What seabird populations tell us about the health of the ocean
[By David Schoeman, Brian Allan Hoover and William Sydeman]
Just as caged canaries once warned coal miners of the risk of carbon monoxide poisoning, roaming seabirds are now alerting humanity to the deteriorating health of our oceans.
Seabirds travel vast distances across Earth’s seascapes to find food and breed. This exposes them to changes in ocean conditions, climate and food webs. This means that their biology, especially their reproductive success, can tell us about these changes on a rare and planetary scale.
We have assembled and analyzed the world’s largest database on seabird reproduction. Our results reveal a key message: the urgency in the northern hemisphere and the desirability in the south.
Ocean systems in the northern hemisphere are degraded and in urgent need of better management and restoration. Damage to the southern hemisphere’s oceans from threats such as climate change and industrial fishing is accelerating, but there are still opportunities to avoid the worst.
Oceans at a crossroads
Seabirds often travel far across the planet. For example, many Sooty Shearwaters breed in New Zealand, but move to the productive waters of the northeast Pacific each year. Arctic terns migrate even further, traveling between the Arctic and Antarctica every year.
Scientists often use satellite data sets to determine, for example, how the ocean surface is warming or how ocean food webs are changing. However, few of these datasets cover the world, and that’s where seabirds come in.
During its long journey, a seabird eats fish and plankton. In doing so, it absorbs signals about ocean conditions, including the effects of pollution, marine heat waves, warming oceans, and other ecological changes.
The reproductive productivity of seabirds (the number of chicks produced per female per year) depends on the available food resources. In this way, seabirds are the sentinels of change in marine ecosystems. They can tell us which parts of the oceans are healthy enough to support their reproduction and which parts may be struggling.
Deciphering the Seabirds Messages
In some cases, seabirds tell us directly about major ocean distress. This was the case in 2015-2016, when an estimated one million emaciated murres died, many of them stranded on beaches from California to Alaska. Seabirds experienced severe food shortages caused by an acute marine heat wave.
In other cases, seabird health may hint at a longer-term, more subtle disruption of ocean ecosystems, and these messages have yet to be deciphered.
In this task, seabird reproduction provides important clues about marine food webs that are otherwise difficult or impossible to measure directly, especially on a global scale. Fortunately, seabird scientists around the world have systematically measured reproductive productivity over the decades.
Our research team included 36 of these scientists. We assembled a database on the reproductive productivity of 66 species of seabirds from 46 sites around the world, from 1964 to 2018. We used the data to determine whether seabirds were producing relatively more or fewer chicks in the past 50 years, and whether the risk of reproductive failure increased or decreased.
In the southern hemisphere, there is still time to reverse the plight of the oceans. Shutterstock
In the northern hemisphere, the reproductive productivity of planktonic birds such as storm petrels and penguins has increased sharply in 50 years, but the reproductive productivity of piscivorous birds has declined sharply.
In the southern hemisphere, on the other hand, the reproductive productivity of plankton-eating seabirds declined slightly, but increased sharply for fish-eaters.
In short, the piscivorous seabirds of the north are in difficulty. Decreased reproductive productivity leads to declining populations, and the low reproductive rate of seabirds (many species have only one chick per year) means populations are slowly recovering.
More concerning, however, were our results on the risk of reproductive failure.
In the southern hemisphere, the probability of reproductive failure was low throughout the study period. The same was true for plankton feeders in the northern hemisphere. But fish eaters in the north have shown a dramatically increased risk of reproductive failure, the most severely since 2000.
Importantly, the increasing risk of reproductive failure was also much higher for seabirds that feed on the ocean surface, such as blacklegged kittens, compared to those that feed on the surface of the ocean. greater depths, such as puffins.
What this tells us
Unfortunately, these results match what we know about man-made damage to the ocean.
First, many pollutants such as plastics accumulate near the ocean surface. They are often eaten by seabirds that feed on the surface, which can hamper their ability to produce chicks.
Likewise, the rate of ocean warming has been more than three times faster and the variation in the number of marine heatwave days twice as large, on average, in the northern hemisphere than in the southern hemisphere over the past 50 years. last years.
Likewise, the oceans of the North have supported industrial fishing for much longer than those of the South. This likely reduced the food supply of northern hemisphere piscivorous seabirds over longer periods of time, causing chronic disruptions in their reproductive success.
But human impacts in the southern hemisphere are accelerating. Warming oceans and marine heat waves are increasingly intense, and industrial fishing and plastic pollution are increasingly pervasive.
We must heed the warnings of our seabird “canaries”. With careful planning and marine reserves that take into account predicted climate change, the southern hemisphere could avoid the worst consequences of human activity. But without action, some species of seabirds risk being lost and the ocean’s food webs damaged.
In the northern hemisphere, there is no time to waste. Innovative management and restoration plans are urgently needed to prevent further deterioration of ocean health.
David Schoeman is Professor of Global Change Ecology at the University of the Sunshine Coast.
Brian Allan Hoover is a postdoctoral fellow at Chapman University.
William Sydeman is an Assistant Associate at the University of California, San Diego.
This item is courtesy of The Conversation and can be found in its original form here.
The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.