Fish cannot be counted like elephants in a national park. Fishery biologists therefore have to calculate the size of a stock based on specific parameters. The size of the annual catch is important. If this declines it could be a sign that the stock size is shrinking. The quantity of sexually mature adult fish, the spawners, is also important because they determine how many offspring are produced. After all, a stock can only sustain itself if the new offspring can compensate for the number of fish that are caught or die of natural causes. Fishery biologists commonly assign stocks to one of several categories: moderately exploited, fully exploited, over-exploited, depleted, or recovering.
The transitions between these status classes, however, are not sharp, for example, the boundary between a fully exploited and overexploited stock. One reason for this is that different fish species react very differently to fishing pressure. Species that multiply in large numbers and reach sexual maturity early can react better to high catch volumes than species that produce fewer offspring and require several years before they can spawn.
But basically a stock is considered to be fully exploited when it is fished to the maximum and an increase in the catch is not possible. If the fishing is intensified at this point, the stock is then pushed into the overexploited status. This stock then continues to decline because there are not enough offspring being produced. The stock is considered to be depleted when the catch is significantly below the historically expected amounts. Many researchers define this situation as the point when only 10 per cent of the highest historical catch is achieved. When a stock is depleted the catch cannot be increased even with intensified fishing, which is referred to as an increase in fishing effort.
A stock is considered to be recovering when the catch begins to rise again after depletion. An example of this is the North Atlantic cod, whose stocks collapsed in the 1960s and recovered again after a fishing ban. The Food and Agriculture Organization of the United Nations (FAO) presently uses three categories to describe the status of the stocks: non-fully exploited, fully exploited, and overexploited.
Fig. 1.8 > The example of the North Atlantic cod shows that a fish stock collapses when not enough mature fish (spawners) are pres-ent to produce offspring. © after http://fischbestaende.portal-fischerei.de