Estimation of noble crayfish (Astacus astacus L.) population size in the Velika Paklenica Stream (Croatia)

Astacus astacus is one of four native European crayfish species living in Croatian freshwaters. It is naturally distributed in rivers belonging to the Black Sea Basin. To some rivers of the Adriatic Sea Basin, however, it had been introduced by man. According to the information from local people it is most likely that A. astacus has been introduced into the Velika Paklenica Stream during the first half of the 20(th) century. Our study stream, draining the Velebit Mountain, is situated in Paklenica National Park, Middle Adriatic. The fieldwork of our study was performed during July 2000 and 2002, respectively. To estimate the population size we used mark-recapture techniques (Jolly-Seber Method, Schnabel Method and Schumacher and Eschmeyer Method). Crayfish were trapped with baited LiNi traps that were set along the stream banks at 10 m intervals and left there overnight. Caught animals were individually marked by piercing holes into the uropods and telson and then released. In July 2000 we caught 233 crayfish in total, from which 70 specimens were trapped more than once. In July 2002 we caught 296 crayfish, from which 86 were trapped more than once. Differences in the population size between the two years are discussed.