Multi-scale spatial variability in fish assemblages associated with Posidonia oceanica meadows in the Western Mediterranean Sea
Introduction
The Posidonia oceanica beds are among the most important Mediterranean ecosystems, and their conservation is a high national and international priority. Posidonia oceanica beds have a multifunctional role within coastal systems that is comparable to that of other seagrasses in temperate and tropical areas. They offer substrate for settlement, food availability and shelter, recruitment and nursery areas, as well as participating in key biogeochemical and geological processes (Orth et al., 1984, Orth, 1992, Proccacini et al., 2003, Nakaoka, 2005).
Fish assemblages associated with Posidonia oceanica seagrass meadows have been extensively studied in the Mediterranean. For instance, community structure and diel variations (Bell and Harmelin-Vivien, 1982, Harmelin-Vivien, 1984), spatial and temporal fluctuations (Harmelin-Vivien, 1982, Francour, 1997), feeding habits (Bell and Harmelin-Vivien, 1983, Khoury, 1984, Harmelin-Vivien et al., 1989), comparisons with other inshore habitats (Guidetti, 2000), effect of bottom trawling (Sánchez-Jerez and Ramos Esplá, 1996), effect of protection (Francour, 1994, Francour, 2000, Macpherson et al., 2002) and methodological bias in sampling methods (Harmelin-Vivien and Francour, 1992) should be considered well known in this area.
Nevertheless, little attention has been given to the variability in the fish assemblages associated with Posidonia oceanica meadows at different spatial scales. Studying spatial patterns is of ecological importance in order to understand the causes of abundance of organisms, and it also provides valuable insights for management and conservation. The topic of variability at several spatial scales, from meters up to hundreds or thousands of kilometres, has been addressed for fish communities other than the one considered here. Specifically, there are numerous examples concerning reef fishes (Choat and Ayling, 1987, Sale, 1998, Chesson, 1998, García-Charton and Perez-Ruzafa, 2001, Guidetti et al., 2002, García-Charton et al., 2004, Chittaro, 2004, Anderson and Millar, 2004, Nuñez-Lara et al., 2005). This is a relevant topic because the patterns that can be observed (and the processes that lead to them) depend on the extent to which the system is examined (Sale, 1998). For example, analyzing multivariate variability in reef fish assemblages at different spatial scales revealed that the greatest variation occurred at the smallest spatial scale, between individual transects (separated by a few meters), and in contrast, variability from site to site (separated by hundreds to thousands of meters) and from location to location (separated by hundreds of kilometres) were comparable (Anderson and Millar, 2004).
Hierarchical multi-scale approaches are conceived for understanding how information is transferred across scales but they are rarely integrated into survey methodologies and analysis of the results. This is probably due to the huge effort that simultaneous sampling at multiple spatial scales with the appropriate number of replicates at each spatial level implies. The marine environment has particular practical, logistical and financial challenges that make it more difficult to obtain this type of dataset (Raffaelli et al., 2005). The analysis of anthropogenic and geomorphologic effects on reef fish communities along 400 km of the coast of the Yucatan Peninsula (Nuñez-Lara et al., 2005) and the analysis of the effect of habitat structure on reef fishes at a large scale of hundreds of kilometres along the north-eastern coast of New Zealand (Anderson and Millar, 2004) are two notable exceptions. Similarly, this specific matter has been studied in relation to Mediterranean rocky reef fish assemblages covering more than 500 km, but at the price of extending the sampling program from June to October (García-Charton et al., 2004). According to this contribution, the variability observed at the largest spatial scale in the structure of the fish assemblages studied seems to be largely determined by differences in local carrying capacity and hydro-climatic conditions, but at small-to-intermediate spatial scales the habitat structure is more likely to be responsible for a large part of the observed differences. The importance of habitat structure has also been studied in the case of Australian seagrasses (Bell and Westoby, 1986a, Bell and Westoby, 1986b, Bell and Westoby, 1986c). Specifically, the effect of physical complexity seems to affect the abundance and distribution of fishes differently depending on the spatial scale (i.e. local scale versus an entire bay).
Therefore, a multi-scale analysis should at least consider the potentially confounding effects of both seasonality and habitat structure (at the intermediate scale, i.e. from more than 1 km to less than 10 km), but a fully factorial experimental design that takes all these factors into account is usually unavoidable. Consequently, a combination of data coming from different sampling programs is proposed, after testing and insuring that the specificities of each program cannot be confounded with spatial variability per se. The two preliminary experimental designs adopted here to allow the datasets to be combined were: (1) the differences between two consecutive years were tested by sampling the same meadow (Formentera) in June 2001 and June 2002; and (2) the differences attributable to the meadow structure (at the within-location scale; in terms of cover and shoot density) were evaluated by sampling two meadows (Mallorca 2000 and Formentera 2001). The absence of any significant correlation pattern related to these factors allows us to combine the data from the three locations (Formentera 2001, Mallorca 2000 and Alacant 2000) in order to carry out a single analysis aimed at determining how much variability is explained by the three spatial scales considered (i.e. <1 km, <10 km and >100 km).
In addition, in terms of multi-scale analyses, this contribution represents the first attempt in the Mediterranean to analyze the correlation patterns between the structure of the Posidonia oceanica meadows and the fish assemblages that inhabit them. Moreover, in the geographical context of the Balearic Islands and the Iberian coast, detailed descriptions of the characteristics of the fish assemblages associated with P. oceanica meadows are scarce (Massutí, 1962, Reñones et al., 1995, Jiménez et al., 1997) despite the fact that these meadows are among the most important habitats in the area since they cover nearly 3100 km2 (Más et al., 1993).
Section snippets
Sampling method
Surveys were carried out over Posidonia oceanica beds in three Spanish locations along the Mediterranean coast, two of them were located in the Balearic Islands (Mallorca and Formentera) while the third was on the eastern coast of the Iberian Peninsula (Alacant) (Fig. 1). A beam trawl was used to sample three randomly chosen sites in Mallorca and Alacant while five sites where sampled in Formentera. Six hauls (replicates) were taken at each site, except in one site of Formentera where only
Results
From the 74 hauls included in the three analyses 5740 fishes, belonging to 49 species and 19 families were caught, representing a total biomass of 81.2 kg (Table 2). Labridae and Sparidae were the most significant fish families in the three locations, although their relative contribution to the overall density differed from one location to another (Labridae 49% and Sparidae 16% in Mallorca, 54% and 21% in Formentera, and 33% and 34% in Alacant respectively). Only one species, Diplodus annularis,
Discussion
Our results revealed that the composition of fish assemblages associated with Posidonia oceanica seagrass meadows of the western Mediterranean tend to show different species-specific relative abundances resulting in a spatial heterogeneity at a large spatial scale. The differences observed between locations 100 km apart are mainly due to changes in the relative density of species rather than differences in species composition. The variation at the smallest spatial scale between individual
Acknowledgements
This paper is a result of the Project POSICOST (1FD97/1654) and “Seguiment de l'evolució dels recursos marins i de les poblacions de les espècies de major interès pesquer a les reserves marines de les Illes Balears: Badia de Palma, Nord de Menorca, Freus d'Eivissa i Formentera” from the Direcciò General de Pesca del Govern Balear. Fishery surveys were authorized by the local authorities (Govern de les Illes Balears and Generalitat Valenciana). We are grateful to B. Oliver, A. Roig, B. Artigues,
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