## 5 Conclusions  
In conclusion, it seems that _A. vistonicus_ can still cope with the constantly changing environment of Vistonis Lake. The latter is concluded by the fact that _A. vistonicus_ CSR is versatile and varies according to the examined tissue, biomarker and season, in order for this species to adapt to its shifting habitat. Therefore, HSPs' induction, Na+-K+ ATPase activity and MAPKs activation seem to consist of a successful adaptive mechanism (Figure 8). However, additional studies on the species' physiological responses, including additional biomarkers, would contribute both to safer conclusions and to further understanding of the species' physiology. At the same time, the estimation of the range, as well as the lower and upper limits, of each environmental parameter, where critically endangered _A. vistonicus_ exhibits its optimal physiological capacity (e.g., Oxygen- and capacity-limited thermal tolerance hypothesis according to [PERSON] et al. [82]) is of great importance. However, further research of the potential correlation between the changing physico-chemical parameters and the synergistic response of different proteins, along with expansion of histological studies, will contribute to understanding the physiology of the endangered species _A. vistonicus_.  
**Author Contributions:** [PERSON] and [PERSON] conceived and designed the study. [PERSON] and [PERSON] conducted fieldwork and collected the samples. [PERSON], [PERSON] and [PERSON] performed the biochemical analyses, and [PERSON] the histological analysis. [PERSON], [PERSON], [PERSON] and [PERSON] performed statistical analysis. [PERSON], [PERSON] and [PERSON] visualized results. [PERSON], [PERSON] and [PERSON] wrote the first draft of the manuscript. [PERSON] and [PERSON] corrected and revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The study was funded by the Management Body of Delta Nestos and Lakes Vistonis-Ismarida, as part of the research project \"Study and management proposals for the two endemic and under extinction fish species of lakes Vistonida and Mitrikou (_Alosa vistonica & Alburmus vistonicus_)\".  
**Institutional Review Board Statement:** All necessary permissions were provided by the Management Body of the Delta Nestos and Lakes Vistonida-Ismarida in the frame of the implementation of the project titled \"Study and management proposals for the two endemic and under extinction fish species of lakes Vistonida and Mitrikou (_Alosa vistonica & Alburmus vistonicus_)\" (code 90981) financed by the operational program \"Environmental and Sustainable Development\" in Greece for the period 2007-2013. Fish handling and treatment were in accordance with the local guidelines for treating animals which comply with the Official Journal of the Greek Government No. 106/30 April 2013 on the protection of animals used for scientific purposes. Animals received proper care in compliance with the \"Guidelines for the Care and Use of Laboratory Animals\" published by US National Institutes of Health (NIH publication No 85-23, revised 1996) and the \"Principles of laboratory animal care\" published by the Greek Government (160/1991) based on EU regulations (86/609).  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to local authorities' privacy restrictions.  
**Acknowledgments:** The authors would like to thank the Management Body of the Delta Nestos and Lakes Vistonida-Ismarida for providing all the necessary permissions for fish sampling and for their technical and financial support. Moreover, the authors would like to specially thank [PERSON], the fisheran who with safety helped us throughout the fieldwork.  
Figure 8: Summarized model of cellular stress responses of the freshwater fish species _Alburmus vistonicus in Vistonis Lake (Northern Greece) under the seasonal variation of the physico-chemical parameters._