Human-produced noise in the ocean is likely harming marine mammals in numerous unknown ways, according to a comprehensive new report from the National Academies of Sciences, Engineering, and Medicine. That’s because there are insufficient data to determine how the ill effects of noise created by ships, sonar signals, and other activities interact with other threats, including pollution, climate change, and the loss of prey due to fishing. The report, which was sponsored by several government agencies and released on 7 October, provides a new framework for researchers to begin exploring these cumulative impacts.
“There’s a growing recognition that interactions between stressors on marine mammals can’t right now be accurately assessed," said Peter Tyack, a marine mammal biologist at the University of St Andrews in the United Kingdom, in a webinar on the report. Tyack also chaired the committee that prepared the study, "Approaches to Understanding the Cumulative Effects of Stressors on Marine Mammals."
Killer whales, for instance, are known to swim away from areas where they have encountered sonar signals of about 142 decibels, a sound level lower than currently allowed by the U.S. Navy for its ships, Tyack said, referring to a 2014 study in The Journal of the Acoustical Society of America that determined the mammals’ likely response. But scientists don’t yet know how other marine mammals might respond. They also don’t know whether or how other factors, such as encountering an oil spill or colliding with a ship, would—or would not—compound the cetaceans’ response to these sounds; or how or whether such combined stressors matter to the animals’ long-term health and overall population.
Perhaps most surprising, Tyack said, scientists’ knowledge about the population size of most marine mammals “is very poor, and too weak to detect declines in time for effective action.”
To begin filling in these many gaps, the scientists are calling on researchers to assess and report the health of individual marine mammals through observation, photography, tissue samples, analysis of waxy ear plugs, and tags that record data as the animals dive. Any changes, such as spotting skinny whales, might provide “early warning indicators” about possible population declines, Tyack said. Such data might have helped scientists figure out why an isolated population of Alaskan belugas, protected from hunting since 1999, has yet to recover.
More information is also needed on the state of the oceans the animals live in and travel through, as well as the types of chemicals, temperature, and sounds they encounter. “We have to watch for larger, indirect effects,” Tyack said, noting that climate changes could reduce prey fish, and thus indirectly harm marine mammals—which might suffer additional stress if they’re traveling in an area of active oil exploration, for instance.
As a first step, the report recommends “building affordable surveillance systems” to detect population changes, including reproductive rates, which can be a simple indicator of a species’ health. Adult female cetaceans, for instance, that aren’t reproducing or are abandoning their young may be a sign of bigger problems—something scientists need to recognize as an alert, Tyack said.
The report calls on federal agencies and other research funders to support efforts to build the surveillance systems and develop case studies on the health of individual marine mammals and populations.