## 4 Conclusions  
The current state of the calibration infrastructure of the underwater acoustic devices, with particular emphasis on marine ecology and fisheries sciences, outlining the countermeasures to effective protection of selected objects of fauna and flora in the considered marine system may be summarized as follows:  
1. The ensuring of reliability and repeatability, leading to improvement of research in the area of marine ecology through traceable calibration of hydrophones and measuring instrumentation enables the identification of the critical parameters in the analyzed marine system and, in consequence, makes it possible to undertake the countermeasures for effective protection of the selected objects of fauna and flora in this system. A complexity of the presented subject is illustrated in Figure 1, explaining the causes and effects relations in marine ecology with stressing the place and role of calibration underwater acoustics sensors and measuring devices, as well as in Figure 2, showing challenges in the research of marine ecology system and ways to solve the formulated problem.
2. The importance of reliability and repeatability of underwater noise research results from the changes in the sensitivity of hydrophones, that is, dynamic pressure sensors, which is mainly caused by adulteration (voltage sensitivity drift), immersion of the sensors during measurements, underwater mounting conditions of the sensors, as well as the waves/tides phenomena, and is presented and discussed in chapter 2.
3. Calibration recommendations concerning hydrophones, based on research studies presented in Figures 3 and 4, cover the analysis of the previously described influencing factors on the reliability and repeatability of underwater acoustic measurements. These recommendations are strictly connected with the Marine Strategy Framework Directive (MFSD) and included in appropriate legal standards and related documents presented in Table 1.
4. The analysis of the low frequency calibration systems worldwide, illustrated in Table 2, shows that commonly accepted and used by numerous well recognized institutions, and confirmed by existing standards, frequency range of calibrations of underwater acoustics devices used in measurement and modelling phenomena in area of marine ecology is defined as a frequency below 1 kHz.
5. The Polish case-study based on the development of metrological infrastructure in the underwater acoustic measurements in GUM (Central Office of Measures, Poland) shows that ensuring the reliability and reliability of measurements in the field of underwater acoustics in civil and military applications with the participation of the Central Office of Measures in Warsaw (Polish NMI) is crucial and will be further developed.  
Conceptualization, K.J.L. and J.M.; Methodology, K.J.L. and J.M.; Software, K.J.L.; Validation, K.J.L. and J.M.; Formal analysis, J.M.; Investigation, K.J.L.; Resources, K.J.L. and J.M.; Data curation, K.J.L.; Writing--original draft, K.J.L.; Writing--review & editing, J.M.; Visualization, K.J.L.; Supervision, [PERSON] and [PERSON]; Project administration, [PERSON] and [PERSON]; Funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript.  
Aforementioned research is supported by the grant entitled \"A concept for constructing metrological infrastructure in underwater acoustics area at the Central Office of Measures\" financed by Ministry of Education and Science within the program: Polish Metrology, grant N\({}^{\circ}\) PM/SP/0057/2021/1, 2021-2022.  
Not applicable.  
Data Availability Statement: Data available on request due to restrictions resulting from provided project. Conflicts of Interest: The authors declare no conflict of interest.