A thousand years ago, the Norse settlers of my home city of York ate cod that weighed as much as eight kilograms. We know this from archaeologists and the fascination they have for medieval waste heaps. But today you would be lucky to find a cod that weighs more than two kilograms. What have we done to our fish stocks to bring about changes like this, and does it matter?
Fisheries policy is designed to allow small fish to grow. This is for the best of reasons - fish that are caught when they are too small will produce a small yield. Moreover, the fish will not have had a chance to reproduce, undermining the fish stock altogether. For example, the avowed intention of the EU Common Fisheries Policy is "to protect fish resources by regulating the amount of fish taken from the sea, by allowing young fish to reproduce , and by ensuring that measures are respected."
As it stands, however, we have "fished down" our stocks, until few large, old individuals remain. To see this, consider North Sea cod. Suppose you start with ten thousand individuals aged one year. If there is only natural mortality, about one thousand of these individuals survive to reach age eight years. A moderate level of fishing mortality brings the number of survivors down to about one hundred individuals. But the levels of fishing mortality we have been applying over the last twenty years bring the number of survivors down to about three.
Mortality acts cumulatively, so, given the way we manage fisheries, few individuals survive to become big and old. Yet left to their own devices, cod are thought to live for at least thirty years. It is a testament to the great efficiency with which we exploit living marine resources that fish like cod are now much more likely to die at the hands of fishermen than for all other reasons put together, once they get into the fisheries.