I popolamenti ittici della Riserva Naturale Regionale Monterano

Abstract River fish assemblages, as well as representing an important research topic related to the community of aquatic systems, are also the target of many actions linked to the implementation of EU Directives aimed at the sustainable management of these systems and biodiversity conservation, such as the Water Framework Directive (EC, 2000) and the Habitat Directive (EC, 1992), the latter specifically referring to indigenous fish species and aquatic communities taken as sensitive biotic components to be monitored in order to devise plans for their protection and safeguarding. Ecologically speaking, fish assemblages represent a fundamental biotic component of river systems, where they occupy the highest levels of the trophic networks and exert a strong influence on the structuring and functionality of the ecosystems. Precisely because of their position in the trophic networks and their sensitivity to environmental changes, fish assemblages are often used as bioindicators to evaluate the ecological status of aquatic systems (Tancioni et al., 2005; Scardi et al., 2006). The existing structure of fish assemblages has been affected by a series of remote natural “filters”, such as zoogeographic factors, which have led to the selection of primary endemic taxa (that is, which are confined to fresh or salt waters) as well as by more recent factors of anthropic origin that, after they have become acclimatized, are added to and often compete with similar primary taxa. An analysis of fish assemblages and fish populations, above all in protected fluvial areas, such as the Monterano Regional Nature Reserve (RM), a site of EU interest (Fiume Mignone-Medio Corso IT 6030001), is a fundamental step in the identification of conservation and sustainable management actions regarding original fish biodiversity and that of the river ecosystem concerned. The hydrographic profile of the protected area consists essentially of the middle course of the Mignone River, with its left tributary Torrente Lenta and other secondary water courses. These lotic environments were studied for the purpose of characterizing the fish fauna both in the distant past (Cataudella, 1977; Bianco, 1989), and in more recent times, in order to draw up the Fish Chart for the fish fauna conservation management plan (Bianco and Santoro, 2004), to prepare the Fish Map (Tancioni and Cataudella, 2009) and the Biodiversity Fish Map of the Province of Rome. During the last two study phases regarding the main stretch of the Mignone and along the Lenta, 8 sampling stations were set up, representing different environmental conditions. At these stations experimental fish catches were performed and the environmental characteristics recorded in order to analyse the composition, abundance and structure of the populations and the fish assemblages in conformity with the ISPRA-MATTM sampling protocol for fish fauna in lotic environments (Scardi and Tancioni, 2007). The experimental catches allowed the identification of 14 fish taxa, 6 of which autochthonous and 8 allochthonous (5 taxa transferred from north Italian river basins and 3 taxa of exotic origin), the latter having been introduced principally in the 1980s. Of the observed taxa, the autochthonous species (chub, S. cephalus, Italian soufie, T. muticellus, Apennine barbel, B. tyberinus, Italian brook goby, G. nigricans, Italian orange-fin roach R. rubilio and eel, A. anguilla) account for 43% of the total. The transferred taxa (north Italian bleack, Alburnus alburnus alborella, Italian orange-fin nase, Chondrostoma genei, Italian red-eye roach, Rutilus erythrophthalmus, Po river goby, Padogobius martensii and Po barbel, Barbus plebejus) represent 36%; the exotic taxa (carp, Cyprinus carpio, goldfish, Carassius auratus and pumpkinseed sunfish, Lepomis gibbosus) make up the remaining 21%. Nevertheless, autochthonous species are more prevalent than allochthonous ones, both in terms of number and weight densities. With reference to the various anthropic pressures exerted on the system (e.g. changes in the hydrological and morphological characteristics caused by the presence of a dam on the main stretch of the Mignone, and by a weir on the final stretch of the Lenta; excessive water withdrawals; pollution of domestic origin) the assemblages appear to have distinct structures. In particular, the “healthier” local assemblages from the conservation point of view seem to be those found upstream from the dams and upstream from the Mignone river weir, as well as in the stretch upstream from the Lenta Torrente weir, where endemic and sub-endemic taxa prevail almost exclusively, as is indicated also in annex II to the Habitat Directive. However, while on the one hand these weirs and barrages have had a positive effect on fauna integrity, as they form an insurmountable barrier for the allochthonous taxa moving towards the upstream sections of the watercourses, on the other they prevent the free movement towards the river of migratory indigenous species like the eel (A. anguilla) which are constrained in the downstream stretches. A. anguilla was also recently included on the IUCN red list as one of the species in serious danger of extinction for which specific conservation and sustainable management plans at both EU and national level are being prepared. As far as the conservation and sustainable management actions regarding fish fauna are concerned, several possible actions have been indentified, to be carried out with both in situ and ex situ approaches: construction of “fish-passages” at the level of the weirs to allow selective eel migration (elvers and glass eels); selective transfer of autochthonous fish species from the less tainted zones to those more heavily colonized by allochthonous taxa; reassessment of minimum instream flow conditions downstream from the weir in order to reduce summer-time concentration of specimens in highly confined areas; launching of allochthonous taxa eradication programmes; launching of programmes to produce authochthonous taxa using ecological techniques in order to ensure, wherever necessary, responsible restocking using selected broodstocks that are ‘certified’ on a genetic, morphological and health basis.