Catalyzing Conservation: An Analysis of Fish Stock Dynamics in a Marine Protected Area before and after Artificial Reef Deployment

: The marine ecosystem’s balance is crucial for sustaining biodiversity and supporting fisheries. Marine protected areas have been increasingly used to enhance marine habitats, yet their impact on fish populations remains a topic of debate. This study focuses on a marine protected area in Kitros, Pieria, in Greece, where an artificial reef was constructed, to understand its influence on coastal fish populations. The objectives were to investigate the changes in fish biomass and abundance, comparing the data from periods before and after the construction of an artificial reef. This research compares the data between 2007 and 2008 with the data between 2016 and 2017, collected with bottom trawl surveys strategically executed prior to and after the artificial reef’s installation. Fish species captured were identified, with their lengths and masses measured. The findings indicate an increase in the biomass and abundance of certain fish species after artificial reef deployment, notably the commercially significant Mullus barbatus and Pagellus erythrinus . The artificial reef in Kitros, Pieria, with its surrounding marine protected area appears to have had a positive impact on the local fish populations over the years, suggesting that it can contribute to marine conservation and fishery enhancement. These results underscore the potential of artificial reefs as tools for marine ecosystem management, offering insights for policymakers and environmentalists into coastal resource management.


Introduction
The Kitros, Pierias marine protected area (MPA) in Greece, characterized by freshwater inputs from the Aliakmonas, Axios, and Loudias river deltas, undergoes significant seasonal environmental changes [1].Factors, such as photoperiod and solar radiation, contribute to the creation of a thermocline in spring and summer.This period also sees an increased freshwater influx due to higher rainfall and ice melt from regional mountains, affecting water column stratification and river runoff in the area [2].Moreover, the area is influenced by two primary water bodies: Black Sea Water (BSW) and Levantine Intermediate Waters (LIWs) [3,4].A primary goal for establishing MPAs, such as the one in the Kitros, Pieria, region of the NW Aegean Sea, is to aggregate various fish species.This aggregation supports local fishing communities by increasing the catch per unit effort (CPUE) and enhancing access to marine resources [5].Artificial reefs (ARs) provide a foundation for epifauna, utilizing organic waste discharged into the sea.They also protect marine life from trawling activities and support artisanal fisheries.Additionally, these reefs improve fish habitat, enhance coastal erosion protection, and offer marine research opportunities while serving as a haven for adult marine species [6].Studies have demonstrated their effectiveness in increasing primary productivity and helping recreational fisheries [1,6].When an AR is established, the area surrounding is declared as an MPA [2].In 2014, an artificial reef was established in the Kitros, Pieria, area.A three-year monitoring program between 2015 and 2017, which employed bottom trawl methods, identified more than 70 fish species in the vicinity of the MPA surrounding the reef [1].Prior to the reef's installation in [2007][2008] with no MPA present, a set of samplings was conducted through bottom trawl as part of an initial preliminary survey.The present study aims to compare the fish populations sampled before and after the MPA's creation.This comparison seeks to investigate any changes in the abundance and biomass of these populations over the years following the artificial reef's establishment.

Artificial Reef Construction
Constructing an AR under current Greek legislation involves a three-stage process.The initial stage entails identifying a potential area, conducting a preliminary field study, and determining the precise location for the artificial reef.Following these steps, requisite approvals are obtained from various authorities, including the Hydrographic Service, the Archaeological Service, and the State Land Service.Subsequently, the construction phase commences, typically spanning 2-3 years, including both the announcement and actual building phases.Post-construction, a scientific monitoring period of three years is mandated to evaluate the reef's impact on environmental enhancement, reduction in marine population mortality, and establishment of a protected area with specific regulations for fishing and other human activities.Within the marine protected area, key regulations included a complete prohibition on bottom trawling and seine fishing, the authorization of fishing nets with a minimum mesh size of 45 mm (stretched), and fish traps with a comparable mesh size, in addition to implementing a four-month annual ban on fish trap usage.Furthermore, minimum catch sizes were established for the most commercially valuable species caught within the MPA.A total ban on spearfishing was also imposed in the area.On average, the entire process, from planning to implementation, took approximately 10 years.This procedure was adhered to for the Kitros artificial reef.The preliminary study was conducted in 2007-2008, construction occurred in 2011-2013, and scientific monitoring was carried out in 2015-2017.The data presented in this work were derived from this entire process.Notably, all studies were conducted by the same entity using the exact same methodology, which was a rare opportunity that eventually led to the data processing and subsequent publication.

Sampling Site
The Thermaikos Gulf, located in the western part of the North Aegean Sea, experiences unique hydrological characteristics.This body of water is primarily influenced by the influx of freshwater from four main rivers: Axios, Aliakmonas, Loudias, and Gallikos.Among these, Axios and Aliakmonas stand out for their complex delta systems with multiple channels [7,8].The Axios River has been pinpointed as a major contributor of pollution, funneling high levels of nutrients from its basin into the Thermaikos Gulf [9].The gulf's water circulation is characterized by saltier water entering from the east and moving in a northwestern direction, while lighter river water flows southward along the western shore [10].The Thermaikos Gulf is typically a mesotrophic zone, yet episodes of severe eutrophication have been recorded, especially during prolonged periods of southern winds [8].Lastly, ref. [11] suggests a decline in the overall health of the ecosystem, evidenced by reduced fish populations and biomass, attributable to overfishing and various environmental stressors.

Seasonal Monitoring
Before the establishment of the AR, an ichthyological investigation was undertaken, encompassing four sampling events between 2007 and 2008.The preliminary sampling took place in May 2007, followed by subsequent ones in September 2007, April 2008, and June 2008.These samplings utilized a bottom trawl method within selected fishing routes in the Coasts 2024, 4 152 Thermaikos Gulf, off Kitros.The trawl employed was a modified bottom trawl featuring a small mesh opening (20 mm mesh size, stretched).The trawl's horizontal length was 20 m, and it was towed at a speed of 3 nautical miles per hour.Trawling operations were conducted using the same vessel during both sampling periods and at precisely identical locations.The trawling was strategically aligned to be perpendicular to pre-set transects and as parallel as feasible to the isobaths.Each sampling comprised three hauls, conducted at depths varying from approximately 27 to 36 m.Hauls were strategically positioned along the edges of the area that would become the artificial reef complex (Figure 1).This study aggregated the average abundance and biomass data from these three hauls into a single sampling value, hence presenting a consolidated figure for each species' abundance and biomass for each sampling date, despite the occurrence of three hauls per date.

Seasonal Monitoring
Before the establishment of the AR, an ichthyological investigation was undertaken, encompassing four sampling events between 2007 and 2008.The preliminary sampling took place in May 2007, followed by subsequent ones in September 2007, April 2008, and June 2008.These samplings utilized a bottom trawl method within selected fishing routes in the Thermaikos Gulf, off Kitros.The trawl employed was a modified bottom trawl featuring a small mesh opening (20 mm mesh size, stretched).The trawl's horizontal length was 20 m, and it was towed at a speed of 3 nautical miles per hour.Trawling operations were conducted using the same vessel during both sampling periods and at precisely identical locations.The trawling was strategically aligned to be perpendicular to pre-set transects and as parallel as feasible to the isobaths.Each sampling comprised three hauls, conducted at depths varying from approximately 27 to 36 m.Hauls were strategically positioned along the edges of the area that would become the artificial reef complex (Figure 1).This study aggregated the average abundance and biomass data from these three hauls into a single sampling value, hence presenting a consolidated figure for each species' abundance and biomass for each sampling date, despite the occurrence of three hauls per date.

Figure 1.
Haul sites (N = 3) for sampling with bottom trawl in the outer Thermaikos Gulf of the Aegean Sea offshore of the coastal zone of Kitros, in the Pieria region of Greece.Field sites were located on edge habitats of the marine area of a previously constructed artificial reef complex (polygon with blue edges) [1].
Following the construction of the artificial reef in 2014, a series of seasonal surveys were executed from June 2015 to September 2017.These surveys formed a part of the three-year monitoring initiative after the reef's establishment [1,2].The sampling stations were the same in both surveys conducted in 2007-2008 and 2015-2017 (Figure 1).

Fish Abundance and Biomass Calculation
After each haul, the catch was identified to the species level [12,13].Onboard, the length frequencies of each species and their abundance in terms of number and weight were recorded.The analyses utilized two primary measures: abundance and biomass.Abundance was expressed as the number of individuals per square kilometer (individuals/km 2 ) and biomass as kilograms per square kilometer (kg/km 2 ).
The trawl's scanning surface area was determined by calculating the door spread, using Carrothers' formula [14,15].area swept (km ) = (doorspread, km) × (towspeed, km h ) × (towduration, h) Following the construction of the artificial reef in 2014, a series of seasonal surveys were executed from June 2015 to September 2017.These surveys formed a part of the three-year monitoring initiative after the reef's establishment [1,2].The sampling stations were the same in both surveys conducted in 2007-2008 and 2015-2017 (Figure 1).

Fish Abundance and Biomass Calculation
After each haul, the catch was identified to the species level [12,13].Onboard, the length frequencies of each species and their abundance in terms of number and weight were recorded.The analyses utilized two primary measures: abundance and biomass.Abundance was expressed as the number of individuals per square kilometer (individuals/km 2 ) and biomass as kilograms per square kilometer (kg/km 2 ).
The trawl's scanning surface area was determined by calculating the door spread, using Carrothers' formula [14,15].area swept(km 2 ) = (door spread, km) × (tow speed, km h ) × (tow duration, h) Fish density (individual/km 2 ) was then determined by calculating the area swept by the trawl net, estimating the density in that area and then extrapolating it to a larger area in km 2 [16] Similarly, the biomass was calculated by using weight data instead of individual number data [16,17] In this study, a similarity percentages (SIMPER) analysis was conducted to elucidate the specific contributions of various species to the observed dissimilarities between sampling events.This study concentrated on comparing the samples gathered before and after the artificial reef was constructed, using the Bray-Curtis dissimilarity index for the evaluation.Biomass values of species, were employed as average figures.For each species, one average value represented the 2007-2008 sampling period, and another average biomass value was used for the 2016-2017 period.Diversity indexes were determined across the sampling periods, before and after the establishment of the artificial reef.A non-metric multi-dimensional scaling (MDS) analysis of fish abundance data (individual/km 2 ), categorized by 8 sampling dates before 2007-2008 and after 2016-2017 regarding the artificial reef construction in Kitros, Pieria, was also performed.

Species Biomass
Species biomass changes before and after the establishment of the marine protected area are detailed in Table 3, highlighting three categories based on the nature of biomass change.
The most notable increase was seen in Mullus barbatus, with the biomass rising from 9.9 kg/km 2 to 5973.2 kg/km 2 .Pagellus erythrinus also saw a significant increase from 6.6 kg/km 2 to 2424.4 kg/km 2 .Likewise, Arnoglossus laterna went up from 37.6 kg/km 2 to 1724.5 kg/km 2 , and Citharus linguatula from 28.5 kg/km 2 to 1788.0 kg/km 2 .The biomass values of Diplodus annularis and Diplodus vulgaris significantly increased from 304.5 kg/km 2 to 3020.8The most notable increase was seen in Mullus barbatus, with the biomass rising from 9.9 kg/km 2 to 5973.2 kg/km 2 .Pagellus erythrinus also saw a significant increase from 6.6 kg/km 2 to 2424.4 kg/km 2 .Likewise, Arnoglossus laterna went up from 37.6 kg/km 2 to 1724.5 kg/km 2 , and Citharus linguatula from 28.5 kg/km 2 to 1788.0 kg/km 2 .The biomass values of Diplodus annularis and Diplodus vulgaris significantly increased from 304.5 kg/km 2 to 3020.8 kg/km 2 and from 1.1 kg/km 2 to 297.1 kg/km 2 , respectively.Other species, like Liocarcinus depurator, Scorpaena notata, and Serranus hepatus, showed notable increases, indicating enhanced habitat and resource availability within the marine protected area.Penaeus kerathurus also saw an increase from 0.3 kg/km 2 to 354.9 kg/km 2 .

SIMPER Analysis
The results demonstrate significant changes in the biomass of certain species after the establishment of the artificial reef.Notably, Mullus barbatus and Pagellus erythrinus, two prominent commercial species in the region, showed an increase in biomass.The average biomass of Mullus barbatus escalates from 1.77 to 8.79 kg/km 2 , and Pagellus erythrinus from 1.6 to 7.02 kg/km 2 , as indicated in Table 4.These species were the main contributors to the dissimilarities observed between the sampling periods.This can be further proven by comparing specific sampling months.Other significant contributors to this dissimilarity included Pomatomus saltatrix, which appeared only in the latter group, and Pagellus bogaraveo and Citharus linguatula, which exhibited an increase in biomass.A noteworthy point is the increased biomass values for species like Arnoglossus laterna, Sparus aurata, Chelidonichthys lucerna, and Diplodus annularis.It is also important to mention that, out of the 52 species in Table 4 contributing to the dissimilarity between the periods, only 8 species showed a decrease in biomass or a complete absence in the post-reef creation period.The completely absent species were Cepola macrophthalma, Platichthys flesus, Leuerigobius friessi, Solea solea, Mullus surmuletus, and Pegusa lascaris.Trisopterus minutus displayed a slight decline, from 3.3 kg/km 2 in the initial period to 2.42 kg/km 2 in the latter.

Diversity Indexes
In the assessment of the Kitros MPA's impact on local fish biodiversity, the investigation included an analysis of species richness, abundance, and diversity indices before and after the reef's establishment (Table 5), with a consideration for seasonal variation influences.The pre-construction phase, spanning from May 2007 to June 2008, revealed an average species richness of 46.5 and an average total abundance of 210.5 individuals per square kilometer.Diversity indices averaged as follows: Shannon's Diversity Index (H' log_e) at 3.6595, Simpson's Diversity Index (1-Lambda) at 0.9755, Brillouin's Index at 3.345, and Fisher's Alpha at 18.5475, indicating a stable biodiversity level with an even species distribution.In contrast, the post-construction phase, from April 2016 to May 2017, exhibited a slight increase in average species richness to 48.25, with a marked decrease in the average total abundance to 119.25 individuals per square kilometer.Despite the reduced abundance, diversity indices improved, with an average Shannon's Index of 3.70825 and Simpson's Index of 0.980675, alongside a Brillouin's Index of 3.195 and Fisher's Alpha of 27.155, suggesting enhanced and more stable biodiversity outcomes post-reef construction.
The analysis acknowledges the role of seasonal variations, which are evident in the fluctuations observed in species richness and total abundance across the sampling periods.Such variations underline the dynamic nature of marine ecosystems and the adaptability of fish communities to environmental changes.The consistent improvement in diversity indices post-construction underscores the MPA's beneficial impact on local biodiversity, beyond mere seasonal effects.
In summary, the post-reef construction period demonstrated an overall increase in species richness and a fluctuating, but generally higher, level of diversity indices, suggesting that the artificial reef may have had a beneficial impact on local fish biodiversity.

Non-Metric Multi-Dimensional Scaling
The multi-dimensional scaling (MDS) analysis, focusing on fish sampling data before and after the artificial reef construction in Kitros, Pieria, reveals distinct groupings based on the sampling dates (Figure 4).This analysis utilized Kruskal's stress formula 1 with a minimum stress threshold of 0.1.The results are reported in both three-dimensional (3D) and two-dimensional (2D) configurations, each achieving a stress value of 0, indicating a representation of the dataset in the reduced-dimensional space.In the 3D configuration, sampling dates from May 2007 to June 2008, corresponding to the period before the reef construction, are primarily negative along the first axis.This contrasts with the post-reef construction dates from April 2016 to May 2017, which exhibit positive values along the same axis.The 2D configuration also shows a clear separation; the pre-reef samples are clustered on the negative side of axis 1, while the post-reef samples predominantly lie on the positive side.The percentages accompanying each sample in both configurations indicate the contribution of each sampling date to the overall stress, with the pre-reef dates contributing more significantly in the 2D configuration.This distinct separation in the 2D configuration highlights a significant shift in the fish community composition associated with the artificial reef's establishment.The April 2016 and June 2016 samples, for instance, demonstrate a notable shift toward the positive end of axis 1.The 3D configuration provides a more detailed spatial representation, showing a similar pattern of separation but with additional complexity due to the third dimension.These MDS results, particularly the low stress values and clear delineation between the pre-and post-reef periods, strongly suggest that the construction of the artificial reef has a measurable impact on the structure of the fish community in the area.The separation patterns observed in the MDS analysis offer a clear visualization of the ecological shifts attributable to the artificial reef's installation.

Non-Metric Multi-Dimensional Scaling
The multi-dimensional scaling (MDS) analysis, focusing on fish sampling data before and after the artificial reef construction in Kitros, Pieria, reveals distinct groupings based on the sampling dates (Figure 4).This analysis utilized Kruskal's stress formula 1 with a minimum stress threshold of 0.1.The results are reported in both three-dimensional (3D) and two-dimensional (2D) configurations, each achieving a stress value of 0, indicating a representation of the dataset in the reduced-dimensional space.In the 3D configuration, sampling dates from May 2007 to June 2008, corresponding to the period before the reef construction, are primarily negative along the first axis.This contrasts with the post-reef construction dates from April 2016 to May 2017, which exhibit positive values along the same axis.The 2D configuration also shows a clear separation; the pre-reef samples are clustered on the negative side of axis 1, while the post-reef samples predominantly lie on the positive side.The percentages accompanying each sample in both configurations indicate the contribution of each sampling date to the overall stress, with the pre-reef dates contributing more significantly in the 2D configuration.This distinct separation in the 2D configuration highlights a significant shift in the fish community composition associated with the artificial reef's establishment.The April 2016 and June 2016 samples, for instance, demonstrate a notable shift toward the positive end of axis 1.The 3D configuration provides a more detailed spatial representation, showing a similar pattern of separation but with additional complexity due to the third dimension.These MDS results, particularly the low stress values and clear delineation between the pre-and post-reef periods, strongly suggest that the construction of the artificial reef has a measurable impact on the structure of the fish community in the area.The separation patterns observed in the MDS analysis offer a clear visualization of the ecological shifts attributable to the artificial reef's installation.

Discussion
Several species made their first appearance in the area post-AR construction, highlighting the transformative impact of the reef on the local marine ecosystem.The presence of Blennius ocellaris post-reef construction was particularly noteworthy, as this species was

Discussion
Several species made their first appearance in the area post-AR construction, highlighting the transformative impact of the reef on the local marine ecosystem.The presence of Blennius ocellaris post-reef construction was particularly noteworthy, as this species was not previously recorded in the area.This emergence suggests the reef's potential for creating suitable habitats or conditions favorable for species not formerly prevalent.Similarly, Trachinus draco and Lepidotrigla cavillone appeared only after the reef was established and the MPA was declared, indicating possible shifts in habitat preferences or expansions in habitat ranges facilitated by the artificial reef structures.The presence of Micromesistius poutassou, Dasyatis pastinaca, and Raja montagui further exemplifies the MPA's influence on a wider range of species, possibly due to changes in the benthic environment or enhanced food availability.
In the years following the artificial reef's development, a significant increase in biomass was observed in several marine species.Among the top performers, Mullus barbatus experienced a remarkable rise, leading the list of species benefiting from the reef.Pagellus erythrinus also showed a notable enhancement in its biomass, following closely behind.Interestingly, species like Penaeus kerathurus and Pomatomus saltatrix, which were either few or not present in the previous data, emerged with substantial biomass values, indicating the reef's role in supporting new marine life.Pagellus bogaraveo, Citharus linguatula, and Arnoglossus laterna also demonstrated considerable increases, reflecting the positive ecological impact of the reef.Sparus aurata, a species previously absent, made its debut in the ecosystem, further underscoring the diversity fostered by the artificial reef.Chelidonichthys lucerna and Diplodus annularis both showed significant improvements, thus highlighting the protected area's role in enhancing marine biodiversity and biomass.
As described before, the strong increase in the biomass of Mullus barbatus and Pagellus erythrinus from 2007-2008 to 2016-2017 deserves a special mention, due to the importance of these species as commercial catch (Figures 2 and 3, respectively).The populations of these species may also have flourished due to the fishing restrictions imposed for bottom trawlers within the marine area of Kitros, Pieria, after the artificial reef's development.Therefore, setting the probable beneficial effect of the artificial reef aside, population mortality may have been reduced due to trawling restrictions that came with the declaration of the reef's marine protected area.Coastal fisheries with gillnets and trammel nets, however, continued throughout the years, despite the ban on trawling.
Both Mullus barbatus and Pagellus erythrinus prefer softer substrates and gravelly sea bottoms [18,19], like the ones that formed around the area of the artificial reef [1,2].As the substrate within the marine protected area transitioned from muddy to a gravel-dominated environment, these species notably thrived.However, the shift proved less advantageous for species such as Gobius niger and Leuserigobius friesii, which did not benefit as much from the new gravelly substrate.
By definition, pelagic fish live in the pelagic domain, that is, they move freely in the water column where they spend most of their time.The presence of small pelagic fish in this study, such as Trachurus mediterraneus, Sardina pilchardus, and Engraulis encrasicolus, can be characterized as random.These species live across the water column and their presence in the bottom trawl samples may be considered as random [20].
Several studies have researched artificial reefs for aspects like abundance, biomass, and species diversity [21,22].However, using a species composition index is more advisable to prevent skewed interpretations of effectiveness [23].The evaluation of the ARs' impact on restoration, using genuinely comparable and appropriate reference sites, is scarce.The majority of AR deployments suffer from a lack of comprehensive ecological data, posing challenges for thorough ecosystem assessments [24].The results of this study portray differences in the abundance and biomass in local fish populations before and after the creation of the artificial reef in 2014.
Restoring marine ecosystems, including the use of active interventions, is considered beneficial for promoting natural species recruitment and survival, reinstating ecosystem structure and function, and enhancing the abiotic processes that influence community dynamics.This is particularly vital in the context of the extensive degradation of reef ecosystems due to climate change [25].Employing innovative artificial reef strategies in coastal and offshore regions can be used for facilitating habitat restoration.However, this requires a well-structured, pragmatic, and scalable approach that identifies the strengths and weaknesses of these methods.It is important to conduct thorough assessments of local regeneration requirements and constraints to ensure effective restoration efforts [23].
However, it is worth mentioning that the Intermediate Disturbance Hypothesis (IDH) posits an optimal level of disturbance within ecosystems that can maximize biodiversity, supporting a diverse array of species by preventing dominance by any single species.This principle, when applied to marine protected areas (MPAs), suggests that not all disturbances are detrimental; moderate disturbances, whether natural or controlled human Coasts 2024, 4 164 activities, can in fact enhance biodiversity within MPAs.Such disturbances can maintain a balance between early successional and more competitive species, thereby promoting a rich diversity of marine life.This perspective advocates for a management strategy for MPAs that not only aims to minimize human impacts, but also recognizes the potential ecological benefits of maintaining intermediate levels of disturbance to foster biodiversity and ecological health [26].
In the Pieria region, alongside the artificial-reef-designated marine protected area (MPA) near Kitros, a similar initiative was launched near Litochoro in 2017, located 36 km to the south of Kitros, resulting in another MPA.The development of networks of MPAs, as opposed to isolated entities, represents a modern conservation strategy.Such networks enhance marine biodiversity benefits by facilitating species movement across protected zones and promoting genetic diversity.Transitioning from solitary MPAs to comprehensive MPA networks necessitates an expansion of governance models, incorporating both topdown strategies to manage human and ecological connections across the MPA spectrum.This approach must strike a balance between enabling local participation within each MPA and addressing broader challenges to fulfill overarching conservation goals, integrating top-down governance mechanisms.Addressing this balance is critical yet often overlooked in MPA discourse [27].When the possibility of a network of MPAs is discussed, it is perhaps worthy to mention that a study assessed the vulnerability of Mediterranean marine protected areas (MPAs) to the invasion of Lessepsian fish species under current and future climate scenarios.It was found that MPAs, especially in the Levantine Sea, are at a high risk of invasion by these species, with projections indicating an increase in suitable habitats for these invasive fish by 2050.This poses a challenge for conservation efforts in the region [28].This however may not be the case in the region of Pieria as the heavy freshwater inflow from nearby rivers may act preventively against many Lessepsian fish species.As seen in Table 1, no Lessepsian migrant species were caught in the area of Kitros.The only migrant species caught was the northern brown shrimp (Penaeus aztecus) that originates from the east coast of the US and Mexico.As this species is the only migrant species observed in the entire region of Pieria, it is widely speculated that its existence is due to an accidental release from a failed aquaculture attempt.
Constructing artificial reefs is costly and logistically difficult [29].Hence, an evaluation of the scientific basis for reef construction and deployment is critical.In the past, a philosophical assumption was stated underlying the construction of artificial reefs, indicating that regional fish production is limited by a paucity of hard bottom habitats [30,31].However, this assumption may have been supported by short-term descriptive studies of individual reefs [30].
If habitat is limiting, new reefs can potentially increase fish production through an increase in the foraging habitats of adult, juvenile, or newly recruited fishes, an increase in the nesting habitats of adult fishes, and an increase in the number of resting habitats from predators.As a result, stock sizes of economically important species increase, and commercial fishers can benefit.And, since all artificial reefs are colonized by fishes, increasing habitats can mean that local increases in fish abundance and biomass are produced [32].Hence, an evaluation of the scientific basis for reef construction and deployment is critical.
The continued monitoring of both natural and artificial reefs would provide estimates of the population size on individual reefs and the total regional population size.The relative importance of habitat and recruitment could be further tested by increasing the number of artificial reefs within an area and measuring the relationships formed.Any positive relationship would indicate some value in constructing artificial reefs.
There is a lack of knowledge in several important scientific domains.Scientific studies have to focus on ecosystem variability, on the scales to be considered, and the appropriate experimental designs to reveal representative results.Ultimately, the success of a MPA or an AR will reflect the quality of the prior planning and ongoing management [33].
The differences in marine biodiversity and biomass before and after the AR implementation highlight the importance of adopting a holistic management approach.Such approaches are crucial for addressing anthropogenic pressures and ensuring the resilience of marine ecosystems.The case of Cocos Island, as part of the Eastern Pacific Marine Corridor, exemplifies how national-scale efforts can create biological corridors, enhancing the connectivity between MPAs and contributing to the conservation of globally significant marine biodiversity [34,35].
It is, however, worth mentioning that the effectiveness of marine protected areas (MPAs) in meeting their conservation objectives has sometimes been met with skepticism, as some MPAs have not achieved their intended outcomes, despite various promising signs.This discrepancy has prompted several researchers to critically examine MPAs' capacity to prevent biodiversity loss [36][37][38].

Conclusions
In conclusion, the research conducted on the impact of the artificial reef of Kitros and its surrounding marine protected area has yielded significant results.The study's primary objective, assessing the changes in species biomass and diversity post-AR development, was effectively met.The key findings reveal a notable increase in both the biomass and diversity of certain marine species, for example, the highly commercial Mullus barbatus and Pagellus erythrinus, indicating that artificial reefs can positively influence marine habitats.However, the research also highlighted a major gap in understanding the long-term ecological impacts of artificial reefs and their surrounding MPAs.While benefits were observed, the study suggests that continuous monitoring is essential to fully comprehend the effects over extended periods.This is particularly relevant for assessing the sustainability of such interventions and their alignment with broader conservation goals.Based on these results, future studies should focus on longitudinal assessments of ARs, examining their ecological impact over decades rather than just years.This would provide more comprehensive insights into their roles in marine ecosystem restoration and conservation, helping to inform more effective environmental management strategies.Additionally, expanding the research to include a wider range of ecological parameters can further help us understand the multifaceted impacts of artificial reefs.

Figure 1 .
Figure 1.Haul sites (N = 3) for sampling with bottom trawl in the outer Thermaikos Gulf of the Aegean Sea offshore of the coastal zone of Kitros, in the Pieria region of Greece.Field sites were located on edge habitats of the marine area of a previously constructed artificial reef complex (polygon with blue edges) [1].

Figure 4 .
Figure 4. Non-metric multi-dimensional scaling (MDS) analysis of fish abundance data (individual/km 2 ), categorized by 8 sampling dates before(2007)(2008) and after (2016-2017) the artificial reef construction in Kitros, Pieria.The analysis is visualized in both three-dimensional (3D) and twodimensional (2D) configurations.The 3D graph illustrates the spatial distribution of sampling dates in a three-axis system, while the 2D graph provides a simplified view with a clear separation along the first axis.Negative values along axis 1 in both configurations are associated with pre-reef construction dates, whereas positive values are linked with post-reef construction dates.These spatial patterns demonstrate a shift in the fish community composition corresponding to the periods before and after the artificial reef's establishment.

Figure 4 .
Figure 4. Non-metric multi-dimensional scaling (MDS) analysis of fish abundance data (individual/km 2 ), categorized by 8 sampling dates before (2007-2008) and after (2016-2017) the artificial reef construction in Kitros, Pieria.The analysis is visualized in both three-dimensional (3D) and two-dimensional (2D) configurations.The 3D graph illustrates the spatial distribution of sampling dates in a three-axis system, while the 2D graph provides a simplified view with a clear separation along the first axis.Negative values along axis 1 in both configurations are associated with pre-reef construction dates, whereas positive values are linked with post-reef construction dates.These spatial patterns demonstrate a shift in the fish community composition corresponding to the periods before and after the artificial reef's establishment. .

Table 1 .
List of species present during the surveys before(2007)(2008)and after (2016-2017) the creation of the artificial reef at field sites in the outer region of the Thermaikos Gulf in the Aegean Sea, offshore of the coastal zone of Kitros, in the Pieria region of Greece.

Table 2 .
List of species abundance /km 2 during the surveys before(2007)(2008)and after (2016-2017) the creation of the artificial reef at field sites in the outer region of the Thermaikos Gulf in the Aegean Sea, offshore of the coastal zone of Kitros, in the Pieria region of Greece.

Table 3 .
List of species biomass (kg/km 2 ) during the surveys before(2007)(2008)and after (2016-2017) the creation of the artificial reef at field sites in the outer region of the Thermaikos Gulf in the Aegean Sea offshore of the coastal zone of Kitros, in the Pieria region of Greece.
The Mullus barbatus biomass obtained in June 2016 is 22,219.8kg/km 2 , showing 1335.7 kg/km 2 and 1.66 kg/km 2 for June and April 2008, respectively.(Table 3).Similarly, the Pagellus erythrinus biomass values were 2920.8kg/km 2 and 5818.8 kg/km 2 in April and June 2016, respectively, showing 15.58 and 8.39 in April and June of 2008.

Table 5 .
Diversity indexes are shown for each sampling period.S shows number of species present, N shows similarity percentage analysis (SIMPER) using average biomass values for species caught in 2007-2008 and 2016-2017.