This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Perhaps our species’s greatest misconception about the sea was that it is inexhaustible. The idea seems rather silly now, in a world where most people are familiar with the word “overfishing.” But men once gazed into the deep and imagined that it teemed with life so plentiful that we could take and take without ever running out.
Now we know that you can take too much. Unfortunately, it’s only because we’ve run our own real-life experiments and have to live with their results.
One of these experiments truly ran out of control: the cod fishery off of Nova Scotia, Canada in the northwest Atlantic. Cod catches increased through the 1950s and 60s, peaking at 800,000 tons in 1968. From there, catches dropped significantly — a warning that the population wasn’t doing so hot — with fishermen only able to catch less than 150,000 tons of cod in 1978. (That seems like a lot, but compared to 800,000 tons, it's nothing!) From there, catches continued to drop but it took nearly two decades for the Canadian government to take control, closing the fishery in 1993.
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The cod’s own prey, forage fishes, thrived following the fishery collapse. As the cod reached their record lows in the mid-1990s, forage fish numbers climbed, peaking at a whopping 900% of their pre-collapse abundance. The forage fish, in turn, greatly reduced their own prey, the zooplankton, allowing the photosynthetic phytoplankton to thrive.
This pattern of alternating gains and losses down the food chain caused by the loss of a top predator is called a trophic cascade, and has been seen in other systems. Scientists hypothesized that this type of trophic shake-up would lead to a new stable ecosystem, with any imbalances eventually reaching equilibrium and forage fish ruling the seas.
And for a several decades, this seemed to explain what happened off the Scotian shelf. As forage fish decimated the zooplankton, they also consumed cod eggs and larvae, suppressing their return to dominance. Forage fish reigned and the cod couldn’t recover, despite the abundance of their own prey. Maybe this was the new stable ecosystem.
But the past decade has finally seen some changes. Forage fish peaked at 10 million tons, more than double what scientists estimated the ecosystem could support. They subsequently decreased in abundance, as there wasn’t enough zooplankton food to support so many organisms. The forage fishes’ bodies were smaller and of lower quality, with less fat and protein — a reflection of how little food was out there.
This depression has given the cod and other predatory fish their chance, and researchers from Dalhousie University in Nova Scotia reported last week that they seem to be coming back. Cod, redfish (Sebastes spp.) and haddock (Melanogrammus aeglefinus) populations are higher than they’ve been since the early-1990s and the weights of individual fish are on the rise.
Although the ecosystem is still far from stable, this suggests that the ecosystem may be reverting away from a forage fish-dominated system back to its original predator-dominated ecosystem. The authors note that haddock seem to be more prevalent than cod currently, so it doesn’t mean that the ecosystem will necessarily return to normal. Haddock could reign instead of cod, which would effect the species composition of the entire system.
If the trends continue, the message here is hopeful: That these systems can return to their original stable state if given enough time to recover. The fishery has been shut down for nearly two decades, giving the community a rare amount of space and time to reorganize and work itself out. This Nova Scotian community indicates that simply leaving a system alone for a while can help significantly.
The other factor necessary for recovery here was that there were still enough cod, haddock and redfish around to generate the eggs and larvae to recover. Not all systems have been so lucky. A a literature review published a few weeks ago summarized how the eradication of top predators -- from jaguars to seastars -- affects communities, examining many types of ecosystems around the globe. Overall they found that ecosystems entered a new stable state with widespread changes including nutrient flow, plant abundance, and species diversity. If you have no top predator babies to replenish the lost species, you obviously can have no recovery. (For picures of ecosystems before and after top predator removal, see this Wired slideshow.)
But removing top predators isn't the only way to damage an ecosystem. A second study out last week modelled how harvesting forage fish at maximum sustainable yield -- the most that can be fished sustainably -- affected the trophic structure in five different regions around the world. They found that forage fish removal rippled throughout the ecosystem, altering the abundances of seabirds, marine mammals, and zooplankton alike. Halving exploitation rates would result in much lower impacts, the authors argue, while still retaining 80% of current catches.
I hope that people don't read the story of the Nova Scotian shelf the wrong way: that we can exploit to our heart's content as long as we stop fishing in time. For two decades the industry has been shut down affecting peoples' livelihood -- and the patterns of exploitation seen there are clearly not healthy. Other similar fishery collapses have not recovered because invasive species, such as jellies, have rolled in to take advantage of the mess. The cod's recovery is hopeful but it's safest to treat it as an outlier.
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