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Summary The aim of this paper was to use geometric morphometry as a helpful tool to clarify the overlap between meristic characters (such as the number of anal fin rays) of different fish identification keys employed for the two sister species, Ameiurus melas and Ameiurus nebulosus. Geometric morphometry included shape analysis of 15 landmarks on 432 specimens. The analyses showed significant differences among groups based on the sampling site, while there was no difference among groups based on the number of anal fin rays (used to distinguish species). This suggests that the use of meristic characters for the determination of these two sister species is rather questionable.

Summary This paper presents the status and trends in management of sturgeon species and the development of sturgeon aquaculture in the Lower Danube countries: Romania, Bulgaria, Serbia, Ukraine and Moldova. Sturgeon fishery moratoria and aquaculture development represent first steps in the Lower Danube countries to combat extirpation. Supportive stocking was used as a compensation for the impact of sturgeon fishery and dam construction, but these efforts unfortunately lacked adequate cooperation and coordination between and among region countries. Unsolved problems remain, such as the presence of illegal sturgeon fishery, water pollution, habitat destruction and fragmentation. Construction of fish passes and habitat restoration project developments are two key issues that have yet to be tackled in the Lower Danube region.

Summary A sample of 198 asp (Aspius aspius) was collected over a 28.5 km long section of the Danube River, upstream and downstream from the mouth of the Sava River, in order to provide information on population structure, diet, growth, and condition of this species. Total body lengths of sampled fish were 90–405 mm, and body weights 5–567 g. The sample was predominantly sub-adult fish aged 0+ to 4+ years. Sexually immature asp prevailed over the sexually mature asp. The sex ratio (1.4 : 1) was biased toward males. Male asps in the Danube become sexually mature only in their fifth year of life, and the females even later, as no sexually mature females were caught. The diet spectrum consists exclusively of fishes; of seven determined prey species, most important are bleak Alburnus alburnus, roach Rutilus rutilus, and goldfish Carassius auratus. Highest value of the seasonal vacuity index is in November (Iv = 94.1), and the smallest in July (Iv = 35.6). Trophic niche breadth is largest in May (H = 1.40), and smallest in November (H = 0.00). The largest average number of prey items is found in age class 4+ (2.33), the smallest in age class 0+ (1.00). Von Bertalanffy growth parameters are L∞ = 526.46, K = 0.28, and t0 = 0.22. The length–weight relationship is described as log W = −5.16 + 3.03 log L. Fulton’s condition factor increases with fish length, from 0.70 to 0.83. Regarding seasonal fluctuations, this factor begins to increase in late spring, with the trend continuing until autumn, and then decreasing from autumn to spring.

Summary The presence of Atlantic sturgeon (Acipenser sturio) and ship sturgeon (Acipenser nudiventris) in the Danube has been known through only occasional, chance sightings; significant uncertainty remains as to whether these species are already extinct or still present in the Danube River basin. In this study, five different statistical methods were applied for assessment of their extinction probability. All methods provided a significant probability that the Atlantic sturgeon is extinct, with extinction having occurred somewhere between 1966 and 1970. Although the applied methods provided different results, all indicated that the ship sturgeon is probably still present, but that extinction may occur within a few decades. Sensitivity analyses of these statistical methods projected that new sightings of ship sturgeon in the coming years would not extend the length of extinction by more than a few years. The best management approach for these two species, given the lack of knowledge on their presence in the Danube River basin, could be to apply all feasible protection measures for the other four Danube sturgeon species. These measures could at the same time provide protection for these two species, presuming they are not already extinct or beyond recovery.

Seasonal changes in ovaries and testes of the pike, sampled in the Danube (at 1163 and 1175 km) were analysed in this research. According to the presence of developing and resting oocytes in histological sections of pike ovaries sampled during August, it is possible to determine which female will be sexually mature the following spring. Active vitellogenesis starts in September (with oocyte diameters of 514 ± 54 μm and 453 ± 68 μm, at the upstream and downstream locality, respectively), at which time the differences between the developing and resting oocytes are even more obvious. Active spermatogenesis starts in September and lasts for 2 months, and by November primarily spermatozoa can be seen in histological sections. The reproductive score points to differences in samples from the two localities that can be explained by the differing characteristics of the habitat. Values of absolute fecundity (AF) ranged from 524 to 123 896 eggs, and the mean value of relative fecundity (RF) was 40.4 ± 12.5.

Summary Whereas earlier Amur sleeper (Perccottus glenii) findings in the Danube River were only single specimens, data reported in this study represent the first record on establishment of a local population. An overview of the spread of the Amur sleeper in the Danube River basin is also presented.