Habitat destruction is a major cause of its fragmentation. Since the 1960's, human pressure on seashores, especially along the Gulf of Lion has induced lots of fragmented habitats. Destruction is not the predominant factor for this fragmentation, but the large-scale installation of coastal artificial rocks which creates new habitats. This study investigated on the role of these structures compared to natural areas (lagoons, rocky shores) to maintain fish populations more south, on the rocky coast. The white seabream, infeodated to these rocky areas, is the biological model used. Scuba diving visual census of adults and juveniles, direct tagging and otolith microchemistry were performed to better understand these dynamics.
Coastal artificial habitats have to be considered as important nursery areas. Depending on the year, densities of juveniles are 30 to 109 times higher than those observed on natural habitats, either on the rocky coast or in the lagoon. Such differences could be explained by the presence of only two main groups of adults along the French Catalan coast. The first would supply the coastal artificial rocks and be located at cap Leucate. The second, in the reserve of Cerbère-Banyuls, would export juveniles to the Spanish coast following the dominant north-south current. Moreover, the lagoon has lost its nursery function to the benefit of the artificial coastal habitats.
Our results also show the connectivity during the adult phase between these coastal structures and the natural rocky areas. 20% of white seabreams which had settled on the artificial habitats, left for the rocky coast, mainly in spring and autumn. We also presume that one third of the seabreams living there are those that had settled on the artificial habitats of the sandy coast.
This study underlies the importance presumably of major role played by these artificial habitats to the maintenance and expansion of populations of white seabream in the Catalan coast. The results are possibly a snapshot of the population dynamics in the Gulf of Lion. Habitat fragmentation has a positive effect in this case. It could be the origin of emergence of new species on the coast of Languedoc-Roussillon like the zebra seabream, Diplodus cervinus and the common dentex, Dentex dentex.