Determination of body length regression formulas for the golden grey mullet (Chelon auratus, Risso 1810) based on otoliths found in the feces of the Caspian Seal (Pusa caspica Gmelin, 1788)

The Caspian Seal (Pusa caspica), the sole marine mammal endemic to the Caspian Sea, is listed in the IUCN Red List as a species facing the threat of extinction, a status similarly recognized and/or assigned in all countries in the Caspian region. The study of seal nutrition using caprological methods is of great interest for understanding animal adaptation to deteriorating habitat conditions and assessing the adequacy of their food base in the sea. This method involves analyzing the contents of feces from ichthyophages to detect undigested fish otoliths. The results of studying the seal's diet during periods of haulout on breeding grounds showed that the frequency of occurrence of Chelon auratus otoliths ranged from 2 % to 3–27.7 % of all fish otoliths detected in feces. This study aims to assess changes in Chelon auratus otoliths as they pass through seals' gastrointestinal tracts and estimate formulas for recovering the linear dimensions of fish consumed by these seals. The research indicates that otolith growth slows with fish growth but does not cease entirely. Allometry is evident in the different ratios of otolith length and width to fish length in various size groups and the change in otolith shape as fish grow. To recover the body length of fish consumed by seals based on otoliths found in their feces, applying inverse calculation formulas obtained when studying the relationship between otolith growth and fish body growth using collection material obtained directly from fish is possible. Further research should focus on refining the recovery formula for the length of large fish by dividing the length into several segments, each described by separate formulas describing the relationship between fish body length and otolith length and width growth.