## 3 Results and Discussion
### The Problem of the Caspian Sea Level Decline  
One of the most difficult problems in the ecology of the Caspian Sea is the intensive decline of its water level in recent decades. Changing the Caspian Sea level is a natural process that has a long-term cyclical character [99]. The main reason for fluctuations in the water level of the Caspian Sea, as established by long-term observations [100; 101], is the river flow entering the lake mainly through Volga and Ural Rivers. The most reliable information about the Caspian Sea level has been available since 1990 [102]. From the beginning of instrumental observations and until the 20 th century, the level of the Caspian Sea fluctuated in average around minus 25 m. In the last century, the level of the Caspian Sea, almost until the end of the 1970s, mainly decreased (Figure 18). The total continuous decrease in the level observed in 1930... 1977 made 3.2 m. In 1977, the Caspian Sea level reached its lowest mark for the observation period--minus 29.01 m. The decrease in the level was due to the fact that from the mid 1930s at the rivers of the Caspian basin began intensive water management construction, the impact of which became most noticeable in the 1950s. By the early 1970s, almost all major rivers in the basin were regulated. As a result, the volume of river runoff has decreased; the area of the Caspian Sea water surface has decreased. According to [103], the value of the reduced area was about 50 thousand km\({}^{2}\). In the northeastern part of the North Caspian Sea the coastline lowered by 120... 140 km [104]. The lowering of the level caused large complications in the operation of the ports at the Caspian coast and sharply worsened the conditions of navigation, especially in the Northern Caspian Sea. The salinity of the water of the Northern Caspian Sea increased, which affected the condition of the food supply for small fish and _Acipenseridae_ fish. Since 1978, an intensive Caspian Sea level rise began, which lasted for 18 years (1978... 1995). During this time, the Caspian Sea level rose by 2.5 m, and by 1995 it reached minus 26.62 m.  
The rise of the Caspian Sea level has led to new problems related to flooding of coastal areas. According to studies [103], as a result of the water level rise the area of the flooded territories increased 35... 40 thousand km\({}^{2}\). In some areas, the coastline has advanced by 25... 50 km. About 100 thousand people and industrial facilities were relocated from the flooding zones. In 1995, the Caspian Sea level rise slowed down, and in 1996 a decrease was observed mainly due to low water input coming from the Volga basin. From 1997 to 2001, the mean annual Caspian Sea level dropped by 19 cm. In 2001, it reached minus 27.17 m. The average annual Caspian Sea level in 2005 was minus 26.91 m b.s.l. [102]. Since 2006, the level of the Caspian Sea has been declined. In 2010, it reached minus 27.25 m. By the end of 2014, it reached minus 27.82 m. The value of the current decrease in the level relative to 1995 is 1.08 m. The background Caspian Sea level for the period from 1900 to 2014 amounted to minus 27.28 m b.s.l., i.e., below the average long-term level by 42 cm. The decrease of the Caspian Sea level is most expressed in the northeastern, shallow part, which belongs to Kazakhstan. In fact, the coast of the Caspian Sea in the Kazakh sector is a flat plain with an extremely insignificant slope, from 1 m elevation difference for 10... 20 km. According to satellite images mapping, the drainage area in the northeastern part of the Caspian Sea amounted to 5055 km\({}^{2}\) between 2005 and 2015. The morphometric characteristics have changed: new islands and bays have been formed. In some areas, the Caspian Sea coast retreated by 25 km [102]. For the entire Caspian Sea during this period, the area of the water surface has decreased by more than 11 thousand km\({}^{2}\), and half of it is in the Kazakh part of the Northern Caspian Sea.