Abundance and size structure of Thalassoma pavo (Pisces: Labridae) in the western Mediterranean Sea: variability at different spatial scales

Abstract

The present study was carried out during the summer of 1997 in three locations of the western Italian coast (La Spezia, Ligurian Sea; Ischia, central Tyrrhenian Sea; Ustica, southern Tyrrhenian Sea) to investigate variability in abundance and size structure of the labrid fish Thalassoma pavo along a latitudinal gradient and over different spatial scales. Fish abundance and size were estimated by visual census in shallow rocky reefs. Significant differences were detected in average abundances of T. pavo (with and without the contribution of juveniles) among locations (separated by a distance of hundreds of kilometres from each other) and over the smallest scale examined (i.e. among sites located tens of metres from each other). However, most variability was explained considering the largest spatial scale associated with the latitudinal gradient. Average abundances generally decreased from the southern Tyrrhenian to the Ligurian Sea. In terms of frequency, juveniles (<5 cm total length (TL)) increased from north to south, while the largest fish (16–20 cm) displayed an opposite trend. The intermediate size-classes (6–10 and 11–15 cm) did not show any distinct patterns. In terms of abundance, small T. pavo (0–5 cm) showed significantly higher densities at Ustica, followed by Ischia and La Spezia. Fish 6–10 and 11–15 cm long were significantly less abundant at La Spezia than at Ischia and Ustica, while those between 16 and 20 cm TL did not differ numerically among the three locations. The results of the present study provide suggestive evidence that factors acting on a geographical scale are likely to influence distribution patterns of the thermophilic fish T. pavo, although significant effects have been also observed at the smallest spatial scale examined. Climatic conditions (e.g. water temperature along the latitudinal gradient) and substrate features (e.g. macroalgal cover, physical complexity, slope) are thus likely to affect average abundances and size structures of T. pavo populations along the western Italian coast.