Settlement and post-settlement processes of Mediterranean littoral fishes: influence of seascape attributes and environmental conditions at different spatial scales

摘要

The Mediterranean sea is one of the richest seas in the world. However, it is also one of the most threatened. In this sea, coastal fish populations, particularly sparids and labrids, play an important economic and ecological role. However, the perpetuation of these populations is limited, among other reasons, by juvenile settlement and recruitment. As juveniles habitats, in contrast to those of adults, are found in shallow coastal areas, they are more exposed to human impacts. Three very common habitats are identified in the literature as habitats for juveniles of these species in the Mediterranean: seagrass meadows, erect algae forests and shallow mixed areas of sand, pebbles and rocks. Faced with the anthropogenic transformations of these habitats, it is crucial a further understanding of the factors that influence the distribution of juvenile densities in these habitats. With this aim, juvenile fishes were monitored during the warm seasons of 2011, 2012 and 2013 in Minorca island (Balearic archipelago). Exploratory and inferential analyses of the data highlighted that at lower scales of the seascape, the variability of the juvenile density distribution patterns among a given habitat could be explained by variations of its structure, for Cymodocea nodosa meadows, Cystoseira spp. forests and shallow mixed bottoms of sand, pebbles and rocks. Different species of labrids and sparids responded differently to this factor, presumably because each species find the best compromise between availability of food and shelter (habitat quality) in different habitat structure conformation. Furthermore, in Cystoseira forests, which in Minorca extend until 15 meters depth, data highlighted some taxa-specific preferences for different depths ranges. Moreover, at larger spatial scales, the configuration of the coast, first in terms of exposure, shaped densities of juveniles, presumably affecting the initial larval input; secondly, coast orientation to strong winds influenced juveniles’ dynamics, impeding larval arrival or taking out larvae from juvenile habitats. Consequently, this thesis highlights the importance of considering forcing factors at different spatial scales in order to better explain the density distribution of juveniles. In parallel to this overall result, this thesis defends the importance of taking into account the different scales of the seascape in the management planning, and delves into the issue of preserving juvenile stocks of the three studied habitats.