In the gray great sadness of the sea’s sprawling end, the ocean creatures will depart the earth one by one for a greater paradise. Specifically: a trawling net, a plastic-choked estuary, or a sweltering primordial stew of algae, bacteria, and whatever will eventually evolve to replace us as earth’s dominant species.
First we’ll lose endangered species, such as the and , which have already proven themselves vulnerable to overfishing and changing environmental conditions. The coral polyps will flit off into the ether, leaving carpets of bleached sea bones, and the baby sea turtles will follow streetlights into storm drains and die there confused and alone. Then we’ll finally eat all the .
And then what? For years, every time I buy fish, eat sushi, go for a swim, or watch the Discovery channel, I've thought the same question: If climate change and overfishing and pollution of the ocean continue unchecked, what will be the very last species of fish on earth?
The Survivor's Checklist
“Clearly the things that are stuck to the bottom are going to do really bad,” says Loren McClenachan, an assistant professor at Colby College in Maine, who studied pictures of great catches from Florida Keys fishermen for her doctoral thesis, finding that their size has declined by 88 percent since the 1950s. “I think that’s a big first order split of things—how much things move individually and how big of a range their population has overall.” Coral, for example, are very sensitive to temperature changes, but completely incapable of swimming somewhere better, which is why we’re always reading about bleaching events.
“We know that one aspect that makes a species sensitive to climate change is how broad the temperature range it can tolerate is,” says William Cheung, associate professor at the Institute for the Oceans and Fisheries at the University of British Columbia. Fish are ectotherms, which means their body temperature relies very much on the temperature of the water around them. Arctic (or polar) species of fish in particular are specialized to live in extreme cold temps that are stable throughout the year. “Even a moderate level of warming will affect the biology of polar species of fish,” says Cheung. As for the tropics, which are already the warmest area of the globe, too much warming will leave them barren.
There’s also lifestyle to consider. Even if a fish could move to an area with a more appropriate climate, its usual food may not move at the same speed. It might move to an ecosystem with less to eat, or with a scarier apex predator. “There’s also disruption in timing, so things that spawn when a temperature hits a certain level in the spring, they might do that sooner than their prey,” says McClenachan.
And then there’s size. “Small fish that can reproduce early in their lives and have fast turnover can withstand higher fishing pressure,” Cheung says. “There’s a well-known phenomenon in the ocean called fishing down the food chain—because of higher vulnerability of the organisms higher up in the chain, with overfishing they’re the first that will be depleted, and sequentially the smaller and smaller fish will be dominant.” Fish that are higher in the food chain are also usually more susceptible to foreign materials that accumulate in the body over time, such as mercury.
One other thing: As fish are moving around the globe searching for a suitable temperature and food source, at least some of them will turn into Mr. Magoo, bumping into each other and rocks and what’s left of the coral while trying to find (or avoid becoming) something to eat. Under extreme high carbon dioxide conditions, the ocean acidifies, affecting, among other things, the development of a fish’s otoliths, or ear bones. “We don’t really quite understand how that will play out in ecosystems, but in lab studies there’s been some scary things that have come out of ocean acidification research,” McClenachan says.
In general, the winning sea creature would possess some combination of smallness, large current habitat, resistance to acidification, speedy reproduction, flexible diet, and the ability to move around to seek more habitable environments. And there may be a few winners in different environments around the world. A 2017 study from Cheung’s lab suggests that the Pacific sanddab, blue crab, and Pacific sandlance have staying power in the Pacific Northwest. In the Atlantic, southern species such as are already encroaching in Arctic waters where they were not previously found.
But nearly every scientist believes in one true survivor: the jellyfish.
“If we continue to overfish and there’s unmitigated climate change…it’s a world I don’t want to see, but jellyfish will fill the ocean, along with small fish and plankton and bacteria,” says Cheung. In an acidic ocean with low oxygen levels, simpler creatures with fewer needs may have the best chances. “My Ph.D. advisor called it the rise of slime,” says McClenachan.
The Triumph of the Damned
Why is it that the species most able to survive the coarsening of the environment are always the most disgusting—cockroaches, jellyfish, algae—and not anything desirable or complex or tasty? (Imagine being chased through a Mad Max hellscape by immortal butterflies the size of elephants). I asked McClenachan, who says that big, complicated things that take a long time to grow up and have babies (Ed Note: *cough* humans) will not be at an advantage in a polluted, climate-changed future. Neither will parts of the environment itself, such as coral.
“What you have left over are these generalists like jellyfish that move around, reproduce really quickly, and are adaptable in what they eat,” she says. “I think generalist species that are simple is a good way of thinking about it."
As depressing as this topic is, it is also possible there may never be a last fish in the ocean. Fish stocks and biodiversity are coming back in some overfished areas with good fisheries management, says Cheung. “For climate change, the IPCC report says we have a window of time in the next few years to take proper actions to limit global warming to 1.5 degrees Celsius.” And, if worst comes to worst, adaptation and evolution are slow, unreliable processes, but they happen.
McClenachan suggests looking to the , a species that lives near her home in Maine and whose ranks were suppressed for several centuries because of dams that prevented them from swimming up the local rivers to spawn. Recently, after efforts to remove the dams succeeded, alewives have come back immediately and in the millions. “I find that astounding,” McClenachan says. “Like, for the last 150 years the alewives have gone to the river and been like, Is this the year that I get to have my babies?”
And then one year, it was.