130 years of catch data show that global warming is contributing to population decline in the world's largest puffin colony.
The optimum temperature for puffins in Iceland has been shown to be 7.1°C. Both lower and higher sea temperatures resulted in a decline in offspring production. Photo: Tycho Anker-Nilssen.
Global warming has already resulted in measurable climate change and will lead to even greater changes during the forthcoming decades.
"In order to understand how different species will fare under these conditions, we usually have to investigate how they have previously responded to climate variations. However, it is not easy to measure responses to climate variations!” These are the words of Hanno Sandvik, a researcher at NINA – the Norwegian Institute for Nature Research.
The classic definition of climate is "the average weather over a 30-year period"; and in order to establish how a species responds to changes in the climate, strictly speaking it would be necessary to have included data from several 30-year periods. Such datasets only exist for a very few species.
Long series
However, a group of researchers from Iceland, Norway, Germany and England have managed to present a 128-year dataset on puffin offspring production on the Westman Islands (Vestmannaeyjar) off Iceland.
This is based on the fact that the archipelago’s inhabitants have been hunting puffins since the Middle Ages, for both their meat and their feathers. They hunted the birds from hiding places on the steep cliffs, using long-handled nets and simply hauling them in as they flew by.
“Harvest statistics have been kept since 1880, and these show that harvest could amount to as many as 150,000 birds during the course of a season. The puffins they caught were mainly pufflings which had not started nesting. We have therefore used a statistical model for converting these catch statistics into cohort sizes for pufflings fledged 2-6 years earlier. Puffin hunting ceased in 2010, so we ended up with harvest data for the period extending from 1878 to 2005,” says Erpur S. Hansen, researcher at the Southwest Iceland Nature Research Centre (Náttúrustofa Suðvesturlands).
“So our data also shows the effects of the so-called early 20th century Arctic warming period, which began around 1920. There are hardly any datasets for seabirds that go so far back in time," says Kjell Einar Erikstad at NINA.
New insights
"This data has given us some insights that would have been difficult or impossible to demonstrate with less extensive datasets. The usual method used for quantifying climate effects involves estimating the increase or decrease in offspring production when the temperature increases by 1°C (correlation). However, in this case it turned out that the sign of the correlation changed three times during the 20th century. During the 50-year period after 1920, and again from about 1990, the correlation was negative: offspring production dropped as the sea temperature increased. During the remainder of the period, the correlation was the reverse,” says Hanno Sandvik.
The explanation for this lies in the fact that offspring production is highest when the sea surface temperature is 7.1°C. If the temperature increases or decreases by 1°C, offspring production drops by 55%. The sign of the correlation is therefore determined by whether or not the sea temperature fluctuates above or below the optimum temperature. This pattern is not actually that hard to explain, but it would have been difficult to spot with less data.
These temperature correlations are probably based on the fact that puffins in the Westman Islands feed mainly on the lesser sand eel. The optimum temperature of this species of fish is around 7°C, i.e. its reproduction and survival is lower at other temperatures.
Figure 1: During periods extending between 20 and 50 years, offspring production has varied either synchronously (+) or anti-synchronously (−) with the sea temperature. This sign changed three times during the 20th century.
"Since we can expect further increases in sea temperatures, it can be assumed that puffin populations will continue to decline. This decline in breeding success is also the reason why hunting was banned in 2010. Even though the Westman Islands had 830,000 breeding pairs at that time, and thus were the world's largest puffin colony, they are facing an uncertain future," explains Erpur Hansen.
"There are some slight hopes that puffins will be able to adapt to the changing situation to some extent. The temperature at which puffins produce the most fledglings has actually increased by a quarter of a degree over the last century. This may be due to the fact that puffins are increasingly finding alternative food to lesser sand eels, or maybe the sand eels are undergoing evolutionary adaptation to this increase in temperature. However, according to Hansen and Sandvik, the problem is that the average sea temperature around the Westman Islands has increased more than three times as fast (by 0.8°C).
Figure 2: The optimum temperature for puffins in Iceland has been shown to be 7.1°C. Both lower and higher sea temperatures resulted in a decline in offspring production.
Funding: This research is being funded by the Fram Centre, under its research programme entitled "Effects of climate change on fjord and coastal ecosystems in the north", and the Research Council of Norway.
Reference: Hansen et.al: Centennial relations between ocean temperature and Atlantic puffin production reveal shifting decennial trends.
https://onlinelibrary.wiley.com/doi/10.1111/gcb.15665