Influence of the raising system on phagocytosis in the endangered Danube salmon (Hucho hucho)

Abstract The Danube salmon ( Hucho hucho ) represents the flagship of the Salmonidae family. The species is severely fragmented within the Danube drainage and is included as Endangered (EN) in the IUCN Red List of Threatened Species. Unfortunately, a limited understanding of biological responses to anthropogenic alterations has impeded the conservation efforts for this fish species. In this framework, the aim of the research was to evaluate one of the Danube salmon's most important nonspecific defense mechanism: the phagocytic activity of circulating leukocytes in relation with the raising system. The study was carried out on a number of 30 mature, healthy Danube salmons ( Hucho hucho ), aged 4–11 years (males: n = 15; females: n = 15), reared in two different systems (raceway and mountain lake) on the same fish farm. In spite of few observations in the wild, to date, there are no self-sustainable populations of huchen. The individuals used in our study represented the last and largest population of Danube salmon known to exist in Romania. Carbon particle inclusion test was used to monitor spontaneous phagocytosis. There was no statistically significant difference (p > 0.05) between average in vitro phagocytic activity of huchens reared in the mountain lake and in a raceway system, therefore, we considered that the raising system had no significant influence on the innate cell-mediated response. Efforts must be made for the conservation of this species thus attention must be directed towards rational repopulations or restocking, where acclimatization is taken into consideration. Since this parameter is crucial for tissue homeostasis and resistance to diseases, it should be interpreted in the framework of microbial pollution of the environment.