Comunitat íctica, Fons rocosos, Biodiversitat, Distribució, Censos visuals, Mar Mediterrani
Data de recepció: 04/08/2008 | Data d'acceptció: 26/09/2008 | Data de publicació: 30/10/2008
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Biodiversity and structure of rocky reef fish assemblages in the Sierra Helada Natural Park (South-western Mediterranean Sea)
Biodiversity and structure of rocky reef fish assemblages in the Sierra Helada Natural Park (South-western Mediterranean Sea)
In the present study the fish assemblages in the rocky-bottom habitat of the Sierra Helada Natural Park (Alicante, Spain) were recorded to provide data for future evaluation of any changes induced by long-term management. Visual censuses were carried out along strip transects by Scuba diving on rocky bottoms at depths between 1 and 32 m. In the seven localities sampled, 44 species were recorded. Number of species, abundance, biomass and size structure values recorded did not show differences between high and low protection areas. Species composition was similar to other marine protected areas of the western-Mediterranean. The main differences found between localities can be attributed to the high heterogeneity and complexity of the habitat at smaller spatial scales.
Biodiversidad y estructura de la comunidad íctica litoral del sustrato rocoso del Parque Natural de Sierra Helada (sudoeste del mar Mediterráneo)
En el presente estudio se analiza la comunidad íctica litoral sobre sustrato rocoso del Parque Natural de Sierra Helada (Alicante, España) y se aportan datos de base que permitirán evaluar a largo plazo los cambios debidos a diferentes medidas de gestión. Se realizaron censos visuales en transectos lineales sobre fondos rocosos entre 1 y 32 m de profundidad. En las siete localidades estudiadas se contaron 44 especies. El número de especies, abundancia, biomasa y estructura de tallas fueron similares entre las zonas con alta y baja protección. La composición de especies observada fue similar a los de otras áreas marinas protegidas del Mediterráneo occidental. Las diferencias entre localidades podrían ser causadas por la alta heterogeneidad y complejidad del hábitat a pequeña escala.
Biodiversitat i estructura de la comunitat íctica litoral del substrat rocós del Parc Natural de serra Gelada (sud-oest del mar Mediterrani)
Aquest estudi analitza la comunitat íctica litoral sobre substrat rocós del Parc Natural de serra Gelada (Alacant) i s’aporten dades de base que permetran avaluar a llarg termini els canvis deguts a diverses mesures de gestió. Es van fer censos visuals en transsectes lineals sobre fons rocosos entre 1 i 32 m de profunditat. A les set localitats estudiades es van comptar 44 espècies. El nombre d’espècies, l’abundància, la biomassa i l’estructura de talles van ser similars entre les zones amb alta i baixa protecció. La composició d’espècies observada va ser similar a les d’altres àrees marines protegides del Mediterrani occidental. Les diferències entre localitats podrien ser causades per l’alta heterogeneïtat i complexitat de l’hàbitat a petita escala.
Fishes are the main group of marine vertebrates and an important component of the marine biodiversity of the rocky Mediterranean infralittoral habitats. Their ecological role in the functioning of the ecosystems is often key to the structure and dynamics of marine communities. This may be due to either their position in the trophic chain or to the influence of their interactions with other fish, invertebrates and macroalgae (competition, depredation, parasitism). Furthermore, fishing has been practised in the area since ancient times and is a traditional source of economic activity.
Fish distribution patterns are influenced by biological and physical factors, such as variations in depth (Dufour et al., 1995), habitat structure resulting in differential availability of resources such as food or shelter (García-Charton et al., 2004), climatic differences (Holbrook et al., 1997), predation (Hixon, 1991), competition (Gladfelter et al 1980), episodic disturbances (Chabanet et al., 1995), larval dynamics (Leis & McCormick, 2002), and recruitment variability (Booth & Brosnan, 1995). Additionally, in some places, management can promote differences in the fish assemblage depending on the level of protection applied (Russ, 2002). Many studies have shown significant relationships between fish assemblage and habitat structure (e.g. Jenkins & Wheatley, 1998). Habitat structure may strongly affect fish assemblages and many marine protected areas have been chosen due to their intrinsic complex seascape. The most reasonable way to achieve better management and understanding of the functioning of these rocky habitats, particularly in the case of marine protected areas (MPAs), fishing areas or even artificial structures, is to have the best possible knowledge of the manner in which fish assemblages respond to this habitat complexity. In this way, it may be possible to avoid confusion between variability that is due to natural factors and effects derived from management.
Marine protected areas are being established worldwide at a rapid rate. Locations with important ecological and socio-economic features are being managed to protect habitats and biological richness, and restore fishing stocks and degraded areas (Agardy, 2000). In this framework, the study of spatial pattern of fish assemblages is crucial as a first step to understanding the causes of their distribution and abundance (Levin, 1992) and to provide a basis for monitoring their long-term changes due to both natural and human disturbances (Underwood, 1990). The Sierra Helada Natural Park (Alicante, Spain – Western Mediterranean) was constituted as a Natural Park with an extensive marine protected area (4,920 ha) in 2005 (DOGV nº 4967, 16/03/05). The great heterogeneity of the bottoms, where a large number of the more characteristic benthic communities of the west and central Mediterranean are found, and their good state of conservation make the Sierra Helada an ideal place for the study of fish communities and the factors that determine their structure in the oligotrophic areas of the south-western Mediterranean. It is clear that to manage these marine protected areas and to adopt measures of conservation and regulation, it is necessary to define the criteria of action. To do so, we need to increase our knowledge on the different types of habitats and the species present in the area. The aim of this study was to characterize the fish assemblages inhabiting the rocky reefs in the Sierra Helada Natural Park (western Mediterranean Sea) in order to determine species composition, defining the distribution and quantifying population abundance, biomass and size structure. Moreover, the selection of the study localities in areas that are affected by different management measures should provide a baseline for future evaluations of the changes produced by management and a source of data for future meta-analysis.
Material and methods
Study site and sampling design
The study was carried out in the Sierra Helada Natural Park (Alicante coast, Spain, SW-Mediterranean). The littoral zone is characterised by the large abundance and diversity of rocky infralittoral bottoms, composed primarily of boulders of diverse sizes interspersed with patches of sand and Posidonia oceanica seagrass meadow (DOGV nº 4967, 16/03/05). Wide zones with rocky blocks larger than 0.5 m in height, without patches of Posidonia oceanica and without sand were selected, ranging from 1 to 32 m depth. Seven areas were sampled in summer, between June and August 2007.
We selected three specially protected areas, Benidorm island, Mitjana island and Mascarat-Toix, where a number of activities (anchoring, fishing, motor-sailing, aquaculture) are forbidden but others are allowed (swimming, snorkelling, windsurfing, sailing, scientific research). The other four localities selected in this study have a lower level of protection (L`Olla island, Punta Albir, Punta Caballo and Finestrat) (fig. 1, table 1).
Underwater visual census and data analysis
Fish assemblages were visually surveyed by Scuba diving (Harmelin-Vivien et al., 1985) along each 25 x 5 m strip transect, recording the abundances and individual sizes in 2 cm size classes for each species encountered. Several observers participated in the sampling operations, after several training sessions. It has been observed in previous studies that fish counts, size estimations and habitat measurements do not differ significantly among trained observers (Bell et al., 1985). Small-sized, cryptic species (belonging to families Gobiidae, Callyonimidae, Bleniidae, Gobioesocidae and Tripterygidae) were excluded from the censuses to avoid biases. Each observation was assigned to one of nine predetermined abundance classes proposed by Harmelin (1987), the limits of which coincide with the terms of a geometric series (base 2). Geometric means of each fish abundance class were used for calculations. All censuses were done between 10:00 and 15:00 h, with optimal water conditions (turbidity and swell). These sampling methods have been used extensively in MPAs because they are non-destructive and guarantee that the fish community is not affected by sampling. Interference with previous evaluations of the effects of protection is avoided, and a high degree of consistency of gathered data among observers is maintained over time (i.e., performance to survey the most visually observable fraction of the fish with the same efficiency) (Harmelin-Vivien & Francour, 1992).
After counting fishes, the same observer covered the transect length in the opposite direction to record data on the habitat heterogeneity and complexity within each transect. We visually measured (i) the relative percentage of cover of different substrate types (rock, clumps of Posidonia oceanica over rock, and patches of sand), (ii) the number of rocky boulders (classified by the size of their major length [ML]: small [ML ≤ 1 m], medium [1 m < ML ≤ 2 m], and large [ML > 2 m]) and (iii) the maximum depth and a verticality index (vertical distance between the deepest and shallowest point inside each transect). Fish species biomass was estimated from the abundance data by size-classes within each transect, using weight–length relationships calculated from experimental and commercial fishing data obtained in the same geographical area.
We carried out a total of 93 census in all study localities, 40 of these in low protection areas and 53 in higher ones (table 2). The highest percentages for rocky habitat and maximum depth were in Benidorm Island (97.11 ± 1.04 % and 15.93 ± 1.60 m depth). The lowest values of minimum depth and verticality were in Finestrat (1.75 ± 0.32 m and 1.12 ± 0.37 m, respectively). The highest values of verticality (5.07 ± 1.29 m) and highest number of boulders, blocks and big blocks were in Punta Albir.
We observed a total of 44 fish species (belonging to 17 families); 11 of these species appeared in all localities (annex 1). In terms of species richness, between 15 and 33 species appeared in low protection areas, whereas in high protection areas we observed between 26 and 31 species. High protection areas showed a relatively higher abundance and higher biomass values than low protection areas. The highest mean total abundance was recorded in Mitjana Island (411.14 ± 34.28 individuals/125 m2), while the lowest value was recorded in Finestrat (54.25 ± 1.18 individuals/125 m2) (table 3). The 6 most abundant species in the study area were Chromis chromis, Sarpa salpa, Diplodus vulgaris, Diplodus sargus, Oblada melanura and Coris julis. The highest mean total biomass ranged between a maximum of 13,345.319 ± 1,605.896 g/125 m2 in Mitjana Island, and a minimum of 1,133.85 ± 28.370 g/125 m2 in Finestrat (table 4). The species that showed the highest values of biomass in the study area were D. vulgaris, D. sargus, S. salpa and C. chromis. Ten species (C. julis, Symphodus ocellatus, S. roissalii, S. tinca, Thalassoma pavo, Serranus scriba, Diplodus cervinus, D. vulgaris, O. melanura and S. salpa) were very common and appeared in every site. Other species like Sardina pilchardus, Mugil spp., Boops boops, Scorpaena spp., Phycis phycis, Symphodus doderleini, Mycteroperca rubra, Lithognatus mormyrus and Pagrus spp. were recorded with very low frequencies.
Large sparids showed the highest abundance values for size 2 (table 5), and they were most abundant at Mitjana Island. Small sparids, however, were widely distributed, and the highest value was size 2, also at Mitjana Island. Large serranids were less abundant than smaller serranid, and both had highest values of size 2. Large labrids were frequent throughout the study area, with highest values of size 3 and 4. A similar occurrence was observed for small labrids where sizes 2 and 3 recorded the highest abundance values. The other commercial species, red mullet (Mullus surmuletus) and brown meagre (Sciaena umbra), exhibited the highest abundance values of size 2, but size 4 was recorded frequently in high protection areas in both cases. Biomass recorded for large and small sparids showed a maximum value in size 2 and the highest sizes were recorded in Benidorm and Mitjana Islands (table 6). Large serranids showed the highest values of biomass in size 2, and small serranids were more frequent in size 3. Large and small labrids exhibited higher biomass values in sizes 4 and 3, respectively. The highest size of red mullets was recorded in Mascarat-Toix, a high protection area, and the maximum of biomass was size 2. The highest size and biomass values for brown meagre were observed in Benidorm Island.
In terms of composition, the rocky reef fish assemblages of the Sierra Helada Natural Park were similar to those recorded in other marine protected areas in the western-Mediterranean sea (e.g. Reñones et al., 1997; Forcada-Almarcha, 2004). Species richness, total abundance and biomass values recorded did not show great differences between high and low protection areas as regulations limiting fishing were not yet l enforced. The main differences found between localities can be attributed to differences in the heterogeneity and complexity of the habitat (García Charton et al., 2004). Those factors changed at a very small scale, conditioning the distribution and presence of fishes, and should be taken into account in order to assess the effect of protection without bias. Our findings suggest that the intrinsic complexity of the habitat in Mitjana and Benidorm Islands may favour the appearance of targeted species such as big serranids (Epinephelus spp.) or large sparids (Dentex dentex) when regulations are enforced.
This study is a part of the Project: “Evaluación de la Biodiversidad de Peces Marinos e Impacto de la Pesca Deportiva en el Parc Natural de Serra Gelada”, funded by the Servicio de Conservación de la Biodiversidad, Consellereria de Medi Ambient, Aigua, Urbanisme i Habitatge. We wish to thank the Club Náutico de Benidorm and Club Náutico de Altea for their support and facilities during this study. We are also grateful to the Director of the Park, D. Eduardo Mínguez, and the wardens for their support and valuable assistance during the field work.