Abstract
<jats:p>Abstract The Bosphorus (İstanbul Strait) is one of the world's most important marine ecological corridors, connecting the Black Sea with the Sea of Marmara and providing essential feeding, roosting, and resting habitats for numerous waterbird species. This study evaluated the seasonal abundance of the Great Cormorant (Phalacrocorax carbo) and the European Shag (Gulosus aristotelis) along the European shoreline of the Bosphorus using standardized coastal transect surveys. Field observations were conducted following identical survey protocols, and Spring surveys conducted in 2012 and 2025 using identical sampling protocols were compared. Bird counts were conducted at representative coastal localities using combined stop-walk and stop-and-go transect methods. Both species exhibited pronounced spatial variation among localities rather than a consistent temporal trend. Population increases were recorded at several localities, whereas declines occurred at others, indicating spatial heterogeneity rather than a uniform temporal trend. These findings suggest that local habitat characteristics and seasonal food availability strongly influenced the observed distribution patterns. These findings demonstrate that standardized long-term coastal monitoring provides reliable information on temporal changes in cormorant populations and further emphasizes the ecological significance of the Bosphorus as a major feeding, resting, and movement corridor linking the Black Sea and the Mediterranean basin. Continued long-term monitoring will improve our understanding of the population dynamics of piscivorous waterbirds and provide an important scientific basis for the conservation and management of this internationally important coastal ecosystem. These findings demonstrate the value of long-term standardized monitoring for understanding population responses of coastal piscivorous birds in ecologically important marine corridors.</jats:p>