Nature's spring: Cod bounce back

Fish Image copyright Other
Image caption The Grand Banks now yield a very mixed trawl

It's the summer of 1497, and explorer John Cabot, freshly arrived from Europe, eagerly samples the waters off what are now eastern Canada.

"The sea there is swarming with fish, which can be taken not only with the net, but in baskets let down with a stone, so that it sinks in the water," wrote the Milanese ambassador to London to his Ducal boss, in an account of the voyage.

Fast forward to 1993, and the cod fishery on the Scotian Shelf off Nova Scotia has to be closed, because the fish are so scarce that you'd be hard pressed to catch any with an ocean-sized vacuum cleaner, let alone a net - a situation found elsewhere in the region, most notably the fecund Grand Banks.

The collapse of cod stocks here to less than 1% of their former abundance remains the exemplar of human over-consumption of resources and of nature's response to it.

Image copyright SPL
Image caption Snow crabs have taken over as a dominant species - and a lucrative harvest

But this week there's a bit of good news - well, sort of - in the shape of a study in Nature journal suggesting the cod, as well as some of the other big predatory fish we love to eat, could be on their way back.

The statistics revealed in the paper by Kenneth Frank and colleagues, from the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, are fascinating.

Cod and their predatory fellows such as haddock and saithe (aka pollock) used to be top of the food chain around here, eating large quantities of smaller "forage fish" such as herring, capelin and sand lance.

Removal of the top predators took the pressure well and truly off these species, which responded with an astonishing 10-fold increase.

This in turn depleted the stuff they eat - typically the tiny animals of zooplankton type - which crashed.

So the herring explosion couldn't and didn't last, with numbers plummeting within a few years down below their former abundance.

The graph that you get if you plot their subsequent abundance against time is pretty much what you'd see if you pulled on a spring and then released it... oscillations from maximum to minimum and back again, with the amplitude of those oscillations getting smaller and dying away.

Image copyright Nature
Image caption Forage fish show an oscillating, response when their predators are fished out

While all of that may be scientifically interesting, it doesn't give the answer to the question that's top in the minds of conservationists, fishermen and ardent human piscivores alike: will the cod come back?

Formerly, investigations of the new ecosystem that sprang up in the wake of cod's decline suggested it might never rebuild.

Forage fish were eating so many of the baby cod that they would never come back, it was thought.

This is where the oscillations come in. When the forage fish are in a big dip, the cod have a window in which to re-establish themselves - and the Bedford Institute paper suggests they have grasped the opportunity.

Image copyright SPL
Image caption Will commercial catches ever reach their former levels?

"Cod are a third of the way back - haddock better, and saithe even better than that," Dr Frank told me.

But there's a catch. The cod and haddock are much smaller than they used to be - a five-year-old haddock on average weighs half as much as in the boom years before the fishing moratorium was imposed nearly 20 years ago.

Why this should be isn't entirely clear. Exploitative fishing can alter the genetics of local stocks by selecting out big ones - in the North Sea, for example, cod are reaching maturity earlier and at smaller size as a result.

Or the fish's feeding might have changed. The explosion in forage fish was accompanied by an increase in snow crabs - so both the amount of food available for the predators and perhaps the way in which they have to eat it may be very different now.

Whether the Scotian Shelf and Grand Banks ever return to their former glory is far from certain, especially with other issues such as climate change threatening to disrupt their feeding and perhaps their breeding too.

Perhaps the ecosystem here hasn't been pushed into an alternate stable state - but in other parts of the world it may still be happening, with jellyfish ruling where fish predators formerly reigned.

The irony of the Canadian situation is that for years, curbs on fishing didn't seem to be working.

And on their own, they still wouldn't be restoring the system. That has needed the extra bounce of nature's ecosystem swing.