Changes in sterlet (Acipenser ruthenus L.) catch and length frequency distribution in the Serbian part of the Danube River during the twentieth century

Abstract

Data from a sample of 262 sterlet (Acipenser ruthenus L.) specimens from the Serbian part of the Danube River were compared with data collected between 1948 and 1967. The recent sample was caught between September 2002 and May 2003 at three localities (km 1173, km 1162 and km 1123) using drag nets. According to pectoral fin spine section analysis, all the specimens were in their first, second and third years of life. Comparison of the length-frequency distribution of these specimens with the data for the sterlet catch in the period 1948-1967 showed a recent shift towards lower length classes, as well as changes in the shape of the length-frequency distribution curve. The current sterlet catch is mainly based on individuals in their first and second year, while fish from the second, third and fourth years of life were dominant in the past. Sterlet catches in the lower part of the Danube River (area of Iron Gate I and II) showed maxima of 23 615 kg and 17 960 kg in 1969 and 1984. These maxima were probably connected with dam construction. Total catch and the catch in the lower part of the Danube River showed significant decreases after 1990. The study indicates unfavourable conditions for sterlet populations in the Serbian part of the Danube River in the second half of the twentieth century. Unfavourable factors include habitat destruction, over-fishing and capture of sexually immature specimens. A better understanding of sterlet migration and behaviour is necessary for more efficient future protection of this valuable species.