A “hot blob” in Northern Pacific caused a mass die-off of sea birds

Between the summer of 2015 and spring of 2016, an estimate of one million seabirds called murres died. The reason of their death was revealed in a recent study. The birds, mostly adult, died of starvation, but the underlying reason was the heatwave of 2014-2016. In addition, 22 colonies experienced reproductive failure during and after the die-off, which was an additional blow to their populations. A die-off of such scales is unprecedented in murres, especially considering its duration over multiple years. Over 60 000 bird carcasses were washed to the coast. In Prince William Sound, Alaska, over 4 500 dead birds were found every kilometer.

What are the murres?

Murres are large auks that live all around the polar region, including UK, Canada, and Russia. They are very good divers, diving as deep as 180 m. Murres breed in large colonies and feed on forage fish up to 20 cm long. They have very high energy requirements, needing to eat at least 50% of their body mass, or 0.5 kg, in fish every day. This makes them more vulnerable to variations in food supply and starvation.

The mass mortality event coincided with the most powerful ocean heatwave on record. This created a “hot blob”, a body of water stretching from California to Alaska, where average temperatures were 3-6°C warmer than normally.

The effect of the heatwave on the ecosystem

A direct consequence of the heatwave was a large decline in phytoplankton, the basis for all marine food chains. However, with higher temperatures, the energetic demands of ectotermic fish increase. That’s because the fish cannot regulate their temperatures and can optimally live just in a narrow range of temperatures. Once outside this range, like during the heatwave, their metabolism takes its own course. This goes both for forage fish, such as cod and haddock, on which murres feed, as well as for large predatory fish.

This situation resulted in pressure on murre’s food resource from all sides. On one hand, there was greater competition for food between murres and predatory fish due to the latter’s increasing needs. Yet simultaneously, there was less food available overall, as less plankton was available for foraging fish, while their energetic demands increased. This pressure did not harm only murres but also many other species. There have, for example, also been reports of a die-off of baleen whales.

With climate change, such heatwaves will only become increasingly more common. In fact, another “heat blob” has already formed in the Northern Pacific, while murre populations have not even recovered yet. Rapid action against climate change is needed if we wish the natural world not to suffer from its consequences.

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