Antonieta Požar-Domac

Banks of the coral Cladocora caespitosa (Anthozoa, Scleractinia) in the Adriatic Sea

Cladocora caespitosa (Linnaeus, 1767) is a colonial scleractinian coral belonging to the family Faviidae and occurs in the Mediterranean Sea and the IberoMaroccan Gulf. The coral branches build big irregular colonies, up to 1 m in diameter, which fuse into banks and produce reef-like structures (Fig. 1C). Large banks of C. caespitosa, both living and fossil, have been found at several sites in the Mediterranean Sea: near the Tunisian coast, and in the Aegean and Ligurian Seas (Laborel 1961; Zibrowius 1980; Peirano et al. 1998). In the Adriatic Sea, Abel (1959) also describes extensive colonies of C. caespitosa in the channel of Lim near Rovinj whilst Pax and Müller (1962) mention banks of C. caespitosa near Mljet Island (Fig. 1A). Our recent surveys are focused on four locations where banks occur in the Adriatic Sea: in the Lim channel and near Prvić Island in the northern part of the Adriatic; near Pag Island in the central part of the Adriatic; and in Veliko Jezero (National Park ‘‘Mljet’’) in the southern part of the Adriatic. In the area where the banks occur, the annual sea temperature range—from 6 C in winter (the Lim channel) to 29 C in summer (Mljet Island)—is rather wide. The C. caespitosa bank in Veliko Jezero occurs at depths from 4–18 m and covers an area of 650 m and is thus one of the largest bank of C. caespitosa found in the Mediterranean Sea. The strong sea currents, which occur as a result of tidal exchange in the channel, appear to favor the growth of the bank. The colonies of the C. caespitosa from Mljet have similar growth rates to those in tropical reefs (growth rates range from 2.8 to 6.2 mm/year) measured with x-radiography and ‘‘Alizarin’’ staining method (Schiller 1993a; Kružić and Požar-Domac 2002) and slightly higher growth rates than those measured in colonies of the C. caespitosa in the Ligurian Sea (1.3–4.3 mm/year) by Peirano et al. (1999). The bank harbors a high diversity of associated species, especially sponges, polychaetes, and molluscs. Coral bleaching during summer was observed only in shallow waters (4–10 m depth) and in only a few colonies.

Impact of tuna farming on the banks of the coral Cladocora caespitosa in the Adriatic Sea

Anthropogenic activities such as industrial and urban sewage discharges, trawl fishing and coastal works, and introduction of the tropical algae Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forsskal) J. Agardh have caused a major decline of the colonial coral Cladocora caespitosa (Linnaeus, 1767) in the eastern part of the Adriatic Sea (Kružić 2005). There is now evidence that the recent increase in fish farming in the Adriatic Sea is imposing further anthropogenic pressure. C. caespitosa is the only reef-building coral in the Mediterranean (Kružić and Požar-Domac 2003). Near Iz Island (Central Adriatic Sea) a C. caespitosa bank occurs at depths of 12–16 m and covers an area of 150 m. This bank is located 300 m away from tuna fish farm cages. Fish farming induces high organic and nutrient loading in the surrounding water. These nutrients enhance phytoplankton and macroalgal blooms, which reduce light reaching the benthos. During an algal bloom, the coral bank was covered with algae (Fig. 1a), and underneath the coral had started to bleach (Fig. 1b). Subsequently, there was a heavy bloom of pluricellular filamentous algae, principally Acinetospora crinata (Carmichael ex Harvey) Kornmann. In recent years the occurrence of benthic mucilaginous aggregates has become an increasing problem along the Adriatic coastline and in many other areas of the Mediterranean. These aggregates appear as small, yellowish tufts in early spring and continue until the end of summer, forming, under favorable environmental conditions, extensive patches on the seabed. A heterogeneous algal community of diatoms, blue–green algae and fragments of macroalgae grows inside the benthic aggregates (Lorenti et al. 2005). The macroscopic development is caused by few filamentous and associated species: Nematochrysopsis marina (Feldmann) Billard and Chrysonephos lewisii (Taylor), two fast-growing multicellular benthic chrysophytes, together with a free-living form of the brown alga Acinetospora crinita (Fig. 1d). Between 2001 and 2005, as a result of these impacts, more than 90% of C. caespitosa colonies on the bank died (Fig. 1c).