You may be aware of the story of the Cod Fisheries off the Canadian Grand Banks. These were some of the most productive cod fishing grounds in the world until overfishing resulted in a collapse in stocks in the early 1990's, at which point (to the sound of a stable door slamming shut) cod fishing was banned in the area to allow stocks to recover.
A recent paper by Kenneth Frank et al entitled "Transient dynamics of an altered large marine ecosystem", published in Nature, described the results of recent research into the Grand Banks marine ecosystem. The results are fascinating and provide reassurance that, given time, fish stocks to recover. We'll get to that in a moment, but first let us set the stage, so to speak.
The marine ecosystem in the Grand Banks can be broken down into three main parts.
At the top of the food chain are the large predator fish such as Cod and Haddock.
Below them are smaller fish such as Herring, Sand Lance and Capelin (no, BFTF had never heard of Sand Lance or Capelin before either). There are known as "forage fish" species.
And below these are the many plankton species.
Thus Cod feed on Herring and Herring feed on Plankton (it's a bit more complicated than this, obviously, but life is short so let's stick to the essence of the story).
Unsustainable fishing practices resulted in overfishing and a collapse of cod stocks in the early 1990's. With no Cod to keep numbers in check, the populations of the forage fish species exploded by some 900% (see graphic below).
But these high populations of forage fish were themselves too large to be sustained by the available plankton, so they in turn collapsed and entered into a "damped oscillation" of population peaks and troughs (see page 3 of the paper). The authors note that the period (time from peak to peak) is related to the life span of the forage fish and that
"Such eruptions followed by crashes involving fast growing, highly opportunistic species are known to occur in other ecosystems freed from predatory control"
Critically, part of the diet of the forage fish was the larvae of the large predator fish, which is why the stocks of Cod remained depressed for so long after the population collapse.
With the stocks of forage fish now moving back towards historical levels, it has finally been possible for Cod and Haddock stocks to recover.
The researchers note that, prior to the collapse, the dominant predator species was Cod, whereas the dominant species is now Haddock. Indeed, Haddock stocks are back up to historical levels, the stocks of Cod are still only 35% of those prior to the collapse. Only time will tell whether this change in the relative proportions of Cod and Haddock is a temporary or a permanent phenomena.
The authors comment that there are a number of factors that could still delay fish stock recovery (e.g. jelly fish blooms, the appearance of invasive species or eutrophication). Having said that, however, the authors have the encouraging view that
"These uncertainties notwithstanding, the answer to the critical question of whether or not such profound changes in the dynamics of large marine ecosystems are reversible seems to be ‘yes’."
You can see a press release about this research here