Seasonal Variation of Fish Assemblages in the Loano Artificial Reef (Ligurian Sea Northwestern-Mediterranean)

BULLETIN OF MARINE SCIENCE, 55(2-3); 401-417, 1994 SEASONAL VARIATION OF FISH ASSEMBLAGES IN THE LOANO ARTIFICIAL REEF (LIGURIAN SEA NORTHWESTERN-MEDITERRANEAN) Marco Relini, Giovanni Torchia and Giulio Relini ABSTRACT Thirty months after the construction of the Loano artificial reef a study of fish abundance was started. The method used both direct censuses at monthly intervals and limited fishing activity with longlines and trammels, The monthly census was carried out by two divers in the central part of the artificial reef made up of pyramids, each consisting of five concrete cubic blocks with 2-m sides, The census was carried out in two main phases: first the observer counted the fish moving around him for 15 min and then he explored the holes and the internal corridors. The second phase consisted of a 25-m crosswise movement between two pyramids, Observations were carried out over a period of 2 years, from February 1989 to February ]991, and 66 species of fish and invertebrates of fishing importance were caught and/or observed in the Loano artificial reef; 41 species (of which three were Crustaceans and two Cephalopods) were censused during the above-mentioned dives. The most numerous families were Sparidae and Labridae, followed by Serranidae, The predominance of the first two families seems to be one of the general characteristics of rocky sea-bed fish communities in the northwest Mediterranean, Seasonal changes in the qualitative and quantitative assemblages of fish are described. An important contribution to these changes was the appearance of juveniles, mainly in the latter half of the summer and in the autumn, It was in this period that the maximum fish density occurred, while the minimum was registered in June. The number of fishes increased with the age of the artificial reef and the evolution of macroben- thos settlement. In particular the development of algae improved the reef's attractiveness to the Labridae species. Over the last two decades various artificial reefs have been built in the Medi- terranean (a list can be found in Bombace's study, 1989), but only for some of these are data available on the assessment of the fauna which is important for fishery. Only on a small number of these reefs has a census been carried out using the techniques of underwater observation. To the northwest of Marseilles, Bregliano and Ody (1985) used monthly observations over a full year's cycle to study fish in two zones with artificial reefs at 15- and 23-m depths and in a rocky natural environment. The authors did not find substantial differences between the zones with regard to the communities of fish; these were dominated by Labridae (J 2 species) and by Sparidae. They were, however, able to confirm a notable increase in the density of fish after the construction of the artificial reefs. In Israel, close to Haifa, Spanier et al. (1985) studied the colonization by fish of a reef constructed with old car tires, which showed what was initially a very large presence of fish with a density that levelled off slowly. Unfortunately, the study was limited to only 7 months from the immersion of the structures (from May to December). Close to Haifa, Diamant et al. (J 986) studied fish in a natural zone around a wreck using both the technique of underwater observation and rothenon to poison the fish. On the basis of the data gathered between 1975 and 1983 the authors were able to point to a greater specific richness on the wreck- 42 species, compared to the 25 and 26 of the two natural zones, rocky patch reef and vermetid platform walls. Some species of great commercial interest, such as 40] 402 BULLETINOFMARINESCIENCE,VOL.55, NO,2-3, 1994 Epinephelus aeneus, E. alessandrinus, Dicentrarchus labrax, Sciaena umbra, proved to be present only on the wreck. Using three techniques, Duval and Duc1erc (1986) studied the fauna of two reefs, at Gruissan and Saint Ciprien, situated on the Languedoc-Roussilon coast in the south of France. Unfortunately, the authors are only able to provide a list of the fish surveyed during dives (41% of the total population), captured with trawls (73% of the species) and with gill nets (65%). Charbonnel (1990) studied the fish communities of several artificial reefs in France using the Harmelin- Vivien et al. (1985) census technique. He reported that the fish populations to be found in artificial habitats are very similar to those on rocky bottoms. On the basis of the data furnished by a few catches and occasional observations by scuba divers, lists of the fish present on the reefs in the Marconi Gulf, eastern Liguria (Relini et aI., 1986) and near Varazze, western Liguria (Relini, 1982) have been provided. Using only the technique of catching fish, in Italy, some artificial reefs in the Adriatic (Bombace et aI., 1990; Bussani, 1981) and in Sicily (Arculeo et al., 1990) have been studied. Direct censuses of fish in natural environments in the Mediterranean are reported in a paper by Harmelin (1987). In particular, the observations carried out in the Posidonia meadow (Harmelin- Vivien, 1982, 1983) in a reserve to the south- east of Banyuls and in the marine park of Port-Cros are of great interest, also because they provide us with terms of comparison with censuses in protected zones containing artificial reefs. To assess the fish population on the Loano artificial reef (LAR) the visual technique developed by Harmelin- Vivien and Harmelin (1975) Harmelin- Vivien et al. (1985) was used. The aim of the LAR is to protect the Posidonia meadows because of their importance in maintaining biological diversity, safeguarding the coast from erosion and promoting small-scale fisheries. The second series of ob- jectives is to protect the area off Loano from illegal trawling, to provide hard substrata suitable for the settlement of benthos organisms and to offer shelter and protection to eggs, juveniles and moulting animals (Relini and Moretti, 1986). STUDY SITE The study site is located in the central part of the LAR (Fig. IB) in an area of about 350 ha between 5 and 45 meters depth in the western Ligurian Riviera, northwest Mediterranean. The LAR is composed of two parts (Relini and Orsi-Relini, 1989, 1990): a central main group of 30 pyramids (Fig. IC), in a 100 X 200 m area (16 to 23 m depth) and a grid of single cubic blocks (200 of 1.2 m side and 250 of 2 m side) distributed around the central portion of a 3 X 1.5 km rectangle (Fig, 18). The census site consists of two pyramids, 25 m apart, at 18 m depth, Each pyramid is made up of four cubic blocks (perforated concrete blocks of 2 m side) at the base and one at the top (Fig. ID). The distance between the basal blocks is about 80-]00 cm and the holes have diameters of 20 and 30 cm. The basal cavities are 30 cm in width and depth. METHODS To estimate the fish population of the artificial reef, we used the above-mentioned Harmelin techniques of visual censusing (Harmelin-Vivien and Harmelin, 1975; Harmelin-Vivien et aI., 1985) from February 1989 up to September 1991, on at least a monthly basis. Though direct censuses cannot cover the entire fish population, as some species will always remain concealed to SCUBAdivers, underwater observation can be an effective method in the process of understanding the dynamics of population formation and seasonal variation on an artificial reef. Considering the limited time available for safe diving activity and on the basis of a preliminary analysis-which showed that there were no great differences between two and three pyramids in terms of the number of species detected-we decided on our sampling zone. This covered a total area of 312 m2 including two pyramids at 18 m depth (80 m) of artificial reef) and the distance that separates them, in a volume of water of 1,026 m), RELlNl ET AL.: SEASONAL VARIATION OF FISH AT LOANO 403 500 m I.........f !......! 1......1 ow B -1-~-~-?-9T9-C?-I~I I I I I I I I I -0- -0- -0- -0- I I I I ; -0- -0- -0- -0- I I I I ; -0- -0- . -0- -0- -~=~~?=~~~=~=9=9~j~I -- C D Figure 1. Location of Loano in the northwestern part of Italy (A). General view of Loano artificial reef (B) with main reef in the central part. The structure of the main reef (C) composed of thirty pyramids. Each pyramid consists of (D) five large blocks. First, two SCUBA divers placed themselves at the apex of the pyramids. From this position, they counted the fish present in the mass of water around the structure (phase A). This operation, which lasted 8 min, enabled them to carry out a count up to two m around the complete perimeter of the pyramid (5 m mean visibility) in a volume of water of 323 m'. Once this first phase was complete, the two divers moved slowly around the perimeter of the pyramid, and through one of the internal passageways. The aim here was to record the presence of the more sedentary and concealed crcatures, which are often underestimated in a visual census (Harmelin-Vivien et aI., 1985). The second phase lasted approx. 10 min (phase B). The third phase consisted in swimming through another corridor, which this time was rectilinear, 25 m long and approx. 6 m wide. In this way the divers reached the second pyramid, where phases A and B were repeated. Further information about the species present on the LAR was obtained using trammel nets and longlines (Relini et al.

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