Stock growth

The growth potential of the stock may be estimated from data on the composition of the stock and the mean growth rates of each age group, due allowance being made for differences of growth rate of the two sexes and in different areas over which the stock is distributed.

Grp Marine Growth

Figure 9.34 North Sea haddock recruitment, 1960-1990.

(From Shepherd, J.G. 1990. 'Stability and the objectives of fisheries management: the scientific background. Lab. Leafl., MAFF Direct. Fish. Res., Lowestoft (64), © Crown Copyright. 1990.)

Figure 9.34 North Sea haddock recruitment, 1960-1990.

(From Shepherd, J.G. 1990. 'Stability and the objectives of fisheries management: the scientific background. Lab. Leafl., MAFF Direct. Fish. Res., Lowestoft (64), © Crown Copyright. 1990.)

Recruitment

In the North Atlantic, the number of young fish recruiting to the stocks of various species each year is very variable and so the stocks can fluctuate widely. Haddock, for example, exhibit more variability than most other stocks as shown in Figure 9.34.

This might logically be thought to relate directly to the fecundity of the mature adult females. Fecundity usually varies with size, large fish producing more eggs than small ones (Figure 9.35) so if there are more large adults in the population, the number of eggs produced will be greater. However, only a very small fraction (much less than 1 per cent) of the eggs spawned each year survive the first year of life. So it is actually slight changes in this huge death rate that cause big changes in the number of survivors that are then recruited into the adult population. The major factors causing the changes in death rates and thus controlling recruitment appear to be environmental, chiefly temperature, food supply and movements of the water. The prediction of recruitment rate is therefore very uncertain unless methods are available for sampling the younger age groups before they join the fishable stock. As previously mentioned, this phase of the life history is often the least well known.

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Length of plaice (cm)

Figure 9.35 Relation between numbers of eggs and length for North Sea plaice. (From Wimpenny, 1953, by courtesy of The Buckland Foundation.)

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Length of plaice (cm)

Figure 9.35 Relation between numbers of eggs and length for North Sea plaice. (From Wimpenny, 1953, by courtesy of The Buckland Foundation.)

Total, natural and fishing mortality

Estimates of total mortality (death from all causes), natural mortality (death from causes other than fishing) and fishing mortality may be made by relating an analysis of stock composition to an analysis of the composition of the catch of the fishery. Total mortality can be derived by noting over a period of years the numbers of fish in each group of the stock. Examination of the commercial catch reveals the numbers of fish removed each year from each age group by fishing, i.e. the fishing mortality. From these, it is possible by subtraction to determine the rate at which the numbers of fish are reduced from causes other than fishing, i.e. the natural mortality.

Mortality rates may also be calculated from the data provided by tagging experiments. If the rate of recapture of tagged fish is known, an estimate may be made of the mortality directly attributable to fishing and the overall mortality from all causes, the natural mortality rate again being the difference between the two.

9.5 The regulation of fisheries

The primary purpose of regulating commercial sea fisheries is an economic one, being to ensure ample supplies of good-quality fish for consumption and to safeguard the profits of fishing (Cushing, 1975; Gulland, 1977; Gulland and Carroz, 1968; Nikolskii, 1969). Fishing is a risky enterprise, and profits must be reasonably assured to attract the large investments needed by an up-to-date industry. Conservation measures for fish stocks are therefore designed principally to preserve the fisherman's livelihood by ensuring that there are plenty of good-sized fish for him to catch, rather than simply to protect the fish. Where it is considered that overfishing is taking place, it may seem obvious that the cure is to catch fewer fish, but the fisherman may not see the matter in such simple terms. The immediate consequence of any reduction of fishing is likely to be some reduction of his earnings. It may be difficult to convince a man whose income is derived from such a precarious occupation that, by reducing his catch, he will eventually be more than reimbursed by larger yields from a recuperated stock. He may feel less confidence in the predictions of fishery scientists than in his own ability to maintain his livelihood by increasing his efforts to catch fish so long as the stocks last. If he does not catch the fish himself, he may well suspect that someone else will; and, unless restrictions can be easily enforced on all fishermen, he is probably right. Regulations to control fisheries are therefore likely to be opposed by the very people they are designed primarily to benefit. Unfortunately, conservation measures avoiding any short-term loss to fishermen are unlikely to be effective. Many of the existing fisheries' management systems, including those applicable in the European Community (see Section 9.5.2), have been unsuccessful or only partly successful. The management of many fisheries on a sustainable basis is still not a reality.

It has been argued that the dangers of overfishing are easily exaggerated because the fishing industry is inherently self-regulating. If overfishing occurs, earnings and profits fall, boats are laid up, men leave the industry, the fishing intensity reduces and stocks are able to recover. Considering that stocks undergo large natural fluctuations (see Section 9.4.4 Recruitment) that cannot be controlled, it might well be thought that the economic checks of laissez-faire enterprise would be as effective as any that science can devise for ensuring that fish stocks are not dangerously depleted.

Experience has not supported this argument. When landings decline through overfishing, there is a tendency for the value of the catch to rise due to scarcity. This provides an incentive for even greater fishing effort, especially if there are other concurrent food shortages. Stocks may therefore become over-exploited to such an extent that the general good would be better served by limiting fishing to a level that would allow some stock recovery, and eventually larger quantities of better fish.

9.5.1 Methods for protecting fish stocks

The principal means used to attempt protection of stocks from over-exploitation fall into two main groups: technical e.g mesh size restrictions, and direct, e.g. restricting fishing effort. Technical measures are a necessary and effective means of helping to protect stocks, but not on their own. In countries where technical methods alone have been relied on, stocks have invariably fallen and fisheries have become uneconomic. Minimum mesh sizes, for example, cannot usually be practically set high enough to prevent the depletion of spawning stock by increased fishing effort as the stock declines. It is also necessary to restrict the quantity of fish caught by implementing direct methods such as limits on catches (Shepherd, 1993).

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