Introduction
Rhodolith beds are distributed worldwide in sedimentary infralittoral and circalittoral bottoms (Foster 2001; Teichert 2024) and are common in European seas (Donnan and Moore 2003a, b). They can be found at depths ranging from 5 to 35 m in the Atlantic Ocean (Hall-Spencer 1998; Foster 2001; Peña et al. 2014) and at shallower and greater depths in the Mediterranean Sea (Basso et al. 2017; Pierri et al. 2024). Rhodoliths-forming species play an important ecological role in marine ecosystems, being considered bioengineers (Foster 2001; Nelson 2009; Straube et al. 2024), as they increase the structural complexity of the sedimentary bottoms, creating habitats that provide substrate and refuge for a wide variety of species during key vital stages such as settlement and nursing, which increase the biodiversity and density of benthic and nekton-benthic communities (e.g. Kamenos et al. 2004; Teichert 2014; Peña et al. 2014). Moreover, rhodoliths beds are one of the main biogenic sources of calcium carbonate on the planet, with a potential role in climate regulation (Amado-Filho et al. 2012; Neves et al. 2021).
Rhodolith beds are widespread habitats in the Western Mediterranean coastal detritic bottoms at depths between 25 and 120 m (Ballesteros et al. 1993; Giaccone et al. 1994; Birkett et al. 1998) and are characterized by high amounts of free-living calcareous red algae of the subclass Corallinophycidae (Rhodophyta, Florideophyceae) (Basso et al. 2015). These species, which can be highly diverse in the Mediterranean (Joher et al. 2016), form unattached nodules with sizes ranging from 2 to 250 mm of mean diameter (Basso et al. 2015).
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Most of rhodolith-forming species are characterized by their slow rate of growth and carbonate deposition, about 1 mm/year (e.g. Bosence and Wilson 2003; Basso 2012). Hence, renewal rates are low (e.g. 10 to 15 years; Ballesteros 1989) and deposits can accumulate over 100s to 1000s of years. Thus, rhodolith beds have been considered a non-renewable resource (e.g. Foster 2001; Basso 2012). Because of their low resilience, several international initiatives have aimed at their conservation through the regulation of the main human threats such as commercial extraction of rhodoliths as fertilizer, fishing with benthic towed gears, water and sediments pollution from aquaculture and other coastal human activities (Hall-Spencer and Moore 2000a, b; Donnan and Moore 2003a, b; Hall-Spencer et al. 2006; Sanz-Lázaro et al. 2011; Basso et al. 2015).
Another habitat found in the coastal detritic bottoms of the Western Mediterranean but with a reduced distribution is the Laminaria rodriguezii forest. This deep-water kelp (Heterokontophyta, Phaeophyceae, Laminariales), endemic of the Mediterranean, grow on rhodolith beds which act as a hard substratum at depths between 50 and 120 m, mainly around 70 m (Boisset et al. 2016). Laminaria rodriguezii is characterized by a branched holdfast with numerous haptera and stolons, a short stipe to 4–10 cm long, a main undivided young blade up to 150 cm long by 30 cm wide and, in some cases, an older blade separated from the young one by a constriction (Boisset et al. 2016).
Laminaria rodriguezii occurs in areas of very specific biotic and abiotic parameters, which are still only partly understood (Žuljević et al. 2016). Its upper bathymetric limit possibly depends on temperature (kelps are generally considered as macroalgae with temperate and cold-water affinities), while the lower limit is presumably determined by light availability. In the Mediterranean, in spite of the oligotrophic waters, the species can form dense kelp forests (up to 210 fronds 2500 cm− 2; Linares et al. 2015), and generates a complex three-dimensional structure that can provide refuge, settlement and nursery habitat for a wide variety of marine species, including fishing resources, as well as other key ecosystems services such as nutrient cycling and carbon removal (Eger et al. 2023).
Both rhodolith beds and L. rodriguezii forests, which in some areas overlap as the former constitute the hard substratum needed for the development of the latter, constitute important marine habitats of high conservation value. The first ones are considered a sensitive habitat and two of the most common rhodolith-forming species, Phymatolithon calcareum and Lithothamnion corallioides, are included in Annex V of the EC Habitats Directive (Council Directive 92/43/EEC), as species of community interest whose collection in the wild and exploitation may be subject to management measures. Laminaria rodriguezii is also considered a priority conservation species according to the Barcelona Convention and included in the Annex II as an endangered and threatened species. According to Ballesteros (2006), as this species develops best in rhodolith beds, from where it has almost disappeared due to trawling activities, coralligenous bottoms now constitute its only refuge.
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The Menorca Channel (Balearic Islands, Western Mediterranean) presents a diverse and wide distribution of benthic habitats and species in a good conservation status (e.g. Barberá et al. 2012; Grinyó et al. 2018). Among the most important habitats, rhodolith beds predominate in the sedimentary bottoms of the continental shelf between 50 and 90 m depth. In some areas, these bottoms present an erect stratum with L. rodriguezii (Pérès and Picard 1964; Pérès 1967; Ballesteros 1994; Ordines and Massutí 2009; Joher et al. 2015), which is one of the most important populations of this brown alga in the Mediterranean (Joher et al. 2012).
Bottom trawling has negative impacts on rhodolith beds and, specially, on L. rodriguezii forests, due to the direct effects of extraction and mechanical destruction and indirect effects like increased turbidity that promotes their burial (Bordehore et al. 2003; Verlaque et al. 2019; Fragkopoulou et al. 2021). Indeed, changes in the benthic communities and in the composition and morphology of rhodolith-forming species have been reported due to bottom trawl impacts (Barberá et al. 2017; Ordines et al. 2017; Cabanellas-Reboredo et al. 2017; Farriols et al. 2022a) and higher abundances of L. rodriguezii have been found in areas with low or no bottom trawl fishing pressure like the Menorca Channel (Joher et al. 2012, 2015), Columbretes islands (Linares et al. 2015) and a non-trawled area in the Adriatic Sea (Žuljević et al. 2016).
The benthic habitats of the Menorca Channel were mapped in 2014 under the umbrella of the project LIFE + INDEMARES (Moranta et al. 2014; Requena and Gili 2014). Thereafter, in 2016, up to 1959 km2 between 50 and 100 m depth were declared Fishing Protection Zone (FPZ), as result of the implementation of the Council Regulation (EC) Nº 1967/2006, concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean Sea, which considers the rhodolith beds and coralligenous bottoms as protected habitats. In this area, bottom trawling was forbidden, which represented the effective protection of 80 and 95% of the rhodolith beds and coralligenous bottoms in the Menorca Channel, respectively (Fig. 1). In this work, we compare the distribution of rhodolith beds and L. rodriguezii forests in the Menorca Channel before and after the declaration of the FPZ in order to assess the results of this protection.
Fig. 1
Study area of the Menorca Channel (Balearic Islands, Western Mediterranean), showing the Site of Community Importance (SCI; continuous blue line) and the Fishing Protected Zone (FPZ; discontinuous blue line) established in 2014 and 2016, respectively, as well as the benthic habitat mapping between 50 and 100 m depth, developed during the LIFE + INDEMARES project (Moranta et al. 2014). The areas were differences in density of rhodoliths (continuous red line) and Laminaria rodriguezii (discontinuous red line) have been estimated and stations were measurements of L. rodriguezii blades have been made are also shown (yellow triangles). The Isobaths are 50, 100, 200, 500 and 800 m depth
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Materials and methods
Study area
The Menorca Channel is a large shallow submarine promontory, that present a regular relief and gentle slope (Canals and Ballesteros 1997; Acosta et al. 2002) and separates the islands of Majorca and Menorca. It is the area with the widest continental shelf of the Balearic Archipelago (western Mediterranean), corresponding to almost 20% of the coastal continental shelf bottoms off the Balearic Islands between 50 and 100 m depth (Barberá et al. 2012). The bottoms of the continental shelf of the Menorca Channel are formed by calcareous biogenic sedimentary material (Alonso et al. 1988), consisting mainly of sand and gravel. On the slope, finer sediments predominate, mainly compacted fine sands and silts (Requena and Gili 2014).
Like the other channels of the Balearic Archipelago, the Menorca Channel plays an important role in the regional circulation of the Western Mediterranean (Pinot et al. 2002). It forms a natural boundary between (i) the Algerian Sub-basin to the South, with warm and less saline waters of Atlantic origin, and subject mainly to forcing due to density gradients; and (ii) the Balearic Sub-basin to the North, with colder and more saline Mediterranean waters, affected by atmospheric forcing, mainly wind. The Menorca channel is very exposed to predominant winds from the North and North-west (Ordines et al. 2011).
The insignificant quantities of nutrients and terrigenous sediments from land runoff, due to low rainfall and the absence of rivers, make the waters around the Balearic Islands highly oligotrophic and transparent (Estrada 1996; Bosc et al. 2004), allowing algal communities to grow down to depths of 90–100 m (Ballesteros 1992, 1994). These particular conditions make the soft bottoms of the continental shelf of the Menorca Channel a suitable area for the development of rhodolith beds and L. rodriguezii forests (Barberá et al. 2012). The presence of these bottoms, together with other species and habitats of conservation interest such as coralligenous and sponges, gorgonians and coral gardens in rocky bottoms (Grinyó et al. 2018), justified that in 2014 the Menorca Channel was declared Site of Community Importance (SCI), within the framework of the Natura 2000 network. This SCI covers a surface of 3353 km2 and a wide variety of marine ecosystems, from the coast line to waters up to 1000 m depth (Fig. 1).
Fishing, both professional and recreational, is one of the main human activities carried out in the Menorca Channel (Massutí et al. 2023), with a great potential impact on the seabed, its species and benthic habitats. Although the impact of some small-scale fisheries should not be underestimated on rocky bottoms (e.g. trammel nets and bottom longlines), trawling is considered one of the largest anthropogenic impacts that can strongly modify the soft bottoms marine ecosystems. However, the number of trawl fishing vessels has remained historically very low in the Balearic Islands, including the Menorca Channel, compared to nearby areas off Iberian Peninsula (Quetglas et al. 2012). This could explain the fact that sensitive habitats such as rhodolith beds are widely distributed in the Menorca Channel and that even L. rodriguezii, a species much more vulnerable to bottom trawling than rhodoliths, is well represented in these soft bottoms (Barberá et al. 2012). In addition, the professional fishing sector of the Balearic Islands has been in a clear recession since the middle of the twentieth century, with a reduction of up to 75% in number of vessels since 1950 (Quetglas et al. 2017).
During recent years, other relevant changes affected the trawl fishing effort of the Menorca Channel: the declaration of the FPZ in 2016 and the implementation of the European Commission multiannual plan for the fisheries exploiting demersal stocks in the Western Mediterranean (Regulation EU 2019/1022). The implementation of this plan involved an effort regime that gradually reduced up to 40% the fishing days between 2020 and 2024.
Sampling
We used information from 12 oceanographic surveys partially or totally conducted in the Menorca Channel during the period 2009–2023 (Table S1). Biomass data of rhodolith-forming species and L. rodriguezii were obtained from samples collected with epi-benthic sledge, while presence/absence of both species and rhodoliths cover were estimated from submarine images of the seabed and its benthic biota. To compare the distribution and density of these species before and after the implementation of the FPZ in 2016, the following periods were considered: 2009–2015 and 2017–2023.
Two epibenthic sledges were used: (i) a standard beam trawl with 2 and 0.5 m horizontal and vertical openings, respectively described by Jennings (1999), whose efficiency was estimated by Reiss et al. (2006); and (ii) another beam trawl with horizontal and vertical openings of 1.3 and 0.88 m. The mesh size of both samplers was 10 mm and the towing speed ranged between 1.8 and 2 knots. A GPS and a SCANMAR system allowed to control the depth and the arrival and departure of the gear to the bottom, and thus, to estimate the sampled area by using the horizontal opening of the beam trawl and the distance covered during the haul, which varied between 102 and 872 m2.
Samples collected with both epibenthic sledges were sorted on board, identified to species level or to the lowest possible taxonomic level, counted and weighed. Unidentified specimens were preserved for further identification in the laboratory. The biomass of rhodolith-forming species (Lithophyllum racemus, L. corallioides, Lithothamnion minervae, Lithothamnion valens, P. calcareum, Spongites fruticulosus and unidentified Corallinophycidae) and L. rodriguezii per each sampling station was calculated from fresh weight. To do that the percentage of each algae species identified was estimated from a subsample and applied to the total biomass of algae in each sample. Then, biomass values were standardized to kg 250 m− 2, using the horizontal opening of the beam trawl and the distance covered during the haul.
The biomass of rhodolith-forming species and L. rodriguezii was also transformed into presence/absence data and complemented with data from visual observations that were made using two Remote Operated Vehicles (Bleeper EVO and Nemo ROV models), the photogrammetric sledge HORUS (a remotely operated towed vehicle (ROTV)), and an underwater drop camera (IPSE model). The ROV and ROTV transects were carried out with the vehicles moving at a maximum speed of 0.5 knots.
Images from ROV and ROTV were recorded along transects (Fig. 2), while images from IPSE were recorded at fixed stations between these transects in order to improve the spatial distribution of data for mapping purposes. This visual information was digitally recorded and used to characterize the habitat type and to estimate the algal cover (%). For more information on sampling and methods for each survey see references in Table S1.
Fig. 2
Images of the bottoms of the Menorca Channel, Balearic Islands, Western Mediterranean obtained with the Remotely Operated Towed Vehicle (ROTV) during a scientific survey conducted in November 2021 showing different covers of ramified (A and B; sampled at 59 m depth) and nucleated (C and D; sampled at 60 m depth) species of rhodoliths and Laminaria rodriguezii (E and F; sampled at 71 m depth)
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Additionally, during the MEDITS_ES05_2022 survey (Table S1 and Table 1), individual length measurements of young and old blades of L. rodriguezii were taken in two sampling stations (Fig. 1), in order to characterize the population structure of this species in the Menorca Channel. Almost 100 non-broken specimens were randomly selected for each sampling station in order to measure their young and old blade lengths, and fresh weight.
Table 1
Number of sampling stations considered from each scientific survey according to the time period, before or after the establishment of the Fishing Protection Zone (FPZ) in the Menorca Channel (Balearic Islands, Western Mediterranean) during 2016, and the sampling methodology used to obtain them: Beam Trawl (BT), underwater drop camera (IPSE), Remote Operated Vehicle (ROV), Remotely Operated Towed Vehicle (ROTV). The number of samples identified as rhodolith beds (Rb), Laminaria rodriguezii forests (Lf), both or neither of them, are also displayed
Time period | Sampling method | Survey | Total stations | Rb-only stations | Lf-only stations | Rb + Lf stations | No Rb nor Lf presence |
|---|---|---|---|---|---|---|---|
before FPZ | BT | CANAL0209 | 60 | 31 | 0 | 29 | 0 |
before FPZ | IPSE | CANAL0209 | 122 | 33 | 11 | 23 | 55 |
before FPZ | BT | EQUIPAR0410 | 30 | 9 | 0 | 21 | 0 |
before FPZ | BT | INDEMARES_CANAL0811 | 50 | 22 | 0 | 28 | 0 |
before FPZ | ROV | INDEMARES_CANAL0811 | 25 | 16 | 0 | 7 | 2 |
before FPZ | BT | MEDITS_ES05_14 | 3 | 3 | 0 | 0 | 0 |
after FPZ | BT | MEDITS_ES05_17 | 2 | 2 | 0 | 0 | 0 |
after FPZ | BT | INTEMARES_CANAL0419 | 40 | 16 | 0 | 24 | 0 |
after FPZ | BT | CIRCA-LEBA-1121 | 7 | 4 | 0 | 2 | 1 |
after FPZ | HORUS | CIRCA-LEBA-1121 | 6 | 3 | 1 | 0 | 2 |
after FPZ | BT | CANAL_04_22 | 8 | 3 | 0 | 5 | 0 |
after FPZ | BT | MEDITS_ES05_22 | 2 | 2 | 0 | 0 | 0 |
after FPZ | BT | CANAL_04_23 | 15 | 3 | 0 | 12 | 0 |
after FPZ | BT | MEDITS_ES05_23 | 11 | 2 | 0 | 9 | 0 |
after FPZ | BT | SOSMED-1023 | 36 | 18 | 0 | 17 | 1 |
after FPZ | HORUS | SOSMED-1023 | 130 | 79 | 0 | 46 | 5 |
Data analysis
To analyse differences on the distribution of rhodolith beds and L. rodriguezii forests, the benthic habitats mapping developed by the LIFE + INDEMARES project for the continental shelf of the Menorca Channel between 50 and 100 m depth (Fig. 1), provided by Moranta et al. (2014), was used as the distribution of habitats previous to FPZ implementation in 2016. Their distribution in recent years has been mapped using presence/absence data from beam trawl, ROV and ROTV sampling using the QGIS free and open-source software 3.28 version (QGIS.org 2023). With this software, we also estimated the surface of both habitat types before and after FPZ establishment.
We used ANOVA test to compare before/after FPZ implementation biomasses of rhodolith-forming species and L. rodriguezii. Prior to the use of the ANOVA, Shapiro-Wilk test was applied to check for normality. When this assumption was not met, a univariate permutation analysis of variance test was applied with the RVAideMemoire package in R. This test calculates p-values using permutations, and does not assume normality. To do so, we selected a sub-area with enough comparable samples for the two time periods (Fig. 1). The statistical analyses were carried out with R, version 3.1.1 (R Core Team 2014).
Fishing pressure
To detect changes in the distribution of the bottom trawl fishing effort along the Menorca Channel before and after the establishment of the FPZ, we analysed the data collected by the Vessel Monitoring by satellite System (VMS) from 2009 to 2023. Previously to this analysis, VMS data were filtered to avoid the inclusion of signals produced when the boats are not fishing (sailing and manoeuvring). To do so, we took into account only the VMS signals produced from 05:00 am to 05:00 pm (the period when trawlers are allowed to fish in the Balearic Islands) and with an instantaneous velocity from 2 to 3.6 knots (the velocity range during fishing operations in the Balearic Islands).
Then, mean annual number of VMS signals per cell in a 0.01º resolution grid was mapped for four years of the time series considered (2010, 2014, 2018 and 2022) and for the difference between the sampling periods before and after the FPZ implementation (2009–2015 and 2017–2023, respectively). The temporal evolution of the fishing effort of the bottom trawl fleet in the SCI Menorca Channel, as number of days, was also calculated from VMS data.
Results
For the two time-periods considered (before and after FPZ implementation), a total of 143 and 121 beam trawl samples were analysed, respectively, from which 65 and 50 were identified as rhodolith beds, and 78 and 69 as L. rodriguezii forests (Table 1). A total of 2 out of the 121 stations sampled in the period after the implementation of FPZ did not present rhodolith beds and nor L. rodriguezii forests, whereas from the 143 stations sampled during the period before the implementation of FPZ the number of stations that did not present any of these habitat was zero. Regarding the video stations, from a total of 147 and 136 before and after FPZ periods, respectively, 49 and 82 were identified as rhodolith beds and 30 and 46 as L. rodriguezii forests, respectively. A total of 57 and 7 stations before and after FPZ periods, respectively, did not present either habitat.
Based on presence/absence data, results showed changes in the distribution of both rhodolith beds and L. rodriguezii forests, with wider distributions for both habitats after the FPZ implementation (Figs. 3 and 4). The area of the sea bottom covered by rhodolith beds increased from 650 km2 in the period previous to FPZ to 691 km2 in the period after FPZ, while the area with presence of L. rodriguezii fields increased from 274 km2 in the period previous to FPZ to 423 km2 in the period after FPZ.
Fig. 3
Comparison of the distribution of rhodolith beds in the Menorca Channel (Balearic Islands, Western Mediterranean) before (Moranta et al. 2014) and after (present data) the declaration of the Fishing Protected Zone (FPZ) in 2016. Samples from beam trawl and submarine images with rhodoliths are presented with circles and squares, respectively; and samples of both samplers that did not contain the species are presented with crosses. Samples obtained before the closure (preFPZ) are in red and samples obtained after the closure (postFPZ) are in green. Isobaths are 50, 100, 200, 500 and 800 m depth
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Fig. 4
Comparison of the distribution of Laminaria rodriguezii in the Menorca Channel (Balearic Islands, Western Mediterranean) before (Moranta et al. 2014) and after (present data) the declaration of the Fishing Protected Zone (FPZ) in 2016. Samples from beam trawl and video with L. rodriguezii are presented with triangles and squares, respectively; and samples of both samplers that did not contain the species are presented with crosses. Samples obtained before the closure (preFPZ) are in red and samples obtained after the closure (postFPZ) are in green. Isobaths are 50, 100, 200, 500 and 800 m depth
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The biomass of rhodolith-forming species and L. rodriguezii before the establishment of the FPZ ranged from 0.001 to 284 kg 250 m− 2 and from 0.009 to 15.6 kg 250 m− 2, respectively, while for the after FPZ period the biomass values ranged from 0.09 to 212 kg 250 m− 2 and from 0.001 to 25.2 kg 250 m− 2, respectively. When these data were compared for the sub-areas selected, results showed that biomasses of both rhodoliths and L. rodriguezii showed significant higher values after the establishment of the FPZ in all the sub-areas (Fig. 5). While the mean values of rhodolith-forming species changed from 34.7 ± 4.4 kg 250 m− 2 to 49.6 ± 5.41 kg 250 m− 2 (F-value: 4.741 and p-value < 0.05), the values for L. rodriguezii increased from 1.2 ± 0.3 kg 250 m−2 to 3.0 ± 0.9 kg 250 m− 2 (F-value: 4.326; p-value < 0.05) before and after FPZ implementation, respectively.
Fig. 5
Mean values (± standard error) of biomass (in kg 250 m− 2) of rhodolith-forming species and Laminaria rodriguezii, before (preFPZ; in red) and after (postFPZ; in green) the implementation of the Fishing Protected Zone (FPZ) in the Menorca Channel (Balearic Islands, Western Mediterranean) and results of the permutation analysis of variance. Significant differences are shown as: * (< 0.05), **(< 0.01), *** (< 0.001)
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The length of a total of 189 blades of L. rodriguezii that could be measured in the period after FPZ implementation, showed a mode in the 15–20 cm interval, with a maximum length of 130–135 cm and the 50% of the specimens ranging from 10 to 30 cm (Fig. 6). The individuals with an old and young blades separated by a constriction (Fig. 7) represented 67.2% of the total measured individuals.
Fig. 6
Size distribution of individuals of Laminaria rodriguezii sampled in the Menorca Channel (Balearic Islands, Western Mediterranean), during a research survey developed in 2022. The mean values only for the young blades size intervals (A), and the measuring criteria for a single young blade (B) and a double young and old blade (C) with a constriction in between (arrow) are given
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Fig. 7
Individuals of Laminaria rodriguezii collected in the Menorca Channel (Balearic Islands, Western Mediterranean), during two research surveys developed in June 2022 (A, B, C) and October 2023 (D, E). A, small individual with complete young and broken old blades; B, medium size individual with only young blade; C, large individual with complete young and broken old blades; D and E, individuals attached to rhodoliths through their holdfast. For individuals A and C, constriction between old and young blades are indicated with an arrow
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The bottom trawl fishing effort exerted in the SCI Menorca Channel during the study period showed a decreasing trend since 2015 both in the continental shelf and the slope (Fig. 8). This decrease was more pronounced in the coastal continental shelf, between 50 and 100 m depth, ranging from 888 fishing days in 2015 to 278 fishing days in 2023 (Fig. 8).
Fig. 8
Temporal evolution in number of fishing days during the study period (2009–2023) in the shallow shelf (50–100 m depth), deep shelf (100–200 m), upper slope (200–500 m) and middle slope (500–800 m) of the Site of Community Importance Menorca Channel (Balearic Islands, Western Mediterranean), estimated from Vessel Monitoring by satellite System (VMS)
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Regarding the spatial distribution of this fishing effort, no fishing signals were detected inside the FPZ for the years 2018 and 2022 compared to the years 2010 and 2014 (Fig. 9A, B, C and D), which confirmed the full implementation of the FPZ. The VMS data also showed an increase of fishing effort outside the FPZ, in an area between the southern limits of FPZ and LIC (Fig. 9E), where the bottom trawl effort exerted in the FPZ before 2016 is currently concentrated.
Fig. 9
Mean number of Vessel Monitoring by satellite System (VMS) signals of the bottom trawling fleet in a grid of 0.01º covering the Menorca Channel (Balearic Islands, Western Mediterranean) for the years 2010 (A), 2014 (B), 2018 (C) and 2022 (D). The variation between the periods before and after the FPZ implementation (2009–2015 and 2017–2023, respectively) are also given (E)
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Discussion
Since the implementation in 2016 of a FPZ in the SCI Menorca Channel, where bottom trawling was prohibited, the rhodolith beds and Laminaria rodriguezii forests have experienced an improvement both in terms of the area of the sea bottom covered by these habitats and biomass. The sensitivity of these species to the impacts of bottom trawling (Joher et al. 2012; de Juan et al. 2013; Žuljević et al. 2016; Barberá et al. 2017; Cabanellas-Reboredo et al. 2017) makes this trawling ban the most plausible explanation for the expansion of this kind of algal-dominated detritic bottoms. Indeed, Farriols et al. (2022a) compared four adjacent areas of the SCI Menorca Channel subjected to different levels of bottom trawl fishing effort, one of them that had never been trawled, and demonstrated that rhodolith beds in 2019, three years after the ban, already showed signals of recovery in their biodiversity, density of some benthic species and cover and morphology of rhodoliths.
The present results confirm that the recovery of these habitats has continued during the last four years, and turned into their geographical expansion, with an increase of 6% of the area covered by rhodolith beds and 54% for the area covered by L. rodriguezii forests. The differing recovery capacities observed can be explained by differences in their life history traits. Rhodolith species have a limited ability to recover or re-establish populations after disturbance due to their complex life cycle, which includes a delayed age of first reproduction and very slow growth (about 1 mm/year) (e.g., Bosence and Wilson 2003; Basso 2012). In contrast, kelp species exhibit rapid growth and a high capacity for population recovery due to their microscopic gametophyte serving as a ‘seed bank’ for forest recovery. Additionally, the sporophyte stage of L. rodriguezii can produce clonal sporophytes from stolons (Carney et al. 2013; Schoenrock et al. 2021; Edwards 2022; Veenhof et al. 2022). Although we do not have a pre-impact baseline for comparison, the larger expansion of L. rodriguezii suggests that physical impacts of bottom trawling were more damaging for this species rather than for rhodolith-forming species. Indeed, long blade sizes and high abundances of old and young bladed individuals have been found in some areas of the Menorca Channel in recent years, while small and broken young blades were usually found in the years before the FPZ implementation and are found nowadays in other zones of the Balearic Islands with bottom trawl activity (personal observations). The long, erect, and flexible thallus of L. rodriguezii make it easy to be moved by the groundrope of the net compared to creeping, small, and hard rhodoliths. Moreover, when fishermen fish on shallow shelf bottoms they use nets with lighter groundropes and special rigging adjusted to reduce the capture of rhodoliths (Farriols et al. 2022a). They try to prevent large amounts of rhodoliths in the codend of the net, that would produce abrasion to commercial species, damaging their scales and pigmentation and leading to the subsequent loss in the quality of catches and economic value.
The concurrent recovery of both benthic habitats is not surprising considering their overlap (Ballesteros 2006; Ordines and Massutí 2009) and the fact that rodolith beds provide a hard substratum where L. rodriguezii individuals can grow (de Juan et al. 2023; Fig. 6). Consistently, during our surveys in the Menorca Channel, we found L. rodriguezii mostly growing on rhodolith-forming species. Hence, the expansion of rhodolith beds, along with the reduction of bottom trawl effort, have more likely facilitated the expansion of L. rodriguezii. The currents that predominate in the area flow southwards (Ordines et al. 2011), which could have also favoured the spread of these habitats to the southern part of the Menorca Channel. Currents may be transporting rhodolith forming species southwards, which may be already carrying L. rodriguezii, and also L. rodriguezii clonal blades from the branched holdfast, which are the most common reproduction strategy of this species (Huvé 1955; Reynes et al. 2021; Fig. 6).
As pointed above, the impact of bottom trawling has been shown as a factor influencing the size and shape of rhodolith beds (Barberá et al. 2017; Cabanellas-Reboredo et al. 2017; Farriols et al. 2022a) and would presumably affect also the size of L. rodriguezii. Although unfortunately we do not have information about its size before and after FPZ implementation in the Menorca Channel, we can compare the size distributions of L. rodriguezii showed in the present study from those observed in 2012 in the Columbretes Islands Marine Reserve (Linares et al. 2012), an area where professional fishing is banned since 1990 and which is not subject to any kind of anthropogenic impact since 2008. While these authors found a size mode of 0–5 cm and a high number of small blades (mainly from 0 to 15 cm) accompanied with a decreasing number of individuals as size increased to a maximum of 120 cm, our results showed a higher mode at 15–20 cm, a high number of blades ranging from 10 to 30 cm and abundant individuals with medium-high sizes. These data highlights L. rodriguezii is in a good conservation status in the Menorca Channel, even when compared to an area with a longer-term protection. The authors of the present study that participated in research surveys with benthic sampling in the area before the FPZ implementation, had not seen individuals with two blades separated by a constriction, suggesting that sizes were much smaller.
The expansion of rhodolith beds and L. rodriguezii forests has not only been noticed in the FPZ but also in adjacent areas, where the bottom trawl fishing effort has shifted as a result of the loss of surface available for trawl fishing. However, these habitats were present in the area before the establishment of the FPZ, reinforcing the idea that the fishing effort in this area has remained low. In fact, the lower bottom trawl fishing effort from the Balearic Islands (Quetglas et al. 2012), together with the use of nets adapted to avoid discards by bottom trawlers of the Balearic Islands (Farriols et al. 2022a) have been argued as probable reasons of the long term overlapping between red algae beds and trawl fleet fishing grounds (Ordines et al. 2017). Considering the decreasing trend in trawl fishing effort in the whole Balearic Islands, including the Menorca Channel, as a consequence of the decrease of the number of vessels during the last decades (Quetglas et al. 2017) and of the implementation in recent years of the European Commission multiannual plan for the fisheries exploiting demersal stocks in the Western Mediterranean (Regulation EU 2019/1022), with a reduction of fishing days in bottom trawling (from the reference period 2015–2017) of 10, 7.5, 6 and 7% in 2020, 2021, 2022 and 2023, respectively, it is plausible to assume that the expansion of these habitats will continue while this trend does not change.
Rhodolith beds are fragile and ecologically relevant habitats supporting a high biodiversity (Barberá et al. 2003; Peña and Bárbara 2008, 2010; Sordo et al. 2020), which have also been identified as essential to the development of critical phases of fishery resources (Ordines et al. 2009, 2015). The high species diversity and complex trophic webs they harbour (Steller and Cáceres-Martínez 2009, Peña et al. 2014; Sordo et al. 2020) are generally attributed to their complex three dimensional structure (Barberá et al. 2003). Kelp forests, like L. rodriguezii ones, are structurally and functionally complex habitats that support also a very rich biodiversity (BIOMAERL 1999; Joher et al. 2012, 2015). Therefore, the protection and conservation of both rhodolith beds and L. rodriguezii forests is highly relevant for the sustainability of fisheries and their management within the framework of the Ecosystem Approach to Fisheries. Furthermore the persistence of ecosystem structuring species that support rich and diverse communities will be essential for the maintenance of ecosystem functions under the current climate change scenario (Hoegh-Guldberg and Bruno 2010; Sasaki et al. 2015; O’Leary et al. 2017). In this context the trawling ban seems to be an effective measure and could be adopted in other areas to protect both worldwide distributed rhodolith beds and kelp forest (Fragkopoulou et al. 2021; Jayathilake and Costello 2020).
In conclusion, the spreading of rhodolith beds and L. rodriguezii forests in the SCI Menorca Channel is a signal that the bottom trawl ban has been an effective measure to improve the conservation status of its benthic habitats, communities and species. Because the temperate and cold-water affinities of kelps, this recovery is of utmost importance in the case of L. rodriguezii forests within the current context of climate change. For these reasons, the maintenance of this measure has been included as a recommendation in the management plan to declare the SCI Menorca Channel as a Special Area of Conservation (Massutí et al. 2023). Nonetheless, our results show that both habitats can also coexist with the low levels of fishing effort currently exerted in the area.
Acknowledgements
The authors thank the participants of the CANAL0209, EQUIPAR0410, INDEMARES_CANAL0811, INTEMARES_CANAL0419, SosMed-10-23, MEDITS, CANAL and CIRCA-LEBA research surveys, as well as the captains and crew of the research vessels on which they were carried out: MarViva Med, Francisco de Paula Navarro, Miguel Oliver, Ángeles Alvariño and Ramon Margalef.
Declarations
Competing interests
The authors declare no competing interests.
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