Mass mortality events of seabirds can have massive effects on populations

Most seabirds have a long life span, delayed maturity and spread their reproduction across many years. This life history pattern makes seabird populations particularly sensitive to decreases in adult survival rates. Mass mortality events can therefore have great consequences for such species.

Adult puffin in winter plumage-TANAdult puffin in winter plumage. Photo: Tycho Anker-Nilssen

The Atlantic puffin Fratercula arctica is a long-lived seabird that lays only one egg per year and seldom starts to breed before it reaches an age of 5 years. Even though puffins are one of the most numerous seabirds in the northeast Atlantic, it is now rated as vulnerable on the IUCN Red List of Threatened Species due to significant decline in populations in some of the historically important colonies. This is largely explained by long-term food shortages during chick rearing, and significant differences in adult survival among colonies have not been previously been found. This does not mean, however, that events with unusually high mortality cannot have major effects on populations.

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This study addressed an event where several hundred dead puffins were found beached in Vest-Agder and Rogaland Counties in February and March 2016. The first birds were found the day after the storm Tor, which brought with it the strongest winds ever recorded in Norway. A total of 200 individuals were collected and examined, of which 59% were adult birds. All birds were emaciated and had most likely starved to death. Wing-lengths of birds not in active moult indicated that most of the birds originated from colonies on the east coast of the UK. This was also corroborated by the find of eight ringed birds that originated from the same area. Seven of these eight were adults with a mean age of at least 22 years. The study showed that the adult survival rate of puffins breeding on Isle of May in southeast Scotland that winter was much lower than normal. This demonstrates that mass mortality events can have a large impact at the population level, despite only small numbers of affected individuals being found.

 

Contact person: Tycho Anker-Nilssen, NINA