Distribution patterns of Caspian seal island haul-outs under Caspian Sea regression

The Caspian seal (Pusa caspica Gmelin, 1788) is the sole marine mammal and an indicator of ecosystem health in the Caspian Sea. This allopatric endemic species is listed as Endangered. Aerial surveys conducted in 2020, 2022, and 2023 showed that a shallow, inaccessible area in the northeastern Caspian Sea served as the primary seal concentration site. During spring, up to 98% of all molting Caspian seals hauled out on these island haul-out sites. To determine the distribution patterns of seal haul-outs, we conducted a comprehensive analysis of geospatial data from Sentinel-2 and Landsat-8 satellite imagery across a designated large study area (130×250 km) divided latitudinally into 16 transects. A 2.28 m drop in the North Caspian Sea level from 1995 to 2023 shifted the coastline westward by an average of 44.1 to 163.7 km. The maximum shift reached 175.4 km due to the drying of the Kaydak and Komsomolets bays. Island area decreased more than 15-fold during this period. We found that island evolution under sea-level regression follows a triad: emergence, growth, and merger with the mainland. An ecosystem shift is occurring: the shallow-water ecosystem is transforming into a mainland ecosystem. The rate of coastline shift has recently exceeded 23 km per year. Combined with increasing shelf depths, this accelerates the triad cycle and drives the disappearance of islands in the northeastern part of the sea. Because islands serve as primary seal haul-out sites, their disappearance forces seals to relocate to newly emerging shallow areas characterized by active shipping, fishing, and planned oil and gas development. This inevitably exacerbates the conflict of interests between industrial shelf development and the conservation objectives for this endemic species. Conserving Caspian seal habitats requires establishing ecological corridors along migration routes between existing marine protected areas, alongside rapid and adaptable protection of identified marine mammal haul-out sites. Because the westward coastline shift has caused mass haul-out sites to extend beyond the territorial waters of the Republic of Kazakhstan, coordination among Caspian littoral states is necessary to grant protective status to these waters, including their designation as an Important Marine Mammal Area (IMMA), and to establish a transboundary nature reserve.