Samples of invasive cyprinid fish, the Prussian carp (Carassius gibelio), were collected by fyke nets in Leszczynskie Lakeland (Poland) during the summer and autumn, 2010, and during the spring, 2011. All captured fish were females. For each fish, the total weight () and the standard length () were measured and Fulton’s condition factor was computed. Graphical investigation and the Mann-Whitney-Wilcoxon test showed statistically significant location shift of the distribution from summer to autumn (upward) and from autumn to spring (downward). Relationship between total weight and standard length was described with the mean growth curve . Seasonal parameters ( and ) were estimated with a nonlinear regression approach, that is, numerical optimization methods. Growth was allometric in summer and autumn and isometric in spring. The differences between summer and autumn growth curves and between autumn and spring growth curves were statistically significant. The seasonality exhibited by the condition factor and the growth curve may be due to different spawning, breeding, and feeding activity in the different seasons and to variable environmental conditions. 1. Introduction The cyprinid genus Carassius is widespread across Europe and North and East Asia. At least five species are considered: C. carassius (Linnaeus, 1758) in most of Europe and Western Siberia , C. langsdorfii (Temminck and Schlegel, 1846) and C. cuvieri (Temminck and Schlegel, 1846) in Japan [2, 3], C. auratus (Linnaeus, 1758) in Mainland East Asia , and C. gibelio (Bloch, 1782) in Europe, Siberia, and Northeast Asia [1, 5]. Some authors recognize additionally the species C. grandoculis and C. buergeri from Japan [6, 7]. In Poland the first documented records of C. gibelio came from 1933, when it was found in ponds northward from Lvov (currently Ukraine) and in southern part of Central Poland. Due to escapes from ponds it appeared in open waters and successively penetrated to next drainages . During the last 20 years it has become more abundant and frequent than native Carassius carassius. Nowadays, it is very common on the whole territory of Poland, particularly numerous in lowland lakes, ponds, and rivers. As a result of lake stocking, C. gibelio began to feature in lake fish landings. In recent years, the total catch in inland waters of Poland was tons, including tons of fish from genus Carassius. In Leszczynskie Lakeland, C. gibelio contributed from to to the total weight of fish catch . This fish is turning out to be more and more important in Poland due to its tasty meat and
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