Small creeks in a big lagoon: The importance of marginal habitats for fish populations

Temperate transitional water systems, as in the case of the Venice Lagoon, are characterised by many different shallow-water habitats. The availability of trophic resources and the low predator pressure make salt marshes one of the most important habitats for many fish species, both resident and marine migrant, but several anthropogenic pressures, erosion and relative sea level rise in particular, are causing a significant loss of this habitat. A part from natural habitats, in many small islands of the Venice lagoon, artificial creeks of different size and morphology are present, once used in traditional aquaculture activities or built up as defence lines. Aims of this study is to analyse and compare the structure and composition of fish communities inhabiting small-sized creeks, considering both the natural and artificial ones, in order to evaluate the ecological importance of these marginal habitats for fish populations. A particular attention was given to artificial sites, assessing their ecological value as alternative refuge habitats to natural salt marsh creeks. One year samplings conducted in four sites (two natural salt marshes and two artificial creeks) allowed to describe the local fish communities, which comprised 20 species overall. The analysis of how water parameters and habitat structure influenced the fish communities showed the importance of the connection between small creeks and the open lagoon, but also the refuge function offered by confined systems. High abundances of resident fish species listed in the Annex II of the Habitat Directive were observed and juveniles of eight species of marine migrant fish were found, some of which are of economic importance. This study underlined the presence of significant densities of juvenile marine migrant and lagoon resident fish species in the two artificial habitats. Actually, species richness and density resulted to be, in some cases, higher than in natural salt marsh systems. Results of this study emphasize the ecological importance of these marginal habitats for many fish species, of both conservation and economic importance Thus a proper management and restoration strategy of these sites is needed to maintain their functionality and to buffer the disappearance of natural salt marshes.