Addressing the International Fishery Exhibition in London in 1883, T.H. Huxley said:
I believe that it may be affirmed with confidence that, in relation to our present modes of fishing, a number of the most important fisheries, such as the cod fishery, the herring fishery and the mackerel fishery, are inexhaustible. And I base this conviction on two grounds, first, that the multitude of these fishes is so inconceivably great that the number we catch is relatively insignificant; and, secondly, that the magnitude of the destructive agencies at work upon them is so prodigious, that the destruction effected by the fisherman cannot sensibly increase the death-rate.
Shortly afterwards the landings of fish from the seas around north-west Europe increased to an extent that Huxley could not have envisaged. Sailing vessels were superseded by ships with powerful steam engines, enabling the use of much larger nets and the replacement of the old beam trawl by the far more effective otter trawl, and giving fishermen a new independence of wind and tide so that they could fish longer and more often. Within 30 years of Huxley's pronouncement there was already evidence of reduction of stocks of certain favourite demersal species such as cod, haddock and plaice on the more intensively fished areas of the north-east Atlantic. Between the two World Wars the decline became more apparent, and in 1942 E.S. Russell wrote:
A state of overfishing exists in many of the trawl fisheries in north-west European waters. Two things are wrong. First, there is too much fishing, resulting in catches below the possible steady maximum, and secondly, the incidence of fishing falls too early in the fishes' life resulting in a great destruction of undersized fish which ought to be left in the sea to grow.
Since World War II the catching power of fishing vessels has been further augmented by several innovations. The change from steam to diesel power has raised the power and speed of ships. The development of nets made of stronger, lighter and longer-lasting materials has encouraged the use of even larger nets. The availability of synthetic fibres of various densities, both lighter and heavier than water, facilitates the control and correct orientation of nets in the water by selection of different densities for different parts, and the use of transparent fibres invisible in water has increased the efficiency of certain nets. Sophisticated sonic techniques have been invented for the detection of fish shoals, and for net handling. Modern navigation systems such as GPS (see Section 3.3.5) allow accurate return to good fishing areas and accurate deployment of nets. Refrigeration equipment installed on fishing boats has allowed them to range far afield and continue fishing until full without deterioration of the catch.
The rise in world landings of fish since the end of World War II has been remarkable (Figure 9.25). Over the period 1948-1968 the increase was around 7 per cent per year, bringing the world total annual catch of sea fish from under 20 million tonnes in 1948 to over 60 million tonnes in 1970. Around 1970, the annual catch stopped rising. Between 1980 and 1989 it rose very slowly, and over the past few years appears to have declined slightly. Greater efforts to catch fish have not recently resulted in significantly heavier landings and there is now much concern at the intensity of exploitation of many fish stocks and the risks of diminishing returns through overfishing.
In order to understand what is involved in 'overfishing', we will briefly consider the effects that fishing is likely to have on the size and composition of fish populations. First, we will take the case of a stock of fish which is subjected only to very light fishing. This population can be regarded as having grown to the limits
Figure 9.25 World landings of sea fish, 1938-1992.
(Source: FAO yearbooks.)
Figure 9.25 World landings of sea fish, 1938-1992.
(Source: FAO yearbooks.)
imposed by the food supply, which restricts both the number of fish surviving and the size to which the individual fish grow. Scarcity of food prevents all the fish from making as much growth as they would if they were better fed, and the slow-growing, older fish compete for food with the rapidly growing younger specimens. The small catches taken by fishermen are likely to consist mainly of the larger, older fish; but due to age, undernourishment or disease, these may not be of good quality, and therefore fetch correspondingly poor prices on the market.
A stock of fish in this condition may be regarded as 'underfished'. The population is overcrowded. An undue proportion of the food is consumed by old fish of poor market quality at the expense of young fish, and none can realize its full growth potential. This stock could support a larger, more profitable fishery of better-quality fish if more fish were caught. A reduction in the size of the population, particularly by the removal of older fish, would promote a better growth rate throughout the remaining stock and improve the condition of the fish, the process being analogous to a gardener thinning out his plants to encourage the best growth and quality of his specimens.
Considering now the opposite case, a stock of fish subject to extremely heavy fishing, this population is likely to consist mainly of young, small specimens because the fish are caught as soon as they reach catchable size. Landings of undersized fish are likely to fetch poor prices because these fish carry little edible meat, being mainly skin and bone. This stock is obviously 'overfished'. Too many fish are caught too early in life. Young fish make rapid growth and, if left longer in the sea, would soon reach a more valuable size, providing heavier landings and better prices. Productivity and profits would eventually improve if the amount of fishing was reduced.
However, in these conditions the fisherman is tempted to try to increase his profits by catching even more fish. This is a vicious spiral which can only lead to a further reduction in the size of the stock, and a further dwindling of the fisherman's income. Indeed a point may be reached at which so many fish are caught before they have lived long enough to spawn that the reproductive capacity of the stock becomes severely impaired, leading to a catastrophic decline in numbers through failure to produce enough young and even a danger of extinction.
We can therefore distinguish two aspects of overfishing. There is what may be termed growth overfishing, where catches are poor because too many fish are caught before making optimum growth but recruitment of young fish is not seriously affected. There may also occur recruitment overfishing when the stock fails through depressed intake of recruits resulting from a reduction in the numbers of spawners.
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