Part III
Where Mountains Meet Waves
Africa colonized the world 150,000 years ago, when hominids traveled by land to Europe, the Middle East, and Asia. But how did humans settle North and South America, two continents detached for millions of years from the cradle of civilization? Once it had been supposed that early people trekked from northeast Asia during the last ice age, when sea level was hundreds of feet lower, crossing a formerly submerged land connection called Beringia to populate a vast emptiness by foot. Archaeological evidence shows the plains of North America were occupied soon after the glaciers retreated, opening the proverbial gates to the New World some 13,000 years ago.1 But a discovery in southern Chile flattened that theory like a glacier bulldozes a spruce forest: there, in a site known as Monte Verde, people had been thriving for several thousand years before the ice sheets melted.2 Maybe early peoples invented cross-country skis to cross miles of glaciers, but a more plausible theory soon emerged. Hominids did not walk to the New World, but rather paddled the salty seas and hopscotched down the Pacific coast all the way to Tierra del Fuego, following a ribbon of marine abundance fancifully dubbed the Kelp Highway.3
Seafaring has a long history: ancient mariners from Southeast Asia reached Australia some 50,000 years ago, and modern humans boated from what is now Japan to the Ryukyu Islands 10,000 or 15,000 years later.4 Hominids may have walked out of Africa, but when they reached the Pacific Ocean, they fashioned boats and piloted them across the waves. Voyaging by water overcomes many of the obstacles of journeying by land. Mountains need not be climbed, nor raging rivers forded; snakes and sabre-tooth tigers do not lurk behind every wave. And critically, familiar foods are abundant and readily gathered. Where there are dense groves of kelp, rooted in the seafloor and tickling the surface with green fronds, there is sustenance. Abalones, oysters, mussels, and snails all can be foraged with ease, using only a pair of hands. Spears, nets, and hooks can be wielded to catch fish such as sheephead and sea bass, who swim in thick schools among the kelp. Nearby seabird rookeries provide copious quantities of eggs, while seal and otter colonies offer abundant meat.
Bob Steneck, an accomplished marine biologist, has studied the kelp highway for years.5 “The very first people who got to Daisy Cave [on California’s Channel Islands] were eating bivalves—you know, clams and snails—and then over time they developed fish hooks and other things that allowed them to stay on the coast. You could actually be a migrant to the New World, living well on easy resources that are along the coast.”6 Tracing the migration route farther south, Steneck reveals what botanical archaeologists discovered: “In the Monte Verde site in Chile … [they] were able to show Durvillaea, a type of kelp, in a cave that was consumed by the earliest human colonists in Chile 16,000 years ago. So direct evidence that they’re actually literally using the kelp.” Paddling away from Asia, primordial humans could have easily followed the crenellated coast of Beringia, glided past the fjords of Alaska, and bobbed down to southern California, reliably feeding themselves the entire way on the bounty of kelp forests, one of the most productive ecosystems on the planet.7
Even where kelp gives way to rock and sand, oceanic shorelines boast abundant resources, for people and for fishes. Compared with open ocean waters, where nutrients come only from the steady churn of photosynthesis, productivity near the coast is supercharged by nutrients flowing from streams, marshes, and the shoreline itself. Where rivers disgorge their upland waters into the ocean, the estuary teems with life. Nearshore fishes like tarpon feast on the crabs and snails and plankton that flourish where salty water meets fresh. Long-distance migrants like salmon gather in these brackish waters, preparing for the journey upstream to breed. Away from the great river mouths, rocky shores host tide pools brimming with shellfish, urchins, seaweeds, and fishes who have adapted to the coast’s crashing waves and wild swings in temperature and salinity. Offshore breezes here also blow surface water out to sea, which is rapidly replaced by local upwellings that further fertilize these waters. Even bird and seal colonies make a donation: their fishing triumphs are soon released as abundant, nitrogen-rich guano that washes into the ocean with every rainfall.
In tropical latitudes, where mangrove forests replace kelp, early mariners would have encountered equally concentrated sources of food: crabs, mussels, fishes, and seabirds all congregate in mangroves. Their tangled and interwoven roots stabilize soft shores, trap nutritious leaves and fruits falling from above, and serve as a sheltered fish nursery for snappers, barracudas, and dozens of other species. Beyond and between the mangroves, where these tropical waters wash over shallow bottoms, saltwater flats can be found: mazy patchworks of marshlands, inlets, tidal streams, and seagrass meadows. Seagrasses, the only true plants in the ocean, carpet sunlit shallows and act as fishy stepping stones between mangroves and coral reefs. This lush underwater lawn—another of the planet’s most productive ecosystems—supports an abundance of crabs, snails, sea turtles, manatees, and juvenile fishes, including many commercially important species.8 Full-grown reef fishes often commute from nearby patches of coral, transporting valuable nearshore nutrients along with them. Meanwhile, over the salt flats, all manner of predators and prey slip through a thin skin of brightly lit water, like the bonefish and tarpon so widely sought by anglers.
Exuberant abundance along shorelines and in nearshore habitats may have nourished seafaring humans and undoubtedly supports an extraordinary diversity of fishes. Opportunities and challenges abound in these diverse, productive, and entrancing environments, prompting a suite of adaptations that include mouth breathing, water archery, and mysterious migrations over thousands of salty miles. Shorelines are where the ocean ends, but it is where life on Earth began. Let us pick up a paddle and follow the Kelp Highway, exploring the world’s coastlines and the fishes who call the edge of the sea home salty home.