Rock shag inshore island colonies at risk in an invaded scenario

BALZA, ULISES; HARRIS, SABRINA; SÁENZ SAMANIEGO, RICARDO; TORRES, MÓNICA; ROSCIANO, NATALIA; LILJESTHRÖM, MARCELA; RAYA REY, ANDREA

Virtual

Congreso; III World Seabird Conference; 2021

World Seabird Union

Viability assessments require long time series which are usually uncommon and often expensive. Rock shags (Phalacrocorax magellanicus, RS) inhabit the southern coasts of Argentina and Chile, and 8 % of the Argentinian population occurs along the Beagle Channel (BC). They nest in cliffs in several islands, and tourism activity does not seem to affect the number of breeding pairs. The Fuegian archipelago is also inhabited by several invasive species, one of which, the American Mink Neovison vison is a well-documented seabird nest predator. In this study We attempt to estimate the effect of invasibility on the viability of the RS population in the BC to recommend local management measures. We used counts of breeding pairs of RS in the BC, from 1994 to 2019 (yearly since 2007), to estimate the intrinsic growth rate (λ) for the overall population. We explore associations between λ and distance to the closest island of the archipelago where American Mink is present. We clustered sites in 'inshore' islands (2.5 km, n= 17), and calculated the absolute values of viability for the population for the next 50 years. High inter-annual variability was recorded, especially on offshore islands. Overall population decreased 2 % annually, from 691 breeding pairs in 1994 to 455 in 2019, and mean probability of extinction in 50 years was 17 %. This negative trend could be attributed mainly to the loss of the inshore population, while variation in growth rates were mainly due to variation in offshore colonies. While in 40 % of the inshore islands a local extinction was recorded, on the offshore islands the records of local extinctions and colonization were similar. In cormorants, site fidelity is usually high, thus a rapid response to disturbance is not expected. Although we know of a few colonies in the Chilean part of the channel that could be harboring emigrants from these islands, we were able to identify islands which are susceptible to be managed by Argentine government, with specific expected outcome in case of success.