Danube sterlet morphometrics and genetic – guidelines for restocking programs

Abstract

Despite sterlet (Acipenser ruthenus L.) being a less important resource regarding caviar production, their populations have experienced a decline during the 20th century throughout its range, mainly due to poorly regulated fishery, pollution, habitat fragmentation and habitat loss. Stocking programs are implemented throughout Danube River basin, with Upper Danube populations being dependent on continuous stocking efforts, while commercial exploitation of wild stocks in the Middle and Lower Danube has to be compensated with stocking of larvae, fingerlings and juveniles. Selection of proper specimens for stocking programs should be carefully conducted, since it can lead to deleterious impact, such as reduction of effective population size, inbreeding and outbreeding depression, and loss of locally adapted alleles. Therefore, natural populations should be examined genetically both before and after release of hatcheryreared juveniles. Our research on Middle and Lower Danube sterlet, suggests that genetic variability should be attributed almost entirely to individual variability, with a weak population structure and no clear evidence of a bottleneck and inbreeding within populations. Also, specimens used for the supportive stocking in the Tisza River in Hungary originated from the Danube River, so the information about gene flow between these rivers should be carefully considered. Additionally, most of breeding programs are focused on genetic diversity and do not acknowledge complexities of wild populations fitness architecture. Although Middle and Lower Danube dams are recent, in regard to sterlet population time, they create more lentic conditions that do not suit sterlet rheophilous nature. Both our previous and current morphometric research suggests that hatchery-reared sterlet specimens are not necessarily suited for stocking of certain Danube River sections. Lower Danube section have a different water flow regime and suspended sediment discharge than those in Middle Danube and in Lower Tisza River, so the sterlet specimens are differently adapted to their environment, which is in concordance with our findings. We suggest that both shape analysis and genetic analysis should be applied in restocking programs.