Short distance directional migration in the threatened Socotra cormorant: link to primary productivity and implications for conservation

Conservation of threatened seabirds commonly focuses on protection of breeding areas. However, conditions at non-breeding areas also affect population dynamics, calling for a better understanding of seabird migratory ecology. In particular, it is crucial to identify the type of migration and the oceanic conditions determining non-breeding habitat selection. We studied movements of the threatened Socotra cormorant Phalacrocorax nigrogularis breeding at Siniya Island, United Arab Emirates (UAE) (35% of the world population), using platform transmitter terminals (PTTs) deployed on adults during the 2013 and 2014 breeding seasons. Concomitantly, we used remotely-sensed chlorophyll a concentration data (CHL) of areas visited by birds in the Arabian Gulf and Gulf of Oman regions (2002 to 2016 monthly averages), as an index of primary productivity. The migratory pattern of the Socotra cormorant was non-dispersive, fitting with the gregarious habits and group foraging mode of this forage fish specialist. Birds performed a short westward directional migration to islands off western UAE, then moved eastwards to the Strait of Hormuz before returning to Siniya Island. Birds concentrated at a few localities, which therefore represent areas of high conservation priority. During breeding, CHL around the colony was high. During non-breeding, however, CHL around non-breeding areas was low, even though more productive waters were present within foraging range. The mismatch between the non-breeding phase and CHL could be linked to spatial and temporal lags in responses of secondary and tertiary consumers to primary productivity. Effective conservation will necessitate a better understanding of the ecology and distribution of forage fish within the Gulf.