In the winter of 2015, the public noticed thousands of dead sea birds washing ashore
across the western coast of the United States and Canada. The U.S. Geological Survey (USGS) later
linked this massive seabird die off to the North Pacific Marine Heatwave. A marine heatwave occurs
when sea surface temperatures are above normal for an extended period of time.
This marine heatwave caused ocean temperatures to increase far above normal from 2013-2016,
making it the longest marine heatwave on record. This sharp increase in warm water
impacted prey communities, especially forage fish, and contributed to the starvation of an
estimated one million common murres from Southern California to Alaska.
During the heatwave, key forage fish, such as capelin and sand lance, were
less abundant and smaller in size than before the marine heatwave.
Smaller reproducing capelin tend to lay fewer eggs,
which could have reduced their ability to produce offspring. Meanwhile, sand lance
populations were composed of much smaller fish
than usual in some areas. Their lack of growth greatly reduced the amount of fat and energy
available to seabirds that rely on them as food. These changes to forage fish populations may have
been due to bottom-up factors, such as warmer waters coinciding with a decrease in size and
energy content of the zooplankton that fish feed on. At the same time, larger forage fish
individuals were missing from the ecosystem. This may have been due to top-down factors, such as
predation from larger fish that needed to eat more due to increased metabolism in the warmer water.
These combined top-down and bottom-up factors resulted in an overall reduction in
several key forage fish species, and this was the primary cause of the major seabird die-off.
The marine heatwave had the greatest impact on common murres, a seabird that primarily feeds on
forage fish. These seabirds likely died from starvation, as the majority of the carcasses
examined had empty stomachs and no body fat. Additionally, some of the seabirds contained
mostly low levels of saxitoxin, the toxin that causes paralytic shellfish poisoning, which
is produced by harmful algal blooms. Although the primary cause of mortality was starvation,
the algal toxins could have been an additional stressor during the marine heatwave. The USGS
continues to investigate the impacts of harmful algal blooms on seabirds and their food webs.
Seabirds serve as ocean sentinels and are indicators of the health of the marine
environment. The USGS is studying the impacts of marine heatwaves on ocean food webs using
seabirds and their prey as key indicators. Such studies assess the health of northern
waters that are important for commercial fisheries and Alaska Native subsistence.
These efforts allow citizens, scientists, and resource managers to better understand and
anticipate future responses of marine ecosystems to warming global temperatures. If you would
like to learn more about the North Pacific Marine Heatwave, please visit the links below.