## 4 Discussion  
Recent studies on the effect of salinity on fish have focused on examining how salinity affects the growth, mortality, health condition, and osmotic stress of fish farm [9; 13; 14; 22; 45; 46] and hatching eggs [47]. This type of research is necessary because it might help to properly choose salt concentrations and times of exposition of fish to keep them healthy during farming. Salt is commonly used as a disinfect for the prophylactic prevention of disease development in fish farms, and thus might help to keep fish in good health, especially at the beginning of fish fry life. It is a cheaper, and probably healthier, replacement for other chemotherapeutics. However, the effect of salinity on the fish gut microbiome has been poorly investigated. Although there have been studies on the effect of salinity on the gastrointestinal microbiome of a few fish species [15; 32; 33; 48], to the best of our knowledge, there is a lack of studies on the gut microbiome of pike fry _(E. lucius)_. Therefore, it was hypothesized that the gut microbiota of pike living in freshwater differ from those of pike living in different salinity concentrations. What is more, a comparison of the morphological parameters and mortality of fish under osmotic stress was conducted. Our results provide information on the gut microbiota of pike and highlight associations between environmental factors (salinity) and gut microbiota. An understanding of these associations provides information that may be useful for addressing problems during the domestication of these valuable freshwater fish. In our study, the size and weight of pike fry were influenced by long-term salinity tolerance. All examined fish reached a developmental stage where the organs involved in osmoregulation, e.g., the gills, were expected to be fully functional. Although the average length of fish showed little difference between each tank, the mean weight differed to a greater extent (Figure 2A). Generally, along with increasing the salinity, the environment should be cleaner and healthier, which could prevent the excessive development of pathogens, leading to better welfare of animals. However, in our study, the highest growth of fish from the 7% group was related to the lowest density of fish caused by the highest mortality. In these conditions, high-salinity fish had better access to food, and thus showed the highest weight values. was the greatest. It seems that larger pike fry cannot acclimate to these long-term osmotic conditions. This is in good agreement with other studies on pike, where the mortality of larger fish was greater than that of smaller fish [18]. The cumulative mortality of fish from tanks with a 3%o salinity was surprisingly similar to that of the control group. What is more, the BCI was greater than in the control group. These findings suggest that this level of salinity is well-tolerated by pike, which is consistent with reports of this species in coastal areas all around the Baltic Sea, where salinities vary from 4%o to 7%o [18]. The fact that around 97% of fish survived this salinity treatment and had a condition factor similar to that of the controls indicates that the fish in 3%o salinity did not experience a loss of water due to salinity stress. A loss of water is believed to be one of the reasons why fish have died in salinity treatments [18]. Our study indicated that, in the pike gut microbiome, the phyla _Proteobacteria_ and _Actinobacteria_ predominate, followed by lower percentages of _Bacteroidetes_ and _Firmicutes_ (Figure 3), in both control fish and fish in different salinity levels. This observation suggests that these phyla could play important roles in pike gut functioning. Although it is difficult to estimate the contribution of specific bacteria to the function of the whole gut ecosystem, it is reasonable to expect that the overall gut microbiome will be strongly influenced by the predominant microorganisms [28].