Recent increase in annual survival of nesting female Common Scoter Melanitta nigra in Iceland

Despite their unfavourable conservation status, little is known about the population status, trends and demography of Common Scoter Melanitta nigra. Capture-mark-recapture (CMR) data from 154 recapture events of 88 individually marked breeding female Common Scoter Melanitta nigra in the Aðaldalur valley, northeast Iceland, during 2009–2018 generated an estimated annual apparent survival probability of 0.848 (95% CL 0.775–0.901). This exceeds a previous Icelandic CMR estimate (0.783, 95% CL 0.715–0.839) from nesting females during 1925–1958 at the nearby Mývatn (45 km to the SSE) when the population was declining at c. 2% per annum. Spring count data from Mývatn show that numbers of male and female Common Scoter at this important breeding site (thought to constitute over 80% of the Icelandic breeding national total) have increased by c. 1.8% per annum since 1974, and by 4–5% per annum since 2009. A population model showed that the observed change in survival corresponded well with the observed change in population growth rate. We reflect on causes of the increase in mean expected lifespan (from 4.1 to 6.1 years) between 1925–1958 and 2009–2018, speculating whether changes in food supply and/or reduction in gill-net fishing on Mývatn in the last 25 years could have been contributing factors to explain recent increases in breeding females there. An alternative, not necessarily mutually exclusive explanation could also be that the dramatic declines in chronic marine oil spill pollution in European waters could have contributed to higher recent female annual survival. While fully recognising the challenges of comparing survival rates from two sites in two different periods, survival rate has increased significantly between the two studies in a way that could potentially explain the increase in abundance at Mývatn. This could indicate that female survival has increased in the population as a whole. We strongly recommend demographic monitoring as a contribution to monitoring sea duck populations, species that are difficult to count and otherwise monitor annual changes in abundance in any meaningful way across large geographical ranges.