Updated: 15 Aug. 2019
Increasing climatic variation is leading to changes in ecosystems in the Bering Sea. The persistence of marine heat waves, i.e. prolonged periods of elevated sea surface temperature, and earlier sea ice retreat affect the timing and composition of primary production. Declines in phytoplankton biomass and lipid density of zooplankton and forage fish in warmer years have been reported in the Bering Sea. These changes potentially lead to bottom-up effects in the food web. Seabirds such as puffins may suffer visible consequences through mass mortality events. Albeit short, these events represent catastrophic periods of high mortality and can ultimately affect population numbers.
A recent study reported an unprecedented mass mortality event of tufted puffins (Fratercula cirrhata) on St. Paul Island, Pribilof Islands, Alaska, in the eastern Bering Sea. Over 350 carcasses were found at a rate 60 to 80 times higher than normal. Of the carcasses, 79 % were tufted puffins while other birds included crested auklets (Aethia cristatella), horned puffins (Fratercula corniculata) and common or thick-billed murres (Uria spp.). Mortality estimates were calculated based on observed carcass abundance, survey effort, carcass detection, persistence and proportional beaching rates. These models estimated puffin mortality in 3,150 to 8,800 birds, likely representing a significant portion of the Bering Sea colonies.
The main cause of death seemed to be starvation, while harsh weather conditions could have contributed to difficulties finding food. Adding to that, the majority of puffin carcasses were adults in wing molt. When puffins molt they do it in synchrony and are flightless for up to 40 days. This is therefore a stressful period for the birds as the growth of new feathers increases nutritional requirements during a time that they cannot forage. If the birds were in poor body-condition before molting due to e.g. energy demands for breeding, which occurs pre-molting, and lack of prey in the area fill-up energy stores, the vulnerability of molting birds increased. In fact, recent ecosystem changes in the Bering Sea include reductions in forage fish abundance and energy density, and reduced abundance of lipid-rich copepods and euphausiids, which are the main prey of tufted puffins.
Estimating seabird mass mortality events is challenging, especially because the behavior of moribund seabirds is unknown and so it is assumed that they move ashore as the estimates can only be based in beached carcasses. Thus, the magnitude of mortality events can be underestimated if the seabirds move offshore and die off in open ocean. Further, this mortality event is one of the multiple that have occurred in the Northeast Pacific from 2014-2018 which is a flag for broad-scale ecosystem changes.
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Source: Jones, Timothy et al. 2019. “Unusual Mortality of Tufted Puffins (Fratercula Cirrhata) in the Eastern Bering Sea.” Plos One 14(5): e0216532. doi: 10.1371/journal.pone.0216532
Author: Sara Pedro
