When the cod were overfished smaller species were allowed to prosper. Previously the cod had controlled the numbers of these smaller fish (by eating them), but with less predation numbers of these smaller fish increased to a level where they controlled the cod population (by eating young cod).
Food and Interaction Webs
The ways that different species interact with one another can be incredibly complex, and their relationships extend beyond predator-prey considerations. 'Interaction Webs' allow scientists to take into consideration the ways in which population changes (in this case of Atlantic Cod) affect their immediate environment – that is, in ways other than by direct predation.
When these additional ways in which animals can affect one another (and plants too) are taken into account there are often surprises. Consider the case where it was discovered that one particular species of red ant was necessary for the survival of a particular butterfly , and that ant needed a particular soil temperature which itself was influenced by how long the grass grew. It gets even more complicated in this case – the grass length was controlled by grazing animals, subject itself to the economics of farming! This example shows how many types of interaction need to be taken into account before population numbers can be understood.
Unfortunately there is often insufficient data available.
Cod and Haddock Numbers
Queens University released the news in July 2011 that cod numbers in Canadian waters seem to be on the increase. Recovery – but not more than: '34 per cent of the level that was typical when commercial fishing was flourishing in the 1970s and 1980s'.
The same study says that: 'Haddock, on the other hand, now exceeds its historical levels and is playing a more dominant role.'
So the question is exactly how cod and haddock are interacting, and how far will the cod recovery go? To predict the answer this question (rather than simply waiting to see) it will be necessary to gather enormous quantities of data about the ecosystem and look in detail at the ways all components affect one another.
Resource Management
Before resources (in this case a fishery) can be managed effectively there needs to be a lot of basic research. It is not enough to consider the population size of the stock that is being exploited. Nor is it enough to consider a few key species in isolation – full understanding (and therefore truly efficient management) can only come when the way everything affects everything else in an ecosystem is taken into account.
Nobody has all the information required, so for the question: 'will cod numbers eventually recover fully?' it remains: 'wait and see'.
Source:
'Cod resurgence in Canadian waters', Queens University, 27 July 2011.
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