Fish Food Chain Flounders – and Finds Its Way Back

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As we reported last month, one of the biggest obstacles to sustainable fish farming is that raising big, popular carnivores  such as salmon and tuna requires us to fish – and overfish – far down the food chain, in the ranks of smaller species like anchovies. Those are the little critters the bigger fish like to eat — and they eat a lot of them until they’re ready to be turned into dinner themselves.

This is a problem because the tight interconnectivity of food webs means that any hiccup, like a decrease in anchovy population, can destabilize everything above and below it. But new research is showing that nature can sometimes re-balance itself even when the scales have tipped badly out of whack. Indeed, according to a study by researchers at Queen’s University in Ontario and Bedford Institute of Oceanography in Nova Scotia, it is actually self-perpetuating imbalances in the food chain that are allowing cod and other  populations off Canada’s east coast to recover in the wake of the early-1990s collapse of the fisheries.

What happened here, according to study author Kenneth Frank, is called a “prey-predator reversal.”  After populations of groundfish (species that live on or near the bottom of their bodies of water) such as cod collapsed, the populations of smaller forage fish such as herring – prime groundfish prey – soared by a whopping 900%. Those forage fish had to compete with one another for limited supplies of the zooplankton that are their preferred dinner. Cod eggs and the tiny young that hatch from them look remarkably similar to zooplankton, which meant that the forage fish began to feast on the eggs and the babies. So the hunter essentially became the hunted – a very bad thing for an already stressed species. Only when the forage fish outstripped its food supply did the ground fish stand a chance once again. As forage fish numbers went down from the lack of food, the eggs and just-hatched cod stopped being just another meal for herring and could actually grow into  adulthood. “This early-stage recovery represents a long ecological transition for an ecosystem that was pushed out of balance and that is gradually moving back into balance,” study author William Leggett said in a statement.

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But even if this cod turnaround gives us hope that other declining fish species may also get back on their non-feet, we shouldn’t celebrate  yet. It’s important to recognize that the cod suffered a one-two punch, and that will require a one-two rebound. They first had to climb back from the abyss that resulted when their eggs and young were gobbled up by forage fish — which they now seem to have done. They must now set out an even bigger hike up the population path to get back to where they were before humans fished them away. Cod populations are still much lower than they used to be — only 34% of the numbers from the commercial boom of the 1970s and 1980s remain in Canadian waters — and closing the gap further requires regulatory and other steps such as catch shares, removing subsidies, or even simply changing our diets.

Also worth remembering is that this kind of ecological sleight of hand is incredibly unpredictable, and could just as easily have gone the other way — catapulting Canadian cod and groundfish further along the trajectory of collaps . “I would say these are unintended consequences,” Frank said. “Some of these effects you may not be able to anticipate.” Study author John Fisher agreed, adding that their results suggest that the groundfish food chain is “more complicated than [they] gave it credit for 20 years ago.”

Still, it’s nice to know that nature is capable of surprises – the good kind, that is. Frank remembers almost giving up, believing that the groundfish couldn’t recover from the 1990s crash. “I guess if there’s enough time and patience, these systems will return despite being over-exploited to the extent they were,” he said. When species — and the environment as a whole— are battling for their lives, any resilience is a good thing.

Tara Thean is a TIME contributor. Find her on Twitter at @TaraThean. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.

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