Among the multitudes of ocean migrants are likely winners and losers. Species that require conditions to be just right will find it harder to survive—the clownfish that can’t survive without its anemone home; the sea slugs that eat just one type of sponge or seaweed, and nothing else will do. If their food and homes don’t move with them, these animals will be in trouble.
Other species are better suited to travel and more ready to adapt and fit into new neighbourhoods. Many of them share key characteristics. They tend to be generalists and aren’t choosy about their food and habitats, and they either spend prolonged periods drifting as larvae or swim great distances as adults.
Climate change is doing some oceanic travellers a favour. Wandering and Laysan albatrosses set off on great soaring journeys above the waves, riding the winds and gathering food for their chicks. Now the heating ocean is intensifying winds, and albatrosses are flying farther and faster, and coming back quicker to feed their chicks, and as a result more of them are surviving.
On the list of likely survivors are lionfish, which are inadvertently being tested for their place in future seas. In the process, they’re stirring a great deal of unease while also confirming that it’s not an easy matter to predict the environmental impacts of migrant species.
A dozen species of lionfish are native to the Indian and Pacific Oceans. Two that are especially difficult to tell apart just by looking at them are the devil firefish and the red lionfish.c The farthest they naturally roam is Oeno, a tiny, uninhabited Pacific island roughly four thousand miles due west of the South American coast. Now, though, these two lionfish species also live together in a troublesome jumble in the Atlantic Ocean.
In the twentieth century, aquarium keepers enchanted by these striking fish brought them into captivity, adopted them as pets, and put them on public display. No one is absolutely sure where or when it happened, but both devil firefish and red lionfish were set free in the warm waters of the western Atlantic, and they found conditions much to their liking. A single lionfish was spotted in Florida waters in 1985. In the ensuing decades, following multiple releases in different places, lionfish have spread along the US East Coast from Florida to North Carolina, all through the islands of the Caribbean, into the Gulf of Mexico, and as far south as Brazil.
The successful spread of two lionfish species through this new range comes down to various quirks of their biology. Females reach breeding age when they’re one year old and from then on can spawn every few days, laying as many as two million eggs a year. Larvae drift for up to a month, riding currents, getting blown about by hurricanes, and travelling for hundreds of miles. And for fish that are not built for speed like tuna and marlin are, adult lionfish swim surprisingly long distances, at least six miles a day. Also in their favour is the fact that lionfish are not fussy about where they live. Normally considered coral-reef species, they’re also quite at home on seagrass meadows, in mangrove forests, and in estuaries. They’ve been seen a thousand feet down in Bermuda and Honduras, even beneath the murky plume of the Amazon River as it flows into the Atlantic.
When unfamiliar lionfish move into new places, the animals around them respond in different ways. Sharks, groupers, and other predators tend to ignore them and rarely try to eat them. Smaller fish aren’t scared of them. Damselfish carefully tend their seaweed farms and usually chase off intruders, even human scuba divers, but they don’t accost lionfish. Young reef fish that normally swim away when they see and smell predators are not bothered by lionfish. In fact, some have adopted the unfortunate habit of swimming towards them, perhaps mistaking their elaborate fins for somewhere to hide.
For these ambush predators, life in their new home is easy because their prey tends not to swim away from them. Stories have emerged of lionfish in the Caribbean finding it so effortless to hunt they were suffering from fatty liver disease, a condition usually seen in overfed pet fish. In many places, as lionfish arrived, conservationists and fishers watched in horror as the predators tucked into a naive Atlantic smorgasbord.
Similar worries of ecological turmoil were unleashed when lionfish moved into another new region. This one was caused not by releases from aquariums but by the opening of a new swimming route for wild fish. When the Suez Canal was built in 1869, it was a human enterprise on par with tectonic forces. The Mediterranean and Red Seas were connected for the first time since the eastern portion of the ancient Tethys Ocean closed around fourteen million years earlier. Historically, marine species were blocked from navigating the canal by the hypersaline Bitter Lakes midway along it. But since the canal has been successively enlarged to make way for more shipping and move more commodities around the planet, the lakes have been diluted, and hundreds of marine species have made the journey with the prevailing current from the Red Sea to the Mediterranean. Known as Lessepsian migration,d the procession of species includes lionfish from the Red Sea. The first arrivals were briefly spotted in Israel in 1991, but lionfish weren’t seen again until 2012, when a new wave of them began rapidly spreading through the Mediterranean. Within a decade, lionfish had reached the coasts of Lebanon, Cyprus, Libya, Syria, Tunisia, Turkey, Greece, and Italy.
Lionfish are restricted by wintertime temperatures and generally survive only if the sea is warmer than fifty degrees Fahrenheit. The warming Mediterranean has let them survive year-round, and as temperatures continue to rise, their range will expand. By century’s end, it’s likely lionfish will occupy most of the Mediterranean basin and slip through the Strait of Gibraltar and out into the eastern Atlantic.
On the opposite side of the Atlantic, the warm waters of the Gulf Stream currently offer winter refuge for lionfish along the shelf break off the southeastern US coast. Tropical larvae ride the Gulf Stream, and occasionally young lionfish make a summertime appearance as far north as Long Island, New York. If winters become warm enough as temperatures rise, lionfish will gradually make a year-round move farther inshore and northwards on the American coastline.
Early on, when the two lionfish invasions were getting underway in the Caribbean and the Mediterranean, the prognosis for the environment looked bleak. A worst-case scenario predicted that native fish would be replaced wholesale by lionfish, leaving habitats depleted and stripped of biodiversity, ruining fisheries and tourism, transforming ecosystems into something completely different and unappealing. However, as time has passed and more lionfish studies are being carried out, a more nuanced and complex picture is emerging.
The direst impacts of the lionfish influx have been happening in the northern parts of their new Atlantic range. Prey species have been hit hard in the Bahamas, where lionfish exist at densities five times higher than in their native ranges, the equivalent of around two hundred roaming a football field.
Off New Providence Island in the Bahamas, as lionfish numbers increased, native species of prey fish were reduced by 65 per cent. In Exuma Sound, scientists set up experimental patches of corals and saw that where lionfish were present, far fewer young fish of other species came in and settled. Presumably, lionfish were eating almost all the other little fish as soon as they arrived on the coral patches.
Where lionfish numbers are high, it’s possible other species could be wiped out. Lionfish are especially fond of eating fairy basslets, finger-length fish with a front half neon pink and back half golden yellow. The predators keep on hunting these colourful snacks even when there are very few left, raising the chance that the basslet could become locally extinct. There are also major concerns for the social wrasse, a highly endangered species that lives only around a few mangrove islands in Belize. There it makes up more than half the diet of the newly arrived lionfish.
In contrast, farther south in the Caribbean, fish populations seem to cope much better with lionfish, although no one is sure exactly why. In Panama, lionfish are still outnumbered on coral reefs by native predatory fish, and local prey face no greater threat of being eaten than they did before. In Venezuela, native fish living inside a marine reserve were still doing just fine three years after lionfish moved in. When ecosystems are in good health, they seem to better resist the impacts of lionfish.
Gladly, the arrival of lionfish hasn’t always triggered instant environmental disaster, as some people originally feared they would. The situation is complicated and difficult to predict. Even though lionfish are so obviously good at surviving and thriving in new places—with ravenous, unfussy appetites and formidable defences against other predators—no simple scenario exists for how they will influence other species that get drawn into their orbit.
When species move to new places, people often keenly respond. In 2022, a female walrus, this one nicknamed Freya, swam into the harbour of the Norwegian capital, Oslo, and attracted huge crowds of curious onlookers. Government authorities sparked outrage when they killed the thirteen-hundred-pound walrus after she was deemed to be a danger to the public. A statue of Freya has been raised in her honour. In contrast, soon after lionfish began settling into the waters of Florida and the Caribbean, people labelled them evil invaders, and the overwhelming reaction was to do whatever it took to get rid of them.
Thankfully, nobody has suggested releasing another species to try to swallow the proverbial fly, but various other ways of killing lionfish have been tried out. Divers tried persuading sharks and groupers that lionfish was good to eat by offering them a speared lionfish. All that did was give predators the dangerous idea of sniffing around after scuba divers to get a free feed. There are reports of groupers learning to lead divers towards a lionfish and hang around for their dinner to be served on a spike.
Bounties have been put on lionfish to encourage people to catch them. In Belize, the price per lionfish was twenty-five Belizean dollars, and in the US state of Mississippi, people were paid five dollars for every lionfish pulled out of the Gulf of Mexico. The problem is that very quickly the money ran out—there are simply too many lionfish.
More successful are lionfish derbies. During dawn-to-dusk bonanzas, competitive fishers and spearfishers go searching for lionfish. Prizes are handed out for the biggest and smallest and the greatest number brought in. The Emerald Coast Open, held in Destin, Florida, is the largest annual tournament, where competing divers have killed as many as fourteen thousand lionfish in two days. Routine lionfish removals are also carried out, especially in areas popular with tourists. In Honduras, wardens at the Roatan Marine Park hand out spears to visitors and teach them how to hunt lionfish.
There’s also money to be made killing and selling lionfish. They happen to be delicious and are safe to handle and eat once their venomous spines have been cut off. The meat sells for a decent price, similar to that of other high-end reef fish such as snappers. People make frilly jewellery from lionfish fins and leather from their skin.
In Florida, people are being encouraged to trap live lionfish. Many of those captives follow in their ancestors’ wake and find themselves in aquarium displays, only now the spectators gazing at them have thoughts of evil invaders on their minds and no plans to set them free.
While triumphant spearfishers hold up their trophy catches, and seafood diners do their bit by eating unwanted invasive species, it’s important not to forget that these are fish that human actions have turned into problems. Not only did humans originally introduce them to places they aren’t welcome, but the impacts of lionfish are made worse by other human disruptions to the environment. Climate change will encourage lionfish to invade new waters. And if other predators like groupers and sharks weren’t already so widely overfished, they could be more effective in controlling lionfish outbreaks, not necessarily by eating them but by scaring them. While a lionfish is hiding, it’s not hunting and growing fatter and more fecund.
It is too late to do away entirely with lionfish in their new ranges. For one thing, they soon get wise to human hunters and learn to avoid spear-wielding divers, and so they have become harder to catch. Efforts to control lionfish will carry on, and this can help reduce impacts in the most vulnerable locations. The methods may change, especially if somebody invents an effective lionfish trap or successfully sends out platoons of underwater robots to zap them with electric shocks, as has been proposed. But there’s no escaping the fact that lionfish are now a permanent part of these Atlantic, Caribbean, and Mediterranean ecosystems, as are many other species that are on the move. Some, like lionfish, have become a nuisance. The silver-cheeked toadfish is among the Lessepsian migrants that ventured through the Suez Canal and are stirring trouble in the Mediterranean, already as far west as the Strait of Gibraltar. A species of pufferfish, it grows between one and three feet long and has a silver blotch in front of its eyes, and its sharp teeth easily damage fishing nets and bite through fishing lines. It’s no good to eat because it’s laced with deadly tetrodotoxin. Other migrants are far more benign, and they’ve found a place in their new ecosystems and fisheries without too much bother. When asked in 2020 about what lives in their waters, fishers from the Mediterranean island of Cyprus identified the toadfish as a new and troublesome arrival, but they considered various other species as natives, even though they originally came from the Red Sea. Rabbitfish, lizardfish, barracuda, and squirrelfish are all mostly harmless, edible fish and have been living around Cyprus since the 1960s, time enough for local fishers, aged on average in their late forties, to get quite used to them.
This is the sort of story about the changing ocean that will doubtless become more common in the years ahead. It can take less than a generation for fishing communities to become accustomed to new assemblies of species, welcoming in those that are profitable, shunning those that are tasteless, toxic, or otherwise disagreeable. Terms like native and non-native will prove not to be fixed and universal but will slip from species to species and from place to place. Ideas of what belongs where in the ocean will continue to be shaped by human wants and needs. And as species move, some will inevitably disappear altogether. Their ghosts will linger for a while, and then memories will fade too, until perhaps no one will recall they were ever there at all.
As the ocean continues to warm, and species move farther and faster, there are going to be far-reaching changes in the way fisheries operate. Which species are available to eat, and where in the ocean they come from, will alter markedly. At higher latitudes, new species that offer new commercial opportunities are already appearing. Fishers in the North Sea now catch squid and anchovies, which not long ago seemed exotic for the region. Along the US East Coast, trawl fisheries have started targeting species such as fluke (also known as summer flounder), which is shifting polewards as the ocean warms. In the decades ahead, territories in the far north stand to gain tremendously from the reorganising ocean. By 2050, catches around the Arctic could increase by as much as 30 per cent relative to the early 2000s.
Farther south, most notably in the tropics, fishing is going to become much more challenging. In a widening band on either side of the equator, the seas are already becoming too hot for many species to survive. Since the 1970s, the diversity of ocean species living at the equator has been falling. Animals are escaping to cooler waters in the north and south, and none are moving in to take their places. The global pattern of marine biodiversity used to peak at the equator; now there’s a pronounced dip. This is especially true for the swimming pelagic animals, the type that fishers generally target.
Predictions extending to 2050 show that in many countries in West Africa, Southeast Asia, and the Pacific islands, fish catches could be less than half what they were at the start of the century. In these countries, coastal communities rely on wild fish for their nutrition and livelihoods. Not only will there be fewer species to fish, but those that do survive in tropical waters will be smaller, owing to the combined effects of rising temperatures and falling oxygen, making it even harder for fishers to make a living and feed their families.
The changing availability of seafood in the warming ocean will add to growing global inequalities and widen the gap in income and food security between rich and poor. If industrialised nations continue with business as usual and fail to curb carbon emissions, it will lead to the worst possible outcome for tropical countries, which had little to do with creating the climate crisis. However, devastating declines of fisheries at lower latitudes are avoidable. If carbon emissions are sharply reduced and global temperature rise is kept below two degrees Celsius by the end of the century, fishery losses will not be nearly as widespread or extreme.
In contemplating the future of the Anthropocene ocean, it is hard to get a clear idea of how species in new combinations will interact with each other and how the novel ecosystems they create will work. Researchers have built all manner of computer models to project future distributions of ocean life, and most often they focus on one species at a time, leaving out the complex dynamics of food webs. And yet, the ways species respond to each other will influence how reshuffling the ocean will pan out. Migrating prey animals could escape from traditional predators and expand their ranges quickly, or they might run into new enemies and slow down. One study predicts that these kinds of food-web interactions will hamper species’ abilities to shift their ranges and will thereby impede the polewards march through the ocean.
Some species will have a major impact on the ocean when climate change forces them to move. Outsize megafauna, the big-bodied animals, often play outsize roles in their ecosystems. Whales affect the ocean around them in many ways: seabirds feed on shoals of fish that whales drive towards the surface as they hunt; whale faeces fertilise plankton, which nourish food webs and draw carbon down into the deep ocean; and when whales die and sink into the deep, they nurture long-lived ecosystems dotted across the seabed like stepping stones, which are occupied by many animals that live nowhere else, such as Osedax, the bone-eating worms. All these connections could change—links could break and new ones form as whales respond to warming seas and alter the timing and routes of their migrations, which is already happening in many cetaceans, especially in the far north.
The Arctic Ocean is one of the fastest-heating regions of the planet, thanks to a feedback loop tied to the sea ice. As temperatures rise, the bright white frozen surface of the sea melts and reveals the dark ocean underneath; more heat from the sun is absorbed into seawater and less is reflected from the ice, accelerating heating and melting in dangerous synchrony. In response to the rapid warming, Arctic ecosystems are in flux. Species are moving; ecological interactions are changing.
Bowheads are the only baleen whales endemic to the Arctic. In the past, one population used to spend the winters in the northwest Bering Sea, between Alaska and Russia, then migrate north through the Bering Strait and hundreds of miles into the Chukchi Sea and the Canadian Beaufort Sea in the east. Acoustic studies have been tracking the whales’ migrations and finding that some bowheads are no longer going back to the Bering Sea and have started staying in the summer feeding grounds year-round, likely because these waters are not freezing like they used to. Bowheads need open seas to swim through and strain plankton from the water with their enormous mouths. When the sea freezes over in winter, the whales are driven south. As sea ice continues to shrink, more bowheads will stay in the Beaufort Sea, where they’ll encounter more container ships, which are also increasingly taking advantage of the low-ice conditions and taking a shorter route across the Arctic between Asia and Europe. The shifting whale populations will alter the ecosystems they feed in and affect the lives of people in Native Alaskan and Canadian communities who rely on bowhead whales for their traditional subsistence hunting.
Orcas are also penetrating farther north into Arctic seas than they used to. They aren’t adapted to living in frozen waters, where sea ice can get in the way of their tall dorsal fins when they surface to breathe (Arctic whales, including bowheads and narwhals, have no dorsal fins). As the ice retreats, the orcas’ available habitat is expanding, and as they move, these apex predators are impacting the lives of their prey. Around Baffin Island in Canada, newly arrived orcas have started hunting narwhals, the small, long-tusked whales that also live in Arctic seas, in and around the ice. Orcas are now hunting and killing more than fifteen hundred narwhals each year, not enough to threaten the narwhals’ future but certainly a big change in their lives. Even those narwhals that aren’t getting eaten are altering their behaviour. When orcas are fifty miles away, narwhals abandon the open seas and huddle closer to the shoreline, presumably where there’s less risk of attack. Bowhead whales in the region are also responding to orcas by retreating into the dense pack ice, which keeps them safer but takes them away from the open waters where they feed. There’s no way the whales can hear the orcas from so far away, or see them, but somehow they’re responding to an alarm that goes off through the ecosystem. Perhaps narwhals and bowheads are learning to spot signs of panic among each other and know when it’s time to hide.
The climate-driven conversion of the ocean is manifesting in many places as a gradual shift, with species moving bit by bit as the seas warm. Off the North American coasts, for instance, commercial fish catches are changing in line with the rise in local sea temperatures. This can lead to a false sense of security that climate change in the ocean will unfold steadily and in ways that can be forecast and, in time, adapted to. Yet, ocean ecosystems also suffer from sudden shocks, shifting into a completely different state.
One such regime shift has happened in the seas off southeast Greenland, a remote region of the Arctic, four hundred miles west of Iceland and thirteen hundred miles north of Newfoundland. Until twenty years ago, this was one of the most inaccessible parts of the planet because much of it was locked in sea ice. Huge ice packs would drift from frozen seas around the North Pole, a few hundred miles away, and accumulate along the Greenland coast. This was old ice—multiyear ice, as scientists call it—which would have formed when the surface seawater froze several seasons previously and hadn’t melted but had grown thicker over time. Throughout centuries of European whaling in the Arctic, ships rarely made it through the pack ice to this part of Greenland. Indigenous people living in a few small, scattered settlements did not make regular contact with the outside world until the end of the nineteenth century.
A diverse mix of cold-adapted, ice-dependent animals thrived in these frozen seas. There were great shoals of polar cod, with natural antifreeze coursing through their blood and covering their skin to prevent them from turning into blocks of ice. Narwhals chased after cod and other fish, perhaps using their elegantly twisted tusks to sense and skewer prey, although the precise role of their singular, long tooth remains a mystery. Hooded seals, ringed seals, and walruses lived on and among the ice packs, using the frozen platforms to rest on and rear their young.
This whole ecosystem changed when the ice disappeared. Seas in this region have warmed by more than two degrees Celsius, ocean currents have shifted, and the procession of pack ice from the North Pole has petered out. Since 2003, summers off southeast Greenland have become almost completely ice-free. In response, cod and narwhals are quickly moving north and could disappear entirely from the Greenland coast in the next few years. Populations of seals and walruses are also declining, as shown by the falling catches taken by indigenous hunters.
