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Why are cod stocks collapsing?

September 23, 2002

Sudden collapses in many ecological systems are the rule rather than exceptions to the rule.

This is shown by Professor Lennart Persson of Ume'å University, Sweden, in the latest issue of the scientific journal Proceedings of the National Academy of Sciences. Among other things, the article provides an explanation for the collapses in cod stocks in different parts of the world.
Several models have shown that ecological systems can experience catastrophic collapses. On the other hand, these models have not been able to say how common they are.

Lennart Persson and co-author André M. De Roos of the University of Amsterdam have now shown in a model that two fundamental traits of individuals in many organisms promote the occurrence of catastrophic behaviors in ecological systems. The two traits that the authors have examined are the correlation between growth and the amount of ingested food and the correlation between mortality and body size. In simple terms, animals grow more the more they eat, and the smaller an animal is, the greater the risk that it will be eaten. On this basis, Persson and De Roos have been able to show that collapses in ecological systems are rather the rule than exceptions to the rule.

"The mechanism is actually quite simple, but that`s also what makes it so general," says Lennart Persson. "Considering that more than 85 percent of all species on earth exhibit populations with variations in size, these results should be extremely generalizable.

Paradoxically, the number of available prey can increase with the number of predators. This happens because the competition among the remaining prey diminishes, and these prey also grow more quickly. Increased growth yields greater reproduction, and that in turn leads to increased numbers of the small-sized prey that predators thrive on.
If, on the other hand, mortality among predators increases, say as a result of high pressures from fishing, it is a natural consequence for mortality to rise among the prey these predators live on. This in turn means that competition increases among the prey, thereby checking their growth. This leads to a decline in the number of prey of the sizes preferred by predators. This can bring about a collapse in the number of predators.

When a collapse occurs, it comes extremely rapidly, and it is difficult to discover changes in advance. The species that professional fishers capture in their nets, such as cod and full-grown herring, provide no clues to what is under way. On the other hand, the small-sized, sexually immature individuals of the species of prey that are nevertheless too large to be eaten by the predators (sexually immature herring, for instance) and the food that the former live on exhibit differences.

"It is actually these small-sized individuals of the species of prey that need to be monitored, because it is here that you can get a measure of when collapse is imminent," explains Lennart Persson.

These small-sized individuals of the species of prey increase in number when the fishing of predators increases. This occurs since the competition for food increases among the small-sized individuals. They have no possibility of growing larger, so their growth is stunted, and the number of small-sized individuals grows. This means that we must be extremely vigilant.

"If fishing is increased, very little change will be noticed until there is a collapse. In order to promote growth at this juncture, fishing must be cut to extremely low levels. Halving quotas, for example, will not help; total bans on fishing are the only answer then," says Lennart Persson.

This line of reasoning is supported by developments in North Atlantic cod stocks, which have not recovered despite drastically reduced fishing.

Vetenskapsrådet (The Swedish Research Council)