Distribution, abundance, and size structure of the Pacific cupped oyster Magallana gigas in northern Patagonia

In the 1980s, the Pacific oyster Magallana gigas (Thunberg, 1783) was deliberately introduced in the southern region of the Province of Buenos Aires (Bahía Anegada, BA), Argentina. In 2004, its presence expanded 80 km south of the Río Negro estuary along the coast of El Cóndor (EC). Although oysters have demonstrated dispersal capability, there is limited data as regards the EC population since 2011. This research focusses on the present M. gigas population encompassing distribution, abundance, and size structure along a 180-km coastal line from EC to San Antonio Este (SAE). Subsequently, we compared these data with those for the BA population. The presence of M. gigas in the Province of Río Negro was detected in four sites: three of them near the Río Negro estuary (EC, Piedras Verdes PV, and El Pescadero); and the last one in San Antonio Bay. Estimated average abundances near the estuary were lower (range 1.8 10-3 ± 0.6 10-3 and 9 10-2 ± 3.4 10-2 ind. m-2) than BA (105 ± 2 ind. m-2). Presence in SAE was only limited to one site and three adults M. gigas. The BA oyster population exhibited a multimodal distribution, with a significant number of recruits, whereas the PV site displayed a trimodal structure dominated by large specimens. In EC, owing to the limited number of individuals, modal components were less discernible, but small oysters predominated. The current abundance of M. gigas in EC was considerably lower than that in 2011, indicating a population decline. Despite this, the presence of juvenile oysters suggests recent recruitment, emphasizing species resilience. These results show that M. gigas faces challenges when attempting to establish itself in this specific region. Studying the underlying causes would help to understand the factors that limit the expansion of a species considered to be a global invader.