Conservation status of the mediterranean shag gulosus aristotelis desmarestii in the adriatic sea during the non-breeding period: baseline population, trends, threats and knowledge gaps

The Mediterranean Shag Gulosus aristotelis desmarestii is a conservation priority seabird endemic to the Mediterranean and Black Seas that is considered a valuable sentinel of marine ecosystem health. however, knowledge of its population size and dynamics in the Adriatic Sea is limited. here, the majority of the population breeds in Croatia, with a high proportion of birds migrating northwards to the Gulf of Trieste and Venice Lagoon during the non-breeding period. To bridge this knowledge gap, a first coordinated census was undertaken in 2020 and 2021 in the Adriatic region (Italy, Slovenia and Croatia), which supports 20-33% of the total non-breeding Mediterranean subspecies population. In 2020 a total of 4,993 birds were counted while in 2021 the count was 4,871. Italy hosted the largest number of birds (- x = 3,040), followed by Croatia (- x = 951) and Slovenia (- x = 941), with most of the birds ( c. 69%) roosting on mussel farms. To fully comprehend the non-breeding population dynamics, we conducted an estimation of the 13-year (2010-2022) population trend in the Gulf of Trieste. During this period, the population strongly declined (-44.7%) for reasons yet to be identified. In 2012-2014, a population fluctuation coincided with an exceptionally wet winter and spring at the Croatian breeding colonies. It is possible that changes in migration strategy, such as new migratory routes, by this partial migrant may have partially offset the decline of birds in the Gulf of Trieste, as the Venice Lagoon population increased during the same period. Overall, the Adriatic population appears to be declining and yet subject to dynamic changes in its non-breeding distribution. To fully comprehend the reasons behind this trend, further research of both breeding and non-breeding populations is necessary.