## 4 Assessment of the Results
### Methodology for Quantitative Assessment of the Ecological Safety Level  
In this regard, we propose a comprehensive (according to: the topography of the Caspian Sea bed, the pollution level, the composition of the pollution, the kind of anthropogenic impact) zonally distributed assessment of the environmental sustainability based on bioindication methods:  
1. estimation of the average amount of lacustrine biota per volume unit (or water surface area for the objects, for example, shallow water);2. biodiversity of lacustrine fauna;
3. absorption of representative pollutants by the lacustrine ichthyofauna.  
Since all indicators are measured in different units, their relative assessment will be correct, which will allow a convenient way, based on probabilistic methods of analysis, to consolidate these indicators.  
We propose to estimate the average amount of lacustrine biota (Kqb) in relation to the fixed population maximum (base level) by weight per unit volume, i.e.,:  
\[\mathrm{K_{qb}}=\frac{\mathrm{qb_{t}}}{\mathrm{qb_{0}}}, \tag{1}\]  
where qb\({}_{t}\) is the number of biota per unit area obtained on the basis of current measurements, qb\({}_{0}\) is the number of biota per unit area obtained on the basis of basic measurements.  
The biodiversity assessment (Kbio) in this case is a zonal feature, assessed for a certain natural water area of the Caspian Sea (shallow water), which is also determined in relation to the base parameter based on the relative structure of the represented species of marine fauna:  
\[\mathrm{K_{bio}}=1-\frac{1}{\mathrm{n}}\times\sum_{i}\left|\frac{\mathrm{D_{ 0}^{i}-D_{t}^{i}}}{\mathrm{D_{0}^{i}+D_{t}^{i}}}\right|, \tag{2}\]  
where \(\mathrm{D_{0}^{i}}\) is the average share of the i-th species of lacustrine fauna characteristic of the studied area of the Caspian Sea in the base period, \(\mathrm{D_{t}^{i}}\) is the average share of the i-th species of lacustrine fauna typical for the studied area of the Caspian Sea in the current (studied) period, n--number of species of lacustrine fauna participating in the assessment.  
The assessment of water pollution (Kp) is carried out by an indication method based on the species of lacustrine animals most sensitive to certain pollutants. In this regard, the most suitable indicators of water pollution are commercial fish species or other organisms consumed by humans for food, for which the permissible values of sanitary standards, certain substances are determined. Secondly, the migration of lacustrine life over a large territory already averages the impact of local sources of the Caspian Sea pollution (ports, pipeline sections, river mouths, offshore oil platforms), which significantly increases the reliability of the data.  
\[\mathrm{K_{p}}=\frac{1}{\mathrm{n}}\times\sum_{i}\frac{\mathrm{SN_{j}^{i}}}{ \mathrm{SN_{j}^{i}+N_{j}^{i}}} \tag{3}\]  
where, \(\mathrm{SN_{j}^{i}}\) are the sanitary standards for the number of permissible values of the i-th harmful substances for the j-species of lacustrine inhabitants most vulnerable to this pollutant, \(\mathrm{N_{j}^{i}}\) is the amount of harmful substances, n is the number of studied species of ichthyofauna.  
The level of ecological sustainability (ES) of the investigated part of the Caspian Sea is determined on the basis of an aggregated assessment of the three indicators discussed above:  
\[\mathrm{ES}=\mathrm{K_{qb}}\times\mathrm{K_{bio}}\times\mathrm{K_{p}}\times 1 00\% \tag{4}\]  
In this regard, we used probabilistic methods for analyses in the most favorable situation. The level of environmental sustainability is close to 100%. The deterioration of the environmental situation accordingly reduces the level of environmental sustainability of the system (Table 6).  
Thus, a quantitative assessment of the ecological safety level of a part of the Caspian Sea on the basis of bioindication methods requires inexpensive additional systemic studies. Most importantly is the possibility of monitoring and constant ecological observation over an extremely vulnerable lacustrine ecosystem.