Marine protection and meadow size alter fish herbivory in seagrass ecosystems

Abstract

In this study, we examine the hypotheses that the size of the seagrass meadow and marine protection influence the abundance of seagrass herbivores and their associated grazing pres- sure. The effect of meadow size was tested in 9 shallow unprotected meadows of Posidonia oceanica encompassing a wide range of areas. The effect of fishing protection was examined by comparing 9 unprotected meadows (controls) with 3 marine protected areas (MPAs) with at least 20 yr of protec- tion that also varied in meadow size (large >3 ha, medium 1 ha < x < 2 ha and small <1 ha). At each site, we quantified the abundance, size distribution and grazing pressure of the 2 most important her- bivores, the fish Sarpa salpa and the sea urchin Paracentrotus lividus. A strong negative relationship between meadow size and both the abundance and grazing rates of S. salpa was evident in the unprotected meadows, but no effects were detected on either the abundance or grazing rates of P. lividus. Results also showed that populations of S. salpa (mainly large individuals) benefit from fishing protection and tended to concentrate in MPAs (with the exception of Port-Cros), resulting in enhanced grazing pressure. In P. lividus abundances or size distributions did not present any signifi- cant difference between protected and unprotected areas, although most MPAs presented low sea urchin densities. While MPAs are important to preserve seagrass ecosystems, results from this study indicate that it is crucial to account for the size of these habitats, particularly when functional aspects of seagrass habitats are so strongly altered by size.