The Fish that Shrank

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.

The absence of big, old individual cod and other fish is worrisome from the standpoint of the short-term ecological health of the stock, as well its longer-term genetic health. The short-term issue concerns egg production, which in many fish is proportional to body size: large individuals contribute far more to future generations than small individuals. In fact, first attempts at spawning can be relatively unsuccessful, so a fisheries policy that depends for the most part just on first-time spawners (which is what we are approaching now), could be especially flawed.

The policy is also suspect because the success of spawning is notoriously variable from year to year. A bad year for recruitment in a fished-down stock means losing both the new cohort and most of the spawners, because the latter are caught before they can spawn again. By contrast, a stock with lower mortality for large individuals at least maintains an adequate reserve of large individuals available as spawners next year.

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The long-term concern is that fishing is intended to be selective; fishermen aim to harvest the most profitable species and the most profitable sizes within species. Like all other forms of life, fish stocks change genetically when directional selection is applied. The effect of fishing-down is to give an advantage to genes causing slow growth but early adulthood.

Indeed, there is evidence from many fisheries of earlier maturation; such changes take place fast enough to be detected over the decades. For example, early in the twentieth century, you would not find a mature cod in the North Sea less than about 50 centimeters in length, whereas by the 1980's mature cod were as small as 15 centimeters. So there is a lot of under-age sex out there in the sea. By cherishing our large, old fish we would reverse the selection for early maturation while selecting for faster growth through the size classes most vulnerable to fishing.

The genetic changes that we are bringing about in our fish stocks are going largely unrecognized. Yet the precautionary principle places some responsibility on us to hand down the living resources of the marine realm in a form that can be used as much by future generations as by us.

Radical new thinking is needed in fisheries management to overcome the ecological and evolutionary problems generated by current practice. If only one change in the prevailing mindset could be made, it should be to realize that, in the case of fish like cod, big is beautiful. To reconstruct our approach to fisheries management with measures that increase the survival of large, old individuals would be good for the health of our stocks in the short term and would be appreciated by our descendants.