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Put yourself, for a moment, in the fins of a juvenile herring. You were born in the spring, after your mother’s egg and your father’s milt mingled on a strand of eelgrass in a British Columbia inlet, the bay colored a cloudy turquoise by the spawning multitudes. Your parents’ school returned to its open-ocean feeding grounds soon after your conception, abandoning you to hatch and then loiter in the shallows, filter plankton, and dodge hungry cormorants. After two years of maturation, you’re ready to venture out and join a school of your own—but which one?
On your way offshore, you bump into a congregation of fellow Clupea pallasii and join the ranks, trailing your companions to the bay where your adopted school has bred for generations. You deposit your own eggs, then follow your school back to sea, memorizing the route from spawning bed to foraging area. The next spring, you repeat the migration, accompanied by a batch of fresh recruits who, this time, shadow you. The cycle replays, again and again: you and other elders guiding, younger herring following. Your school is as faithful to its inlet as salmon are to their natal streams, except you’re guided by intergenerational knowledge—by social learning.
That is the powerful concept that underpins the adopted migrant hypothesis, the notion that some fish learn migration routes via follow-the-leader. Now a new study, inspired by Indigenous ecological knowledge, suggests that adopted migration may explain the structure of Pacific herring populations—as well as their sluggish recovery from nearly a century of overfishing.
The motivation for the study, published this spring in the American Naturalist, came from a fisherman. Bill Gladstone, a member of the Heiltsuk First Nation, has plied British Columbia’s central coast for decades, harvesting kelp upon which herring lay their eggs and selling the valuable roe clusters. That non-lethal harvest technique long stood in contrast to the method used by non-native commercial fishermen, who netted and cut the roe sacs from around 30,000 tonnes of BC herring annually throughout the 1990s and early 2000s.
As the heftiest, most valuable herring vanished, Gladstone says, the spawning runs began to go haywire: fish vanished from bays where they’d bred for centuries or popped up for a day in places they normally stayed a week. “The small herring became very erratic,” Gladstone says. “They didn’t know what the heck they were doing.”
Gladstone passed along his observation to Anne Salomon, a marine ecologist at Simon Fraser University in Burnaby, British Columbia. “Herring have lost their big chiefs,” Gladstone told her, “and no longer know where to spawn.” The remark intrigued Salomon and Luke Rogers, a marine biologist at the University of Toronto. They began to wonder whether social learning through adopted migration—first proposed in 1997 to explain the behavior of Atlantic herring—was operating on the central coast.
To test whether adopted migration was driving the province’s herring populations, Rogers, Salomon, and their colleagues did what marine biologists do: they developed a model. Among other parameters, they assumed that juvenile herring were more likely to join larger populations—the more fish in a school, after all, the better the odds that others will bump into it and sign on.
When the researchers ran their model, they made two important discoveries. First, they found that adopted migration could create “spatial demographic structure”—in other words, the central coast’s herring stock may actually be a collection of subpopulations, each following learned paths to unique spawning areas. Imagine the region’s herring as cliquish high school kids in a cafeteria: all seem to share the same broad space, yet each clan is fiercely loyal to its individual table, with freshmen flocking annually to the most popular factions.
Second, the team discovered that herring’s spatial separation comes with alarming implications. Fish a herring population intensely enough to shrink its numbers, and it becomes less likely new recruits will encounter the school. Gradually, the group’s numbers dwindle as young members fail to replace the old. Eventually, says Rogers, “They risk going beyond the tipping point and heading toward extirpation.”
This demographic death spiral could inflict lasting damages. Once a population blinks out, the maps stored within its members’ memories disappear, just as a language dies with its final speaker. According to Rogers, that could be why some prime coastal habitats have remained barren even as herring begin to recover from overfishing. Without big chiefs, who leads the recolonization?
Of course, the adopted migrant hypothesis is still just that—a hypothesis. Doug Hay, a retired biologist formerly with Fisheries and Oceans Canada, acknowledges the idea is consistent with some data: larger herring are indeed declining, and tracking studies suggest that British Columbia’s fish do divide into distinct spawning populations. But he’s not convinced that herring migrate by following the leader rather than obeying their own noses—the strategy favored by, say, salmon. “The olfactory mechanism is really well established in other fish,” Hay says. “You don’t necessarily have to invoke a new mechanism.”
Rogers readily acknowledges that herring may rely on multiple cues, and that the adopted migration hypothesis needs empirical study. In their paper, the researchers suggest several techniques for testing the idea, including analyzing the chemical composition of herring scales or ear bones to determine how faithfully the fish return to spawning grounds. Although further research will determine whether herring learn from their peers, Gladstone, for one, requires no further convincing. “We live and die by the herring,” he says. “It’s obvious to us what’s occurring.”