Declining reproductive success in seabirds is a red flag for ocean health

To investigate reproductive success in seabirds, researchers have analyzed more than 50 years of breeding records for 67 species worldwide. The study shows that many seabirds in the Northern Hemisphere are struggling to breed, and the conditions in the Southern Hemisphere are beginning to follow suit.

The experts report that reproductive success has decreased over the past half century for fish-eating seabirds north of the equator. The greater impacts in this region are tied to climate change and human activities, like overfishing.

Study co-author P. Dee Boersma is the director of the Center for Ecosystem Sentinels and a professor of Biology at the University of Washington. She noted that many scientists view seabirds as sentinels of habitat health because their lives and well-being depend on sound conditions both on land and at sea.

“Seabirds travel long distances – some going from one hemisphere to the other – chasing their food in the ocean,” said Professor Boersma. “This makes them very sensitive to changes in things like ocean productivity, often over a large area.”

Furthermore, seabirds congregate at particular sites along coasts to breed and rear their young, which makes them vulnerable to changing shore and surface conditions and restricts how far they can travel for food while still successfully raising their chicks, explained Professor Boersma.

The research shows that the diets of seabirds play a major role in their ability to rear chicks. Reproductive success was found to decline among fish-eating seabirds in the north, and also among surface-feeding birds in both hemispheres, over the course of the study period. 

The experts believe that changing environmental conditions are to blame. Climate change is causing more frequent and more extreme events like heat waves. A long term marine heat wave in 2015-2016 caused one million murres to starve to death. In the ocean, seabirds face other threats as well.

“They have to compete with us for food. They can get caught in our fishing nets. They eat our plastic, which they think is food,” said Professor Boersma. “All of these factors can kill off large numbers of long-lived seabirds.”

“What’s also at stake is the health of fish populations such as salmon and cod, as well as marine mammals and large invertebrates, such as squid, that are eating the same small forage fish and plankton that seabirds eat,” said study co-author William Sydeman of the Farallon Institute. 

“When seabirds aren’t doing well, this is a red flag that something bigger is happening below the ocean’s surface which is concerning because we depend on healthy oceans for quality of life.”

The team found high variability in reproductive success among species, which means that additional research is needed to understand all the factors that shape feeding and breeding among seabirds.

“By knowing what is important to a species for success, we can make the world a better place for its survival,” said Professor Boersma.

The study is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer