Discovery of Two New Colonies of Olrog’s Gull (Larus atlanticus) in the Bahía Blanca Estuary, Buenos Aires Province, Argentina

Abstract

Olrog’s Gull (Larus atlanticus) is a larid species endemic to the South Atlantic coast and is considered Vulnerable in Argentina. Its breeding range is restricted to estuarine islands characterized by low elevation above sea level, sparse vegetation, and proximity to crab beds. Nearly the entire breeding population nests in protected areas with varying categories of conservation in Buenos Aires Province. During aerial censuses, conducted on 17 October 2025 to update population estimates, we recorded two new breeding sites of Olrog’s Gull in the Bahía Blanca estuary: one at the northeastern tip of Trinidad Island (39°05’S, 61°55’W) and the other at the northeastern tip of Ariadna Island (39°14’S, 61°58’W). Both sites are located within the Bahía Blanca, Bahía Falsa, Bahía Verde Nature Reserve. Apparently Occupied Nests, which provide an estimate of the number of breeding pairs, were counted yielding a total of 26 ± 2 (n = 7 photographs) and 22 ± 1 (n = 4 photographs), respectively. The colonies are located close to the tidal line and may therefore be vulnerable to substantial habitat alterations caused by severe storms or human activities such as dredging and recreational use. These sectors are currently outside the reserve’s strictly protected zone. As a result of this finding, they have been incorporated into the ongoing revision of the reserve’s zoning scheme to strengthen effective protection measures during this critical period for the species.