Death of the Caspian seals on the Dagestan coast of the Caspian Sea in the autumn of 2020 and its possible reasons

   The aim of this research was to assess the scale and determine the possible reasons of the death of Caspian seals (Pusa caspica), that were washed up by a storm on the coast of the Republic of Dagestan in early December 2020. The corpses of seals were found on the Dagestan coast from the mouth of the Terek River to the southern outskirts of Derbent, in total about 205 km long.
   The main methods of collecting material were: accounting of dead seals in open (not overgrown with vegetation) coastal areas, measurements and pathoanatomic autopsies of corpses, sampling of internal organs and tissues of animals for histological, virological, helminthological and toxicological studies. Simultaneously with the onshore work, observations of seals and registration of hydro- meteorological parameters in the coastal waters of the Republic of Dagestan were carried out by two research vessels of the Caspian branch of the Federal Research Institute for Fishery and Oceanography («KaspNIRH»): «Caspian Explorer» and «Hydrobiologist».

   As a result, 12 coastal sites with a total length of 49.2 km were surveyed, where 313 corpses of dead seals were registered; the average density was 12.27 ± 2.16 specimens/km. The total number of dead seals in December 2020 was estimated at 2,515 ± 443 individuals. Adult females predominated among the dead animals, most of which were pregnant. The corpses showed no signs of infectious diseases or exhaustion. Judging by their condition, the death of animals occurred during the first two decades of November, at some distance from the shore.
   Conclusion: Basing on the data collected, infectious diseases, helminthiasis, intoxication, accident by catch in the fishing gears, and the impact of an underwater shock wave were excluded from the possible reasons of the death of seals. Acute asphyxia as a result of local release of natural gas, which could form above the sea surface a polluted lens of air unsuitable for breathing, was recognized as the most likely cause of the seals death.

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