The survey results published in May in the Canadian Journal of Zoology suggest that widespread gillnetting after World War II likely led to the harbor porpoises’ decline. Improved fisheries management, including more fishing practices designed to reduce bycatch of marine mammals, likely helped the porpoises recover.
“We don’t really have good scientific information from that period, but bycatch, especially in gillnets, was probably one of the major factors” in the decline, said Thomas Jefferson, a consulting biologist with Clymene Enterprises and lead author of the publication. “All the pieces of the puzzle kind of fit together to indicate that, although other factors may have contributed too.”
The researchers also mention disturbance by vessels, competition with other predators and water pollution as possible factors in the decline. Puget Sound waters are now considerably cleaner than they were 50 years ago, which may have improved conditions for porpoises and the fish they prey on. Recovery of harbor porpoises in heavily urbanized Puget Sound is a major conservation success story, Jefferson said.
The new information – including estimates of more than 11,000 harbor porpoises in Puget Sound – will feed into new NOAA Fisheries stock assessments of marine mammal populations. The stock assessments help NOAA Fisheries develop fisheries regulations that protect marine mammals.
The survey was funded by the U.S. Navy and NOAA Fisheries’ West Coast Region and the study design was a collaboration between the Navy, NOAA’s Alaska and Northwest Fisheries Science Centers, and Smultea Environmental Sciences.
“One of the great benefits of the return of harbor porpoises is that people who live around the Sound can see and enjoy them,” said Lynne Barre, branch chief for protected resources in NOAA Fisheries’ Seattle office. “The scientific information helps us evaluate impacts on the population from human impacts, like fisheries bycatch.”
In the 1940s harbor porpoises were common around Puget Sound, but they began a steady decline until they almost completely disappeared by the 1970s and remained virtually absent in the 1980s and 1990s as well. Increased sightings by local research groups, such as NOAA’s Northwest Fisheries Science Center, Cascadia Research Collective, and the Washington Department of Fish and Wildlife after 2000 suggested that harbor porpoises had begun a resurgence that accelerated in the last few years. Recent aerial surveys have now documented their distribution and provided a population estimate to confirm those observations. The latest research indicates that harbor porpoises have reoccupied even the farthest southern reaches of the Sound throughout the year.
“This represents a remarkable ‘comeback’ for the species, although we do not fully understand either the reasons for the initial decline, nor the subsequent recovery,” the researchers wrote.
“We suspect strongly that their role in the ecosystem was a pretty important one, and that they had a role in regulating populations of prey and predators,” Jefferson said. “Now they have taken up that role again.”
