Multi-scale spatial variability in fish assemblages associated with Posidonia oceanica meadows in the Western Mediterranean Sea

Abstract

Fish assemblages associated with Posidonia oceanica from three locations of the western Mediterranean (Mallorca, Formentera and Alacant, Spain) were sampled in order to assess their spatial variability at three different scales ranging from <1 km to >100 km. Sampling was carried out using a beam trawl. Simultaneous sampling at these three spatial scales with the appropriate number of replicates implies a huge effort which is rarely possible to achieve. Consequently, we propose an arrangement of data coming from different sampling programs, after testing and making sure the specificities of each program cannot be confounded with the spatial variability. The two preliminary experimental designs adopted in order to combine the datasets were: (1) differences between two consecutive years were tested by sampling the same meadow (Formentera) in June 2001 and June 2002; and (2) differences attributable to the meadow structure were evaluated by sampling two meadows (Mallorca 2000 and Formentera 2001). The absence of any significant correlation pattern for univariate community descriptors and multivariate species-specific densities (i.e. individuals per hour) related to both between-year and within-location structural differences of the meadows, allow us to combine data from the three locations (Formentera 2001, Mallorca 2000 and Alacant 2000) in a single analysis aimed at determining how much variability is explained by the three spatial scales considered. The between-location scale (>100 km) is the most variable scale for species-specific densities (multivariate approach). Spatial variability at the smallest scale (<1 km) is also considerable, but the variability corresponding to the intermediate scale (<10 km) was found to be non-significant. This is an expected result for fishes given that the spatial scale of individual transects is not large compared to the high mobility of many of the species considered. The differences observed between the locations placed 100 km apart are due to changes in the relative density of species rather than differences in species composition. Between-location variability in the univarite community descriptors was not significant. However, within-location (intermediate scale) differences were significant for density but not for biomass. This is related to the large number of small individuals found in one Formentera site. These results are consistent with the hypothesis that P. oceanica meadows from different locations in the western Mediterranean might display a similar carrying capacity although large-scale hydrodynamic conditions and meadow structure at the between-location level might lead to differently shaped fish communities (for example, the larger meadow complexity in Formentera might favour smaller sizes, since small species and/or individuals are known to find more shelter and food there), while differences at an intermediate spatial scale remain irrelevant.