The conversationJune 8, 2021 9:54:09 am IST
Just as caged canaries once warned coal miners about the risk of carbon monoxide poisoning, free-flying seabirds are now warning humanity of the deteriorating health of our oceans.
Seabirds travel great distances through Earth’s seascapes to find food and reproduce. This exposes them to changes in ocean conditions, climate, and food webs. This means that your biology, particularly your reproductive successes, can reveal these changes to us on a scale rare across the globe.
We collect and analyze the world’s largest database on seabird reproduction. Our Findings they reveal a key message: urgency in the northern hemisphere and opportunity in the south.
The ocean systems of the Northern Hemisphere are degraded and urgently need better management and restoration. Damage to the southern hemisphere’s oceans from threats like climate change and industrial fishing is accelerating, but opportunities remain to avoid the worst.
Oceans at a crossroads
Seabirds often travel all over the planet. For example, many black shearwaters breed in New Zealand, but travel each year to the productive waters of the Northeast Pacific. Arctic terns migrate even further, traveling each year between the Arctic and Antarctica.
Scientists often use satellite-derived data sets to determine, for example, how ocean surfaces are warming or how ocean food webs are changing. However, few such data sets span the entire world, and this is 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, ocean warming, and other ecological changes.
The productivity of seabird rearing (the number of chicks produced per female per year) depends on the available food resources. In this way, seabirds are 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 in trouble.
Deciphering the messages of seabirds
In some cases, seabirds inform us directly about the great difficulties in the oceans. This was the case in 2015-16, when about a million emaciated common murres, many of them on beaches from California to Alaska. Seabirds experienced severe food shortages caused by an acute marine heat wave.
In other cases, the health of seabirds may suggest a long-term and more subtle alteration of ocean ecosystems, and we are left to decipher these messages.
In this endeavor, seabird farming provides important clues about marine food webs that would otherwise be difficult or impossible to measure directly, especially on a global scale. Fortunately, seabird scientists around the world have consistently measured reproductive productivity for decades.
Our research team included 36 of these scientists. We compiled a reproductive productivity database for 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 over the past 50 years, and if the risk of reproduction failure increased or decreased.
In the Northern Hemisphere, the reproductive productivity of plankton-eating birds, such as storm petrels and auklets, increased sharply over 50 years, but the productivity of fish-eating birds dropped dramatically.
In the southern hemisphere, by contrast, the reproductive productivity of plankton-eating seabirds decreased weakly, but increased strongly in the case of fish-eating birds.
In short, the seabirds that eat fish in the north are in trouble. Decreased reproductive productivity leads to population declines, and the low reproductive rate of seabirds (many species only have one chick per year) means that populations recover slowly.
However, more concerning were our findings on the risk of reproductive failure.
In the southern hemisphere, the probability of reproductive failure was low throughout the study period. The same happened with plankton feeders in the northern hemisphere. But fish eaters in the north showed a dramatically increased risk of reproductive failure, most acute in the years since 2000.
Importantly, the increased risk of reproductive failure was also much higher for seabirds that feed on the ocean surface, such as black-legged gulls, compared to those that feed at greater depths, such as puffins.
What this tells us
Unfortunately, these results are consistent with what we know about human-caused damage to the ocean.
First, many pollutants like plastics accumulate near the surface of the ocean. They are often eaten by surface-feeding seabirds, potentially hindering their ability to produce chicks.
Similarly, the rate of ocean warming has been more than three times faster, and the change in the number of days of marine heat waves has been twice as large, on average, in the Northern Hemisphere as in the Hemisphere. south for the past 50 years.
Likewise, the northern oceans have sustained industrial fishing for much longer than the southern ones. This has likely reduced the food supply to northern hemisphere fish-feeding seabirds for longer periods of time, leading to chronic disruptions in their reproductive success.
But human impacts in the southern hemisphere are accelerating. Ocean warming and marine heat waves are becoming more 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 projected climate change, the southern hemisphere could avoid the worst consequences of human activity. But if action is not taken, some species of seabirds may be lost and the food webs of the oceans may be damaged.
In the northern hemisphere, there is no time to lose. Innovative management and restoration plans are urgently needed to prevent further deterioration of the health of the oceans.
David schoeman, Professor of Global Change Ecology, University of the Sunshine Coast; Brian allan hoover, Postdoctoral Fellow, Chapman University, and William sydeman, Associate Associate, University of California San Diego