We are working in the Slettnes Arctic skua colony since 2014. That year nests were abundant, skuas were aggressively attacking us, and many chicks survived and fledged. The seasons since then have been worse every year. Less nests, less known birds resighted and hardly any breeding success. The main driving force behind these changes seems to be food availability. Arctic skuas do not feed for themselves, but rely on host species such as Arctic tern and kittiwake. Skuas chase their victims until they throw up the fish they just swallowed. There are no consistent monitoring programs on Arctic skuas, but the kittiwake is in strong decline all along the Norwegian coast (see this NINA report). We counted no more than 80 breeding pairs of Arctic skua in the Slettnes colony last week, while there used to be 250 in 1997/1998. The key food for seabirds, but also for large fish and whales in the area are capelin and herring.
Capelin underwent several collapses and rises since the 1980s. The stock estimate is based on the joint Russian–Norwegian acoustic surveys in September each year (see this report). During winter and early spring Barents Sea capelin migrate to the coast of northern Norway (Finnmark) and the Kola Peninsula (Russia) for spawning. During summer and autumn capelin migrate north- and north-eastward for feeding. The last collapses were caused by poor recruitment, most likely in combination with low growth and increased predation pressure. The 1986 collapse was aggravated by high fishing pressure. Since then fishing pressure has been low. Predation by young herring has been suggested to have strong negative influence on capelin recruitment. The recent strong decline in the capelin stock appears to be caused by a combination of the same factors as in the previous capelin collapses: a decrease in individual growth rate of capelin, increased mortality caused by heavy predation from the large cod stock, and lower recruitment. In 2016 there was better recruitment of both capelin and herring, which could have led to a good food situation in 2017. However we have not witnessed that so far. For bird species relying on pelagic species, also the distribution of the fish is important: it should all be within reasonable flying distance of the colony.
So this is our idea of how the bad food situation has affected the colony in the past few years at Slettnes:
- Because of the lack of food there are fewer host birds breeding in the area and feeding out at sea (kittiwakes and Arctic terns)
- As a results fewer Arctic skuas start to breed, resulting in a colony with a much looser structure than in good years
- Arctic skuas are also less aggressive towards predators because they are in worse condition (the body mass of birds that we captured every year is 5% lower in 2015-2017 than in 2014).
- All this gives better opportunities for the Red Fox to enter the colony and predate the skua nests
- The skuas also produced smaller and fewer eggs in 2015-2017 than in 2014.
- The increasing reindeer herd further aggravates the situation: ca 400 animals wander through the area every day, possibly resulting in trampling of nests (and they will also eat eggs when they come across them).
Of course we have also considered our potential disturbing role as researchers in the area. However, especially in parts of the colony where we did not work, the decline is even more prominent than in the area where we caught most birds and spent most time.