Protections help porpoises to populate again
Banning gill nets led to jump in number of shy marine mammals
In the 1980s and ’ 90s, it was a grim trend off Northern California’s coastline: Hundreds of harbor porpoises, shy marine mammals that look like small dolphins, were killed every year in huge gill nets used by commercial fishermen.
The animals would wash up dead on beaches after becoming tangled in the nets, which stretched for up to half a mile underwater. After public outcry and threats of lawsuits from environmental groups, state officials banned gill nets in nearly all Northern California waters near the shoreline.
Now, a new study shows the impact a
generation later: a dramatic jump in the number of harbor porpoises.
Their population has more than doubled since the late 1980s off Monterey Bay, San Francisco and the Sonoma Coast in what scientists are calling an inspiring example of nature’s resilience similar to the recovery of the California condor, the gray whale, elephant seal and brown pelican. The marine mammal has increased sevenfold in Morro Bay, and porpoises have repopulated San Francisco Bay inside the Golden Gate.
“It’s exciting. It’s hopeful,” said Karin Forney, a research biologist based in Moss Landing with the National Oceanic and Atmospheric Administration. “There are many species that have been harmed by human activities, and often if we stop doing what we were doing to hurt them, they can recover. They can
rebound.”
Harbor porpoises are similar to dolphins but smaller. They are one of the ocean’s smallest mammals, generally growing to about 5 feet long, feeding on herring, mackerel and squid and weighing up to 170 pounds. They are about half the size of bottlenose dolphins, which have become more common along the Northern California coast after moving up from Southern California waters during the El Niño winter of 1982-83.
Unlike dolphins, which often leap dramatically out
of the water, harbor porpoises are more reserved. They live in shallow water and are comfortable in harbors and bays, even swimming up rivers sometimes.
Killer whales and sharks sometimes eat them, but their biggest threat is fishing nets. When they become tangled in gill nets — large curtains that hang from floats on the surface, trapping fish as they swim by — porpoises cannot swim to the surface to breathe, and they asphyxiate. The nets also can kill sea otters and diving birds, like common murres.
During the 1980s, studies found that at least 70,000 common murres died in gill nets off Northern California, along with hundreds of sea otters and about 2,000 harbor porpoises. While the porpoises were never added to the endangered species list, biologists at the time worried they might be headed there.
“We were picking up so many harbor porpoises on the beach that had gill net marks on their heads,” said Nancy Black, a marine biologist who now runs Monterey Bay Whale Watch in Monterey. “It was such a distressing thing that was going on.”
Black, who worked at Moss Landing Marine Labs in the late 1980s, began putting the dead porpoises in a freezer. One day she and other biologists lined up about 30 on the ground and called TV crews and other journalists. The stories shocked the public.
California voters passed Proposition 132 in 1990, which banned the use of gill nets and trammel nets, a similar type of fishing
gear, starting in 1994 in coastal waters off Central and Southern California. By 1989, state Fish and Game officials had prohibited them from San Francisco to the Sonoma Coast. And after several high-profile deaths of endangered sea otters and threats of lawsuits from environmental groups, the state extended the ban to Monterey Bay and Morro Bay starting in 2003. That affected about a dozen commercial fishermen who used the nets to catch halibut, sea bass and other species.
Scientists who have tracked the harbor porpoise population from small airplanes say the change since then has been significant.
“I was not surprised to see the populations increasing after the gill nets stopped,” Forney said. “I was surprised by the magnitude of the rebounding.”
Forney is the lead author of a study published Nov. 21 in the journal Marine Mammal Science that updated population estimates comparing various aerial surveys.
The number of harbor porpoises in the Morro Bay area increased from 571 in 1990 to 4,191 by 2012, the most recent survey, the researchers found. In Monterey Bay, the numbers more than doubled, from 1,486 in 1987 to 3,760 in 2013. And from San Francisco to the Russian River, their numbers expanded from 2,957 in 1987 to 7,777 in 2017.
North of the Sonoma Coast to the Oregon border, near- shore gill nets were never allowed. The number of harbor porpoises there has remained stable, with 12,160 in 2016, a small dip from 1990 when the population was estimated at 13,880.
Around the world, from China to Indonesia to the Baltic Sea, dolphins, porpoises and similar animals, such as vaquitas in Mexico, are still killed in large numbers by gill nets. In some areas, fishermen have put underwater noisemakers, called pingers, on the nets to drive them away.
“Gill nets are so good at catching fish,” Forney said. “But there are other costs.”