Caspian Sea: State of the Environment 2019

5. State

5.1. Changes in bioresources There have been significant changes in the state of the biological resources in the Caspian Sea over the past 10–15 years, during which zooplankton numbers and biomass have fallen five- or six- fold in the Middle Caspian and approximately tenfold in the Southern Caspian. The biomass and number of Mnemiopsis leidyi comb jellies on the western shelf of the Central and South- ern Caspian increase significantly from summer to autumn. Thus, the Mnemiopsis leidyi, which was brought to the Caspian Sea with ballast water in the late 1990s, spread rapidly, causing direct and extensive damage to the biodiversity of the sea. It consumed large amounts of zooplankton, which is a food source for sprat, and this led to a decrease in the food source available to Acipens- eriformes, predatory Clupeidae, and others along the food chain (Azerbaijan Scientific-Research Fisheries Institute). Most of the benthic animals on the western coast of the Caspian Sea are molluscs, crus-

taceans and worms. Plankton larvae, such as Mitilyaster, Abra, Balanus, and crab have fallen victim to the Mnemiopsis leidyi in the South- ern Caspian to the extent that, in recent years, Abra and crab numbers have declined signifi- cantly and the species are rarely encountered in the benthos, if at all. No spots created by a high benthic biomass as a result of the mass de- velopment of Abra, Cerastoderma or Nereis are found on the western coast of the Southern Cas- pian, compared with data from previous years (National Contribution). The superior numbers of Abra and crab, which created an abundant benthos in the Southern Caspian in previous years, compared with Ne- reis and Balanus, have not been recorded in recent years. However, the high numbers of worms and molluscs in the benthic fauna have created a decent food source for fish at all tro- phic levels in the western region of the Southern Caspian (Azerbaijan Scientific-Research Fisher- ies Institute). This is illustrated in Tables 5.2 to 5.6, below.

Table 5.1: Biomass of Mnemiopsis (g/m 3 ) in the Azerbaijan sector of the Middle and Southern Caspian

Sea area

2011

2012

2013

2014

2015

2016

Middle Caspian

1.8

3.3

2.5

5.8

8.2

7.6

Southern Caspian

6.83

5.98

4.13

9.34

11.9

6.7

Source: Azerbaijan Scientific-Research Fisheries Institute.

Table 5.2: Biomass of the Southern Caspian zoobenthos (g/m 2 )

Organism

2010

2011

2012

2013

2014

2015

2016

Vermes

3.25

5.16

2.11

1.47

3.98

5.34

1.02

Crustacea

2.07

4.85

12.01

7.69

9.26

8.05

8.75

Mollusca

1.85

1.37

5.1

8.23

14.02

11.26

6.29

Total

7.174

11.38

20.22

17.39

27.26

24.65

16.06

Source: Azerbaijan Scientific-Research Fisheries Institute.

59