Fish versus starfish predation in controlling sea urchin populations in Mediterranean rocky shores

In the Mediterranean, fishing bans generally allow the recovery of populations of sea urchin predators, such as the seabreams Diplodus sargus and D. vulgaris, promoting the transformation of overgrazed communities into ones dominated by erect macroalgae. However, in the marine reserve on Ustica Island (SW Italy) the opposite trend has occurred in the upper infralittoral community, and urchin barrens formed after the cessation of fishing activities. We hypothesized that (1) the natural scarcity of the 2 seabream species leads to a low predation rate on sea urchins at Ustica, and (2) predation rate varies with depth, due to differences in the predator assemblages. Tethering experiments were conducted to test these hypotheses by comparing the predation rates at different depths at Ustica and 2 other locations in Sicily characterized by erect macroalgae communities and low urchin densities. Differences in the assemblages of predators, relative vulnerability of the 2 species of sea urchin present (Paracentrotus lividus and Arbacia lixula), and sea urchin density were also assessed. Despite large variation in predator abundance among locations, there was no difference in predation rate on tethered urchins. Predation by fish was found to be higher in locations where seabreams were abundant, while predation by the starfish Marthasterias glacialis, usually considered a minor sea urchin predator, was found to be the major source of mortality at Ustica. The paucity of predatory fish at Ustica was consistent with the high abundances of urchins and persistence of barrens habitat. The high predation rate on urchins by M. glacialis at Ustica suggests that this species might play an important role in controlling urchin populations in the future.