Population Dynamics of Commercial Fishes of the Caspian Sea after the Introduction of the Ctenophore Mnemiopsis leidyi A. Agassiz 1865

On the basis of long-term monitoring data, we analyzed the population dynamics of commercial marine, anadromous, and semi-anadromous fish species in the Caspian Sea after the introduction of the ctenophore Mnemiopsis leidyi A. Agassiz, 1865. It was shown that the negative impact of M. leidyi had a particularly strong effect on two endemic species—big-eyed and anchovy kilkas, which was caused by food competition between these species and the ctenophore in the Middle and Southern Caspian Sea. After the introduction of M. leidyi in 1999, the average annual spawning stocks of anchovy kilka decreased by a factor of 4, and that of big-eyed kilka, by a factor of 46. It was found that a significant decrease in the average annual spawning stocks of benthophagous fish (Caspian roach and bream) during 2000–2010 coincided with a rapid increase in the abundance of M. leidyi in the Caspian Sea. Between 2011 and 2020, the average annual stocks of these species stabilized, with a slight upward trend. In contrast to zooplanktophagous and benthophagous fish, sturgeons (Russian sturgeon, starred sturgeon, beluga) have shown a continuous decline in their spawning stocks, primarily because of the deterioration in diet quality, loss of spawning grounds, reduction in natural reproduction, low efficiency of artificial breeding, and illegal fishing. The average annual loss of spawning stock between 2000 and 2020 was 2% for beluga, 7% for Russian sturgeon, and 10% for starred sturgeon. Among sturgeon species, only the beluga has shown stabilization of the spawning stock, albeit at a low level. Monitoring data for 1980–2020 confirmed that, owing to the biological characteristics of the common kilka, M. leidyi has not had a significant impact on its population: after the introduction of the ctenophore, the annual stock increase was 2.23%, compared to 0.8% before its introduction. The analysis also showed no influence of M. leidyi on the dynamics of commercial carp stocks, as the mean annual stock of these species did not differ across the various time periods, including the period of rapid growth in M. leidyi abundance in the Caspian Sea.