Assessment of the size distribution of the big-scale sand smelt (Atherina boyeri Risso, 1810) in the diet of the Caspian seal (Pusa caspica Gmellin, 1788) using fish otoliths from fecal samples.

Big-scale sand smelt (Atherina boyeri) is an euryhaline pelagic schooling fish found throughout the Caspian Sea. A specialized fishery does not target this species, and its body length can reach up to 13 cm. A. boyeri serves as prey for many omnivorous and predatory fish and is one of the primary food sources for the Caspian seal (Pusa caspica). The objective of this study was to determine the size composition of A. boyeri in the diet of P. caspica, a declining species of the Caspian Sea. A coprological method was used, involving the collection of seal feces during spring and autumn haul-outs in the Northern and Middle Caspian, followed by examination of fish otoliths. Before this, otoliths from fish were analyzed to assess morphological diversity and growth patterns in relation to Atherina boyeri. Equations were developed to estimate fish length from otolith length and width. Otoliths from feces were classified into three wear categories: lightly, moderately, and heavily abraded. A comparison of otolith sizes from fish and feces allowed the calculation of wear coefficients based on otolith length for each group, which were incorporated into the reconstruction formula. The reconstructed fish length distribution showed that at the Northern Caspian haul-outs, individuals measuring 60-70 mm predominate, while at the Middle Caspian haul-out, individuals measuring 80-90 mm are most common. Overall, this is the first study to confirm that P. caspica feeds on both juvenile and mature A. boyeri, with lengths ranging from 23.1 to 117.1 mm.