Night fell as the two scientists got to work, unfurling long nets off the end of their boat. The jungle struck up its evening symphony: the sweet chittering of insects, the distant bellowing of monkeys, the occasional screech of a kite. Crocodiles lounged in the shallows, their eyes glinting when headlamps were shined their way.
Across the water, cargo ships made dark shapes as they slid between the seas.
The Panama Canal has for more than a century connected far-flung peoples and economies, making it an essential artery for global trade — and, in recent weeks, a target of President-elect Donald J. Trump’s expansionist designs.
But of late the canal has been linking something else, too: the immense ecosystems of the Atlantic and the Pacific.
The two oceans have been separated for some three million years, ever since the isthmus of Panama rose out of the water and split them. The canal cut a path through the continent, yet for decades only a handful of marine fish species managed to migrate through the waterway and the freshwater reservoir, Lake Gatún, that feeds its locks.
Then, in 2016, Panama expanded the canal to allow supersize ships, and all that started to change.
In less than a decade, fish from both oceans — snooks, jacks, snappers and more — have almost entirely displaced the freshwater species that were in the canal system before, scientists with the Smithsonian Tropical Research Institute in Panama have found. Fishermen around Lake Gatún who rely on those species, chiefly peacock bass and tilapia, say their catches are growing scarce.
Researchers now worry that more fish could start making their way through from one ocean to the other. And no potential invader causes more concern than the venomous, candy-striped lionfish. They are known to inhabit Panama’s Caribbean coast, but not the eastern Pacific. If they made it there through the canal, they could ravage the defenseless local fish, just as they’ve done in the Gulf of Mexico and the Caribbean.
Already, marine species are more than occasional visitors in Lake Gatún, said Phillip Sanchez, a fisheries ecologist with the Smithsonian. They’re “becoming the dominant community,” he said. They’re “pushing everything else out.”
On a recent evening, Dr. Sanchez and a Smithsonian biologist, Víctor Bravo, brought seven nets on the lake. Each one was at least 150 feet long and 10 feet wide, with meshes of varying sizes to snare fish by the gills. The scientists set the nets at different spots then stayed on their boat overnight to make sure the crocodiles didn’t eat their catch. Later, they and other researchers would analyze the captured fish in the lab to figure out where they had swum in from and how they fit into the lake’s food web.
Mr. Bravo tied one end of a net to a tree branch. “Vamos!” he called out. The captain threw the boat into reverse, pulling the net taut.
Then, they waited.
The Panama Canal has a long history of sea creatures voyaging through as stowaways on ships’ hulls and in their ballast tanks: oysters from the Indo-Pacific, jellyfish from the Black Sea, worms from mud flats in the Netherlands. As far as scientists can tell, however, the latest watery trespassers aren’t arriving by boat.
As part of the canal’s recent expansion, Panama added a new lane at each entrance, with new locks that can raise and lower today’s king-size cargo ships. Naturally, the new locks are bigger than the old ones. So each time a vessel passes through, more fresh water spills out to the ocean and more seawater sloshes in — and with it, perhaps, more coastal fish.
All that extra seawater washing in has also made parts of the lake saltier. So far, though, the increase in salinity hasn’t been big enough to account for the sudden presence of so many marine fish, said Gustavo Castellanos-Galindo, a postdoctoral fellow at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, who studies the canal ecosystem.
Instead, he and other scientists reckon it’s the combined effect of bigger locks, bigger ships and bigger water volumes that has allowed more fish to swim or drift into the canal. “There’s just more opportunity for them to move in,” said Diana Sharpe, a freshwater ecologist at Harvard.
The fishermen of Gatún, who know the lake better than anyone, say the effects have been sweeping.
From his home in Cuipo, a village of rainbow-colored houses on the lake’s western shore, Félix Martínez González has for decades prowled the waters on a powder blue canoe. On a recent day, he harpooned 16 pounds of fish in six hours. Before the canal was expanded, he would have caught twice as much, he said.
He blames the salt. Rising salinity might be killing off the vegetation where the tilapia and peacock bass like to live. (Another possible factor, Dr. Sharpe said, is that the lake’s fish now have to compete against the marine invaders for food.)
Over coffee on his porch, Mr. Martínez González, who is in his mid-60s, takes the long view. “I’m not worried for myself; I’m worried for the next generation,” he said. “All this affects them, too.”
The canal’s peacock bass are also a popular game fish. But with the population under strain, Oswaldo Alberto Robles, 54, a fishing guide, wonders whether it makes sense for tournaments to keep giving prizes for them. “Imagine 20, 30, 40 boats searching for one fish,” he said. “We’ll just keep running out of them even faster.”
The fish problem is hardly the only headache the canal expansion has created for Panama.
Saltwater intrusion is threatening Lake Gatún’s other main function — namely, providing drinking water for half the country’s people. The canal authority is examining ways to desalinate portions of the lake. It is also planning to dam another river to create a new freshwater reservoir, and in the process displace 2,000 or so people, most of them poor.
To critics, the situation suggests a lack of foresight by the Panamanian authorities: The costly, disruptive canal expansion created problems that only another costly, disruptive project can fix.
“The truth is, before the expansion, the problem of salinity in the lake wasn’t discussed,” said Manuel Cheng Peñalba, a member of Panama’s legislature and former officer for the canal. Now, he said, Panamanians are worrying about drinking water despite living in one of the rainiest countries on Earth.
When asked whether the canal should have been expanded without first securing a new water supply, Ricaurte Vásquez Morales, the canal administrator, emphasized how vital the expansion had been for Panama. Ships were outgrowing the canal’s original locks. The nation had a choice: keep up or “lag behind,” he said.
Juan Carlos Navarro, Panama’s environment minister, used a Spanish expression to describe how the government would resolve the canal’s environmental issues: “I get dressed slowly because I’m in a hurry.” In other words, urgently, but with care.
“We will not get the canal wrong,” Mr. Navarro said. “Panama is the canal, and the canal is Panama.”
When it comes to the fish, however, it’s not exactly clear what getting it right would entail. Adding more fresh water wouldn’t necessarily stop invaders from swimming through the new locks. Putting up electric barriers or curtains of air bubbles might keep some species out but not others. Barriers might also impede ship traffic.
With many invasive species, you can’t predict whether they’ll live quietly in their new homes or “blow up,” said Bella Galil, the curator emerita of crustaceans at the Steinhardt Museum of Natural History in Tel Aviv.
Dr. Galil has for decades studied invasions of non-native species that traveled through the Suez Canal, including jellyfish, mussels, puffer fish, rabbit fish — hundreds of them in all. The jellyfish weren’t known to congregate en masse in their old home in the Red Sea, she said. Yet in the Mediterranean they gather in swarms, stinging children on the beach, clogging fishermen’s nets and plastering the intakes of desalination plants with their sticky bodies.
Regulators have occasionally gotten serious about controlling such intruders, Dr. Galil said. Even then, success is neither cheap nor quick nor assured. “It takes a lifetime,” she said. But “if you don’t start, you are left with a destroyed sea.”
Back on the scientists’ boat in Lake Gatún, it was almost midnight: time for Mr. Bravo and Dr. Sanchez to check their nets. They roused themselves and began retracing their path across the dark water.
At their first stop, they each grabbed one side of the net and pulled it aboard. The night’s first catch was a sea catfish from the Pacific. The second: a snook, another coastal species.
When the scientists reached the end of the net, they lifted it and shook. Tiny anchovies and silversides rained onto the deck. “These are marine species, too,” Mr. Bravo said. “Now these little fish are found throughout the lake.”
He slipped the silvery bodies into a Ziploc bag, which he labeled and tossed into a cooler.
The other nets held similarly mixed-up smorgasbords of species and origins: one snook from the Caribbean and another from the Pacific. A slender, elegant ladyfish and a long-nosed needlefish, both Caribbean. More stubby forage fish, some native, others not.
Smithsonian researchers have been sampling the fish of Gatún this way for more than a decade. They examine the fishes’ eye lenses, their muscle tissue and the contents of their stomach. Dr. Sanchez is analyzing their otoliths, the calcified structures in their inner ear that, like tree rings, record vivid histories of their surroundings.
At sunrise, he and Mr. Bravo hauled up their nets a second time. Dawn’s light was gentle and rosy. The scientists’ boat looked toylike next to the tankers making morning crossings through the canal.
Mr. Bravo pondered the changing lake and the communities that live off it.
“You feel a little sad,” he said, “because many people who have dedicated themselves to fishing for subsistence, for food, who have no other kind of work — they have to fish.” But some of the new trespassers, like jacks, are harder to catch than the species they’re replacing. The new fish are faster, more aggressive — less “stupid,” Mr. Bravo said.
Even now, Gatún might not be done changing. All the jumbling of species could lead some of them to crossbreed, scientists say — with effects on the lake, and the two vast oceans beyond, that are very hard to predict.
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