Juvenile fish in Cystoseira forests: influence of habitat complexity and depth on fish behaviour and assemblage composition

The sublittoral forests formed by the fucoid algae Cystoseira spp. are important juvenile habitats for many Mediterranean fish species. However. the spatial variability of juvenile fish assemblages within the forests and the potential environmental drivers, such as depth and habitat complexity, remain poorly understood. We estimated the density, size and behaviour of juvenile fish assemblages in subtidal (0-15 meters depth) Cystoseira brachycarpa var. balearica forests in north Minorca Island (North-western Mediterranean Sea) over two consecutive autumns (2012 and 2013). Depth and forest complexity, here measured as canopy volume, had both a significant and independent effect on the juvenile fish assemblages in tenns of species abundance composition and body size. Assemblages found in the shallowest depth range (3-4m) were characterized by greater density of the ornate wrasse Thalassoma pavo, while those deeper down (10-12m) had higher density of the rainbow wrasse Coris fulls, independently of its size composition. Juveniles of both species were more abundant in less complex forests; conversely, juveniles of wrasse of the genus Symphodus were more abundant in more complex forests. The smallest sizes of T. pavo occurred in the most complex forests. On the other hand, our results demonstrated that juvenile fish behaviour was unrelated to the complexity of the Cystoseira forests, but mainly related to the body length. The effects of body length on behaviour were however species-dependent. Cryptic and transitory' behaviour was mostly observed in the smallest and largest juveniles of T. pavo and C. julis, respectively, while the behaviour of Sympliodus spp. was unrelated to their body length. Our study highlights the importance of preserving healthy Cystoseira forests and their intrinsically patchy nature, as this habitat, with its mosaic of different degrees of complexity and bathymetric variability, enable the presence of different fish species at various life stages.