Variations in a shallow rocky reef fish community at different spatial scales in the western Mediterranean Sea

Abstract

Fishing grounds off the northern coast of Mallorca Island (western Mediterranean Sea) are subject to very different fishing efforts as a result of both commercial and recreational activities. We analysed the variability at different spatial scales of the fishable community associated with shallow littoral rocky bottoms. Underwater visual censuses (UVC) were carried out in 3 localities that have different fishing pressure. Three sites were sampled within each locality, and nine 50 m long UVC transects were completed within each site. Multivariate correlograms showed that the transects were completely independent statistical units even when located less than 75 m apart. The spatial variability of the fish assemblage is remarkable. A redundancy analysis (RDA) revealed highly significant differences between the species relative abundance of the different localities. The bottom structure and rugosity were not detected to have any effect on the fish assemblage. The variability of the bottom structure at the below-locality level shows that these tests were powerful enough and that the effects of bottom structure were not confused with the effects of fishing pressure. Localities with lower fishing pressure showed significantly higher species richness (8.4 ± 0.3 and 8.9 ± 0.3 species 250 m–2) and abundance (54.6 ± 4.4 and 65.9 ± 4.1 ind. 250 m–2) than localities with higher fishing pressure (6.7 ± 0.3 species 250 m–2 and 41.1 ± 4.5 ind. 250 m–2). Serranus cabrilla and Symphodus tinca were significantly less abundant in the localities with higher fishing pressure. These results suggest that differences in fishing pressure may generate strong gradients of faunistic change even at relatively small spatial scales (5 to 10 km), and that the observed pattern emulates what would normally be expected to happen after protection measures are established in marine protected areas