Summary The composition of fish populations in lake Piediluco has undergone significant changes in recent years, mainly as a result of environmental deterioration due to the eutrophication of its waters and the introduction of some alien species. Lake Piediluco is a natural, regulated lake, which has been used for the production of hydroelectric power since 1908. Over the years, the trophic status of the lake has been profoundly modified: until 1980 it was mesotrophic; by 1989 it had become eutrophic, and now it is deemed to be hyper‐eutrophic. The aim of this research was to analyze variations in the composition of fish populations in relation to the trophic evolution of the lake and the introduction of alien species. The data on fish populations were gathered during four monitoring campaigns carried out in 1988, 1996, 1999 and 2004. To estimate the abundance of the various species, catches were standardized in terms of catch per unit effort (CPUE). The results of the research reveal that the increase of the trophic status of the lake has been accompanied by a qualitative and quantitative change in the composition and structure of the fish community, which is being increasingly dominated by omnivorous cyprinids, such as Scardinius erythrophthalmus and the alien Rutilus aula. This is taking place at the expense of species that are ecologically less resilient and commercially more interesting, supported by provincial management policies as annual repopulation programs and deployment of artificial spawning substrates. Moreover, in more recent years the presence of some new alien species has been recorded, such as Rutilus rutilus and Gymnocephalus cernuus. These species, which were not previously found, pose a further threat to commercial fishing in the lake.

The age and growth of the invasive black bullhead Ameiurus melas (Rafinesque, 1820) were studied in the Corbara Reservoir, an artificial lake located on the River Tiber in central Italy. The results of the research revealed that the population was made up of eight cohorts, the oldest specimens being spawned in 1999. The sex ratio appears to be balanced, although females predominate in the younger cohorts, while the percentage of males increases among older specimens. The differences between sexes in the slope of the length-weight relationship were not statistically significant; the slope for the whole sample (b = 3.055) was significantly greater than 3, indicating an allometric growth pattern of the black bullhead. There were no significant differences in mean back-calculated lengths at age between the sexes. Analysis of Lee's phenomenon revealed the existence of differential mortality among younger individuals (1 and 2 years old) that favours larger individuals. The Von Bertalanffy growth function was TL t = 35.69 {1 -exp[-0.18 (t + 0.27)]}. The specimens caught displayed mean relative weight (W r ) values distinctly below 100 at all ages, while no significant difference emerged between the sexes with regard to condition. RÉSUMÉCroissance du poisson-chat Ameiurus melas (Rafinesque, 1820) dans le Lac Corbara (Ombrie -Italie) L'âge et la croissance du poisson-chat invasif Ameiurus melas (Rafinesque, 1820) ont été étudiés dans le Lac Corbara, un lac artificiel situé le long du cours du fleuve Tevere, en Italie centrale. Les résultats de la recherche ont révélé que la population était composée de huit classes d'âges, les spécimens les plus âgés provenant de la fraie de 1999. Le sex-ratio semble être équilibré, bien que les femelles prédomi-nent dans les classes d'âges plus jeunes, tandis que le pourcentage de mâles augmente parmi les spécimens plus âgés. Les différences entre les sexes en matière de pente du rapport longueur-poids n'étaient pas significatives ; la pente de tout l'échantillon (b = 3.055) était significativement supérieure à 3, indiquant que le poisson-chat a un modèle de croissance allométrique. Aucune différence significative n'a été constatée entre les longueurs rétromesurées moyennes des différents sexes au même âge. L'analyse du phénomène de Lee a révélé l'existence d'une

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