The recent invasion of Rutilus rutilus (L.) (Pisces: Cyprinidae) in a large South- Alpine lake: Lago Maggiore

Very recently, a common cyprinid fish species at northern latitude, the roach (Rutilus rutilus L.) has been observed in the large south-alpine Lago Maggiore for the first time. The colonization of the lake has the characteristics of an invasion. In order to investigate this new fish population, benthic multi-mesh and commercial gill nets were used to sample fish in a wide gulf in the central part of the lake (Golfo Borromeo) from February to October 2006. In total 779 roach (TL: 5.5 – 38.8 cm) were caught, aged, sexed and stomach analyzed. Fecundity was assessed by counting the eggs in ripe gonads of females of age 3 and 4. The roach have become clearly the dominant species in the catches, constituting over 70% of the biomass and 70% of the numbers caught. None of the roach exceeded 8 years in age. They grow very fast (Φ' = 2.55) and have high fecundity (Fabs = 13740 ±4700 S.D. at age 3 and Fabs = 14768 ±5642 S.D. at age 4). The diet spectrum is wide and characterized by a strong seasonality. In early spring algae (benthic and planktonic algae), benthos and detritus prevail, whereas in summer zooplankton is the most frequent food item. In autumn roach switch back to benthos and detritus. Algae occur more frequently in young specimens while zooplankton, benthos and detritus in adults. The roach population is practically unimpacted by fishing. The minimum legal mesh size (25 mm) of gillnets, used by commercial fishermen, selects roach at the mean LT of 21.2 cm, which is over its length at maturity. The results suggest that roach established a viable population in Lago Maggiore less than a decade ago and rapidly became one of the dominant (if not the dominant) fish species. The possible consequence for the fish fauna in general and for the ecological status of the whole lake is briefly discussed.