Interactions between fish resources and Cormorants Phalacrocorax carbo in the Grado and Marano lagoon (NE Italy)

The aim of this study was to determine the importance of different environments for Cormorant Phalacrocorax carbo biology in the Grado and Marano lagoon (Friuli Venezia Giulia, NE Italy, Upper Adriatic Sea), and to estimate the levels of fish removal within such areas. Data were collected on Cormorant abundance and the amount of fish consumed in two fishing valli (Valle Noghera and Valle Artalina) and in three tidal areas (Goppion, Cavanata sea, Grado and Marano lagoon). The number of Cormorants per 100 ha was relatively low in all the study areas. The highest density of feeding cormorants in November (24 birds/100 ha) was in Valle Noghera. The latter is ascribed to an isolated event of 50 birds in social fishing activity. During the rest of the November survey time (72% of the total), lower densities were noted (2.5 birds/100 ha). Fish consumption was also relatively low. In Valle Artalina the maximum was 6.8 kg/100 ha in December. The highest fish consumption was estimated in Valle Noghera in November (41.9 kg/100 ha) and in the Grado and Marano lagoon in January (7.6 kg/100 ha). In fish farms, the highest fish consumption is recorded in November and December, exclusively within wintering basins and canals. So these areas should be covered with wire nets. Furthermore, considering the high variability and irregularity of fish removal, the use of active deterring methods (e.g. gas cannons) can give good results, especially if applied during social fishing events.