An international team of 40 researchers sharing bird-breeding data from their respective countries discovered that seabird reproductive success has plummeted in the Northern Hemisphere, where birds feel the effects of climate change and human impacts more acutely than in the Southern Hemisphere.

The study, published Thursday in Science, highlights a need to consider hemispheric differences when planning conservation measures.

Seabird populations have been shrinking for years. A 2015 study, for example, found that global populations of monitored seabirds declined by almost 70% from 1950 to 2010.

"It's one thing to say the birds are doing well, or they're not doing well in your region or your area, but it's a whole other thing to say, 'Well, what's going on collectively?'" said lead author William Sydeman, a senior scientist studying the ecology of seabirds and small fish at the Farallon Institute, a California nonprofit dedicated to studying marine wildlife ecology. 

Prior research that synthesized global marine ecosystem change has not considered hemispheric variation as much as this current one, according to the study. 

When studies only consider how certain trends may or may not be consistent with climate change, they can only answer a precise yes or no question in a limited way, Sydeman argued. While such studies may be useful for studying coral calcification rates as they directly relate to climate change, for example, they cannot draw conclusions across many habitats. 

"We thought we could take advantage of that and draw a story by looking at the changes in ocean temperature between hemispheres, and ask a very straightforward but important question: Are the birds responding to that?" Sydeman said in an interview with The Academic Times. "And the answer is yes."

By studying 122 time series, or yearly datasets of breeding productivity, of 66 bird species from 1964 to 2018, and weighing them against marine heatwaves, the researchers could see that species breeding success was reflected in such heat waves and other variables. Those include cumulative human impacts from fishing, shipping and pollution, as well as the velocity of ocean warming, or the change of temperature bands around each hemisphere. Sydeman and his colleagues confirmed these bands were moving faster toward the North Pole, which means that areas of higher ocean temperature have been moving northward.

As the Northern Hemisphere showed higher rates of overall oceanic change, seabird breeding success fell.

"Seabirds are built to withstand breeding failures periodically, but they're not built to withstand long-term chronic breeding failure," Sydeman said. "If they missed breeding for one year or two years, that's not something that would concern anybody. But when you have chronically poor breeding success, which is what we're seeing, particularly with those omnivores in the northern hemisphere, that's of concern."

Birds that eat fish and those that eat both fish and plankton seemed to be hit hardest, while those that eat only plankton did not see as steep a decline in breeding success. In all cases, food availability is tied to breeding success, but omnivorous seabirds cannot merely switch to plankton when no fish are present. According to Sydeman, omnivorous seabirds tend to eat plankton during egg-laying, and fish as they're raising their young. 

While food is paramount for seabird breeding success, many factors could hamper it, including pollution and overfishing. But he and his colleagues believe that climate change is the primary influence behind food loss.

Birds are known to be susceptible to effects of climate change. Another recent study described how songbirds are migrating earlier than they used to, as temperatures rise. 

For future research, Sydeman is most interested in figuring out how climate change alters food availability, and how that affects seabirds. According to Sydeman, ocean stratification may be driving a loss in food availability for seabirds, especially in the Northern Hemisphere, with its higher rate of warming. 

Ocean stratification occurs when the surface layer of the ocean becomes warmer and more saline than other areas of the ocean, limiting the turnover of nutrients. The result of this could lead to fish being driven deeper into the ocean, severely limiting the food supply for birds that feed near the ocean surface. 

Sydeman and his colleagues are considering stratification studies using satellite imagery to better understand how the process could influence fish populations. 

"This study is the tip of the iceberg," Sydeman said.

The study, "Hemispheric asymmetry in ocean change and the productivity of ecosystem sentinels," published May 27 in Science, was authored by W. J. Sydeman, S. A. Thompson, M. García-Reyes and J.A. Thayer, Farallon Institute; D. S. Schoeman, University of the Sunshine Coast and Nelson Mandela University; B. A. Hoover, Chapman University; F. Daunt, M. P. Harris and S. Wanless, UK Centre for Ecology and Hydrology; P. Agnew, Oamaru Blue Penguin Colony; T. Anker-Nilssen, Norwegian Institute for Nature Research; C. Barbraud and K. Delord, Centre d'Etudes Biologiques de Chizé; R. Barrett, The Arctic University of Norway; P.H. Becker and S. Bouwhuis, Institute of Avian Research; E. Bell, Wildlife Management International; P. D. Boersma, University of Washington; B. Cannell, Murdoch University; R.J.M. Crawford, Department of Environment, Forestry and Fisheries; P. Dann, Phillip Island Nature Parks; G. Elliott, G. Taylor and K. Walker, New Zealand Department of Conservation; K. E. Erikstad, Norwegian University of Science and Technology; E. Flint, U.S. Fish and Wildlife Service, Honolulu, Hawaii; R. W. Furness, University of Glasgow; S. Hatch, Institute for Seabird Research and Conservation; K. Hilwig and H. Renner, U.S. Fish and Wildlife Service, Anchorage, Alaska; J.T. Hinke, National Oceanic and Atmospheric Administration; J. Jahncke and P. Warzybok, Point Blue Conservation Science; J. A. Mills, Kaikoura; T. K. Reiertsen, Norwegian Institute for Nature Research; R. B. Sherley, University of Exeter; C. Surman, Halfmoon Biosciences; P. N. Trathan, British Antarctic Survey; E. Velarde, Universidad Veracruzana; and Y. Watanuki, Hokkaido University.