## 1 Introduction  
In recent years, the intensification of human activity in marine areas has been related to, among others, broadly understood economic development and maritime research, such as maritime transport [1], gas and oil platforms, wind farms [2], underwater cables, port expansion [3], and fishing [4]. Underwater acoustic monitoring of the marine environment with the extensive use of underwater sensors and acoustic devices to protect marine fauna and flora has become a fundamental necessity resulting from the requirements of Directive 2008/56/EC of the European Parliament and of the Council, known as the Marine Strategy Framework Directive [5, 6]. The causes and effects relations in marine ecology area including the place and role of calibration of underwater acoustics sensors and measuring devices are presented in Figure 1. The content in the diagram reflects the main issues and does not exhaust all detailed relationships. Anthropogenic underwater noise and natural underwater noise, which are the main sources of underwater noise, affect not only living organisms in the sea but also each other [7]. The registered threats for marine ecology system impaired echolocation caused by high levels of underwater noise from, for example, an underwater volcanic eruption or high-energy sonar used by active warships, or more widely with increased military operations. As a detailed example, underwater mine explosions, in addition to noise, may cause additional waves, and increased waves and wind strength may increase the demand for propulsion power on ships and thus increase the generation of noise from ships' power plants into the water environment. The main effects provoked by underwater noise and causing degradation of the marine ecosystem are presented in the second column. Currently, this impact on all living species is not sufficiently studied and requires further research [8, 9]. The third column presents the main areas for improving the situation in marine ecology. Not only does the European Union have a Marine Strategy in the area of marine ecology, but similar initiatives are implemented all over the world [7; 10], also as part of the activities of the United Nations under the United Nations Framework Convention on Climate Change (UNFCCC) [11]. The flow of present challenges in the research of marine ecology system and ways to solve the formulated problem is illustrated in Figure 2. The aim of this article is to systematize the knowledge on the influence of factors changing the sensitivity of measurement sensors on the need to provide calibrated sensors and devices in measurements in the area of underwater acoustics for the protection of fauna and flora in the marine environment and the participation of Polish scientific institutions, including GUM (Polish NMI), in the process of maintaining appropriate measurement standards ensuring the repeatability and reliability of underwater noise measurements. A problem to solve was appointed as a necessity to protect marine fauna and flora in the face of existing operating conditions and threats characterizing today marine ecology systems. A main thesis is that the ensuring (leading to improvement) of reliability and repeatability of research in the area of marine ecology through traceable calibration of hydrophones and measuring instrumentation, which enables the identification of the critical parameters in the analyzed marine system, and, in consequence, makes it possible to undertake the countermeasures to effective protection of the selected objects of fauna and flora in this system. Individual elements of actions aimed at improving the state of the natural environment, presented in Figure 2 as blocks, will be successively discussed later in this article. For Figures 1 and 2, the solid single direction arrows mean the relationship of the block near the arrow relative to the block at the opposite end; the solid double direction arrows mean that both blocks are dependent on each other.