Transitional aquatic ecosystems are characterized by much greater levels of environmental variability and ecological productivity than are typically found in marine ecosystems. Fish fauna present in transitional ecosystems must and are able to cope with the high variability and instability of the abiotic environment in order to utilize the abundant trophic resources available in these ecosystems, representing an essential component of the biodiversity of transitional aquatic ecosystems. For this reason, Directive 2000/60/EC declares that fish fauna also contributes dominantly to the definition of the ecological status of transitional waters in the Biological Quality Elements.

This paper describes the results related to the ecological status assessment study of the transitional waters of the Po Delta E-R for an evaluation by analysis of the biological quality element “Fish Fauna” and application of the HFBI index. The study was carried out in 5 lagoon-estuaries, for two seasons (spring-autumn 2024), using a beach seine “for juvenile fish,” as per ISPRA protocol, in order to standardize catch data in terms of sampled area units. All individuals sampled were identified to the species level and classified into ecological and trophic categories. Abundance and biomass values by species, were expressed as density values (individuals/100 m2 and grams/100 m2) for the sampling area. For the analysis of lagoon biological quality, the HFBI multimetric index was used (Catalano et al. 2017; Franzoi et al. 2019). The six metrics required to calculate the Habitat Fish Bio Indicator (HFBI; Catalano et al., 2017) were calculated from fish stock density and biomass data, The HFBI scores, calculated on an annual basis by averaging spring-autumn values, were used to assess the ecological status of lagoon-estuaries

A total of 35 species of nectonic fauna were identified, of which 4 belong to the order Decapoda. The most represented functional group belongs to the lagoon residents, 90% of individuals in spring and 42% in autumn, followed in quantitative abundance by the marine migrants 6.7% in spring and 17.6% in autumn.

The results of the application HFBI index in the transitional waters of the Po Delta E-R, show that out of 16 stations, 4 stations achieve “good” ecological status, 5 ‘sufficient’ status and 7 “poor” status. Assessing the annual average values of the HFBI index, only one lagoon reaches the “good” ecological status, 3 lagoons the ‘sufficient’ status and one results in the “poor” status.

The application of the HFBI index shows that the ecosystems of the Po Delta tradition are in an average state of “sufficient” quality, with only a few areas at good and poor status, no environment has recorded values at “excellent” or “bad”. This first application showed that the HFBI index can be an effective tool for the assessment of the ecological status of transitional waters, capable of assessing and identifying the main anthropogenic threats and pressures that insist on these ecosystems and become a basic element for the management and conservation of these environments.