Looking at our diagram of ovals and arrows, you'd think an explosion of herring and their ilk would be a boon for cod, as it would send more food up the system. But what if the herring are not only feeding cod, but also competing with their young for food—or even making them into food? In that case, an unleashed herring population could prove disastrous for an overfished cod population: the herring provide more food than adult cod can possibly eat, but eat as many young cod as the adults can possibly provide. And now the trophic cascade, beyond resizing all the ovals in our diagram, has actually rewired the arrows: what was prey has effectively become predator. "Regime shift" is just the right term: a different species is on top, and it uses its newfound power to stay there.
It now seems likely that some rewiring has occurred not only in the Baltic, but also across the northern Atlantic, and elsewhere in the world, as well. In fact, it has probably contributed to what many consider the grandest oceanic collapse of all. The world's most productive water—in the simple terms of how much biomass is made there every day—is not off the coast of Peru, where anchoveta presently holds the title of world biggest fishery, but rather off the coast of Namibia. There, the Benguela Current Ecosystem produces more than twice as much phytoplankton as Peru's Humboldt system, and about nine times as much as the California Current system. The biomass of sardines alone in the Benguela Current was once about 25 million tons, or about a third of the entire world's current annual take from the sea.
But after sardines and then anchovies collapsed under enormous industrial fishing operations, so much plankton was left to sink unconsumed that the ecosystem tilted more and more frequently into a poisonous state that had always been a threat: a thick layer of detritus, rotting on the ocean floor, disgorges hydrogen sulfide gas, which fizzes to the surface, stripping out oxygen and killing more sealife along the way. So vigorous is the ocean's belching that even the sea breezes grow fetid and corrosive. Released from predation and competition, and favored by the low-oxygen conditions, jellyfish and bearded goby—a bizarre fish that survives hypoxia and eats sulfidic mud—have taken over, and now, it appears, locked in their advantage over sardines and anchovies. Unfortunately, humans can't eat all that much jellyfish, bearded goby, or sulfidic mud.
So what should be done? Or rather, now that we've realized that trophic cascades and ecological regime shifts can, in fact, happen in the ocean, what should we do differently? The case of coastal Nova Scotia gets at what a hard question this really is. The collapse of groundfish led to an expansion not just of herring, but also, somewhat later, of large shellfish, which adult cod had previously eaten. And in this case, the newly insurgent prey—shrimp and crab—have now exceeded the erstwhile value of groundfish they replaced. So after many fishermen became poor, some recovered, and some others have lately become almost rich. And now it's not even clear what the goal of management ought to be: Bring back the cod, and jeopardize the shrimp and crab? Or should we, instead, view our record of serial depredation—catastrophic though it's been for some—in a more positive light? After all, our history of failings has at least revealed to us various possible configurations the ecosystem can adopt, and such knowledge might—might—enable us to manage with ecological savvy, tilting the system in the very directions we desire, and thus maximizing its value.
As alluringly sophisticated as this prospect sounds, there are reasons to approach it warily. Our record in Management 101: Sardines is spotty at best, so it's not exactly clear why we ought to feel confident in Management 505: Advanced Oceanic Biomanipulation. It was this very course, in fact, that led managers to break the Benguela. Shortly after the sardine population had suffered a severe decline, managers tried to make room for it to rebound by heavily fishing anchovy, the sardine's competitor. But as we know, what they facilitated was not the recovery of sardines, but rather the rise of jellies and bearded gobies.