurae*, but also documented the decline in local benthic communities [111].

After the arrival and establishment of an NIS in MPA, early eradication is often the most effective method; we cite the case of *Caulerpa taxifolia*, a species of green alga that was discharged from a Monaco aquarium in 1984 and that has widely colonized the French Riviera coast [79,112]. In 1992, early detection measures were implemented in the Port-Cros archipelago (PCNP), followed by annual campaigns of removal from 1994. Since 2011, the species is no longer present at the site [113].

In the same way, this approach has been applied to *C. taxifolia* in California through the “shoot first, ask questions later” strategy, where rapid and aggressive eradication efforts are undertaken without extensive preliminary assessment, aiming to quickly eliminate the invasive species before it can establish further and cause additional ecological damage. As soon as it was first spotted in 2000, rapid and effective decisions were made regarding the eradication method to be used, the funding for the operation, and those responsible for its implementation. Seventeen days after its observation, implementation of the eradication protocol began. In the end, the use of bleach (5% sodium hypochlorite) in submerged containers was chosen to successfully eradicate the species [114,115].

Numerous actions for the control of invasive species are carried out in the terrestrial parts of national parks. In the CNP, action plans have been in place for over 10 years, including more than 141 terrestrial invasive plant removal projects, especially *Agave americana* Linnaeus or *Opuntia ficus-indica* (Linnaeus) Mill. [116]. In addition, lists of invasive plant species are included in the Park's charter to prohibit their planting and introduction. Similarly, programs to eradicate *Carpobrotus spp.* (ice plant) and *Rattus rattus* (Linnaeus, 1758) (black rats) have been carried out since 2010 in the PCNP, particularly in the Port-Cros archipelago.

In general, in the marine part of the MPA, this type of action is limited in French MPAs. This can be attributed to a number of factors including a relative lack of knowledge regarding marine ecosystems and the inherent challenges associated with detecting and monitoring species introduced into these environments. The logistical complexities, high costs of underwater operations, and lack of accurate data on the distribution and impact of invasive species also contribute to this situation. In most French MPAs, there is no monitoring or preventive action plan to limit the introduction of invasive species, unlike the practices of many American national parks that incorporate invasive species management into their long-term planning and day-to-day operations. Managing biofouling is particularly important, as 19% of introduced plant species are believed to originate from these vectors [117]. For example, the Curecanti and Glen Canyon national recreation areas (USA) have implemented “boat checks” to ensure that boats are free of zebra and quagga mussels (*Dreissena polymorpha* (Pallas, 1771) and *D. bugensis* (Andrusov, 1897) before entering the park [118]. This method is also employed in New Zealand, where the Craft Risk Management Standard (CRMS-BIOFOUL) has been implemented. These standard mandates that vessels maintain a “clean hull” in order to prevent the spread of invasive marine species. This regulation, which came into effect in May 2018, sets out specific biofouling management practices and provides three options for compliance for vessels. The objective is to reduce the risk of non-indigenous species entering New Zealand’s marine
ecosystems through biofouling, with a particular focus on preventing their establishment and spread [119,120].

Furthermore, measures such as establishing blacklists, which prohibit the import of species recognized as invasive or threatening to native ecosystems—as in Great Britain—or whitelists, which only allow the import of non-harmful species included on a pre-established list—as in New Zealand—are highly effective in limiting the establishment of introduced species. The whitelist strategy, the most restrictive, has significantly reduced propagule pressure and the risks of biological invasion [121–123].

To effectively contain and limit accidental introductions of NIS through their primary vectors, detection and monitoring protocols should be implemented in high-risk introduction areas within MPAs such as ports and oyster farms. For instance, in the Gulf of La Spezia, protocols developed by the Smithsonian Environmental Research Center (SERC) have been implemented in certain ports and marinas. Between 2018 and 2020, 50 PVC panels were deployed annually at five sites, leading to the identification of 79 taxa including 11 non-indigenous species [124]. In aquaculture, sorting and cleaning protocols have been implemented including the immersion of mollusks in specific disinfectant solutions, such as controlled biocides (diluted chlorine or acetic acid), to eliminate invasive species without affecting the cultivated organisms. The effective management of aquaculture infrastructures is also crucial in limiting the establishment of NIS. Measures such as the regular cleaning of structures with high-pressure jets, the application of antifouling paints containing biocides (copper, zinc), and the use of removable structures that allow for the aerial exposure of submerged surfaces contribute to reducing the adhesion and development of invasive species [125,126]. For example, oyster farming in Thau Lagoon (Occitania) is a major vector of non-indigenous macroalgae in the northwestern Mediterranean, surpassing the Suez Canal. Despite its Natura 2000 status, its hard substrates and aquaculture infrastructure promote the establishment of species such as *Codium fragile*, *Sargassum muticum*, and *Undaria pinnatifida*, exerting strong ecological pressure. The lagoon remains a key hotspot for future introductions; however, no regulations currently limit these species [127].

In general, the late eradication of an NIS, once it is well-established and spreading, is significantly more costly and less effective, while also incurring additional expenses due to the ecosystem and socio-economic damage it has already caused [5,128] (see Figure 5). The cost of damage can be colossal, as was the case for the mussel *D. polymorpha*, which cost 3.2 billion USD between 1993 and 1999 in the United States due to damage to thermal power stations [129].

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Recommended strategies for managing NIS in marine protected areas (adapted according to [130]).</figcaption></figure>
Few studies have provided a comprehensive estimate of the total cost of NIS in French Mediterranean MPAs. However, some figures are available for the French Mediterranean coastline. Between 2017 and 2021, expenditure related to NIS monitoring and information totaled EUR 1,183,731 including the costs associated with research, theses, internships, assessments under the Marine Strategy Framework Directive (MSFD), and initiatives such as the "Alien Corse" network.

At a more local scale, the case of *Caulerpa taxifolia* illustrates these management costs. Between 2012 and 2016, the annual surveillance, prevention, and awareness efforts in the commune of Le Pradet (near the PCNP) amounted to approximately EUR 5900. In contrast, over the same period, control measures for this species within the Port-Cros National Park (PCNP) exceeded EUR 150,000, highlighting the substantial economic burden of management once the species has become established [131].

**4. Conclusions**

This study highlights the extensive distribution of non-indigenous macroalgal species within French Mediterranean marine protected areas (MPAs). This widespread presence underlines the fact that MPAs are in no way inaccessible to NIS; their supposed richness in species and the health of their ecosystems do not protect them; moreover, they do not host more species than the outer areas. The degree of protection (national parks with strong regulation of human activities, vs. Natura 2000 areas) has no effect on the number of NIS. It is, in fact, the proximity of pathways allowing the arrival of exotic species that seems to explain the record number of NIS in the Calanques and Port-Cros National Parks: the major ports of Marseilles and Toulon. What matters most would therefore be the vector.

This study also highlights a significant lack of knowledge and monitoring, largely
due to the inherent challenges of observing marine environments. Finally, while MPAs are
designed to protect biodiversity (species and ecosystems) and enhance biodiversity, there
are few stringent regulations to limit the arrival of NIS, a major driver of biodiversity change,
either via secondary pathways from neighboring areas already colonized, or via primary
pathways from native areas. By 2030, the French government aims to increase strongly
protected marine areas to 10% of national waters compared with the current coverage of
0.6% [132]. As far as NIS are concerned, this shift will not result in a decrease in their
pressure in MPAs if habitat monitoring and control measures regarding the primary and
secondary pathways are not undertaken, both within MPAs and outside. This shift presents
a crucial opportunity to strengthen MPA resilience against invasive species pressure and
improve management and monitoring practices in these regions.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jmse13020374/s1, Table S1: Details of the marine protected areas (MPAs) analyzed including identification, classification, and geographical information.

**Author Contributions:** Conceptualization, M.B. and T.T.; Methodology, M.B., C.-F.B. and T.T.; Validation, M.V. and C.-F.B.; Writing—Original Draft Preparation, M.B. and C.-F.B.; Writing—Review and Editing, S.R., A.B., M.P.-B., B.T., M.V., C.-F.B. and T.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work received support from the Port-Cros National Park and the Région Sud [PhD grant].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** A significant portion of the data was provided by the Macrophytes Platform of the Mediterranean Institute of Oceanography (MIO) at Aix-Marseille University.
**Acknowledgments:** The authors are grateful to Michael Paul, a native English speaker, for proofreading the manuscript. We also thank Christine Pergent-Martini for providing us with certain data from the Réseau Alien Corse.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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