Abstract
One of the most degraded states of the Mediterranean rocky infralittoral ecosystem is a barren composed solely of bare rock and patches of crustose coralline algae. Barrens are typically created by the grazing action of large sea urchin populations. In 2008 we observed extensive areas almost devoid of erect algae, where sea urchins were rare, on the Mediterranean coast of Turkey. To determine the origin of those urchin-less ‘barrens’, we conducted a fish exclusion experiment. We found that, in the absence of fish grazing, a well-developed algal assemblage grew within three months. Underwater fish censuses and observations suggest that two alien herbivorous fish from the Red Sea (Siganus luridus and S. rivulatus) are responsible for the creation and maintenance of these benthic communities with extremely low biomass. The shift from well-developed native algal assemblages to ‘barrens’ implies a dramatic decline in biogenic habitat complexity, biodiversity and biomass. A targeted Siganus fishery could help restore the macroalgal beds of the rocky infralittoral on the Turkish coast.
Highlights
The Mediterranean is a microcosm of the major threats to the oceans – historical overfishing, habitat degradation, pollution, introduced species, and global warming [1]
The biomass of herbivorous fish represented between 46% and 57% of the total fish biomass at the study sites (Table 1), and it was significantly greater than the biomass of other fish trophic groups at Kas (ANOVA, p,0.001) and Bodrum (p,0.01)
Sparisoma cretense was the only other major herbivorous fish observed at the study sites, but its biomass accounted for between only 5% and 17% of total herbivore biomass
Summary
The Mediterranean is a microcosm of the major threats to the oceans – historical overfishing, habitat degradation, pollution, introduced species, and global warming [1]. The confluence of some of these stressors helps to create one of the most degraded underwater communities in the Mediterranean – infralittoral barrens. These barrens are impoverished bottoms dominated by bare rock with a few species of encrusting algae, caused typically by the grazing action of abundant sea urchins Paracentrotus lividus and Arbacia lixula [2,3,4]. In 2008 we observed extensive areas (several hundred meters in length) devoid of any erect macroalgae at several locations in the Mediterranean coast of Turkey, but sea urchin abundance was surprisingly low (Fig. 1). Herbivorous fish appeared to be abundant at the barrens, especially the alien rabbitfish Siganus luridus and S. rivulatus, which formed schools of up to several hundred individuals each
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