Four species of sturgeons appear in the Yugoslav part of the Danube River. Among them three are anadromous, beluga (Huso huso), Russian sturgeon (Acipenser gueldenstaedtii) and stellate sturgeon (Acipenser stellatus) and migrate to Danube from the Black Sea to spawn. Only sterlet (Acipenser ruthenus) is exclusively freshwater. There are obvious differences between vernal and winter races of anadromous sturgeons, due to their different migratory patterns. Russian ichthyologist Berg (1934) claimed that there are two races (morphs) in the starlet population as well. This stimulated long discussion in Russian and Yugoslav literature, and it was the starting point for our research on sterlet in the Yugoslav part of the Danube River.

Morphometric analysis was preformed on 159 individuals caught in Danube River among 1123 and 1173 km, on four localities during September and October 2002. Analysis included 24 morphological and 3 meristic characters. K-means cluster analysis indicated existence of two different clusters (groups), one with 95 (59·75%) and other with 64 (40·25%) samples. Significant differences were found for preorbital, preoral and prebarbel length. Mean values (in % of head length) for first group were 50·13 ± 1·71, 57·61 ± 1·87, 39·74 ± 2·20 and for second group 53·50 ± 2·46, 62·07 ± 2·20 and 43·30 ± 2·08 respectively. Findings were confirmed by Principal Components & Classification Analysis. These results speak in favour of existence of two morphs in sterlet population, one with long and pointed rostrum (vernal) and other with short rostrum (winter). According to the literature that is probably influenced by different reproductive behaviour (time separation in spawning).