Seasonal Changes in the Condition Factor and Gonado-Somatic Index of Carassius gibelio From the Kopački rit Nature Park, Croatia

This study’s objective was to explore some of the annual Prussian carp’s (Carassius gibelio) biological characteristics (sex ratio, length–weight ratio, and gonado somatic index - GSI) in the Kopački Rit Nature Park in Croatia. The Prussian carp is an invasive fish species originated from Asia, which became widely distributed throughout Europe and is believed to be responsible for a decline in the number of indigenous fish, invertebrate, and macrophyte populations in different areas. The fish sampling was conducted from September to November 2017 and from February to July 2018, respectively, using electrofishing. A total of 475 fish individuals were caught and measured for the total length, standard length, body height, and body weight. Males made up 34.31% of the population of Prussian carp. The mean value of Fulton condition factor (FC) of Prussian carp in Kopački Rit was 1.77 ± 0.28. Overall, the FC value had a relatively small amplitude, but it exhibited a clear seasonal vari ability. The average GSI values of females (9.91 ± 5.34) were higher than those of males (3.58 ± 1.37) during the observed period. The average GSI values indicate that the majority of females are spawning in the late April and May, which are also the same periods when other cyprinid species are spawning, allowing the Prussian carp to exploit its gynogenesis strategy.