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this activity. The preference of fishery biologists is to view the purpose behind the conservation of marine fisheries as the maximisation of the long-term catch, and thus to extract frcm each stock its Optimum Sustainable Yield (OSY) (Y2,E2 on Figure 2.1). This is approximately the largest average catch a stock can bear over a period of years without causing a reduction in the bicmass (the total weight of the entire stock). The bianass is a function of recruitment (the number of new young fish joining the adult stock) and of growth and mortality, both natural and as a result of fishing. Below the level of OSY any fish left uncaught may not necessarily be harvestable in the future, and represent a permanent loss.

While OSY sounds a biologically-appropriate goal, it is impossible to identify until it has been passed, because the characteristics of an individual stock cannot be known with certainty. When a stock of fish is being exploited by a growing number of fishermen, the weight of the catch will at first increase. This will result in a fall in the total number and weight of fish in the stock and in the average age of its members. These trends will continue until OSY has been exceeded, and the yield of the fishery is declining.

Although OSY is impossible to locate precisely, the relationship between fishery mortality (the percentage of a stock killed by fishing) and stock size has been the subject of increasingly sophisticated study b y fishery biologists since the end of the nineteenth century. Scientists distinguish (after Peterson, 1894 ^) between 'growth overfishing1 and 'recruitment overfishing'. Peterson formulated growth overfishing as occurring when fish are caught while still juveniles, depressing the total catch weight.

Figure 2.1; Hie relationship between o p t i m a susi-^inahle and maximmi economic yields 5

Y, .E, - M E Y Y 2 . E2 - M S Y = OSY Y3 . E3 - TC = TR

The diagram shows a typical yield curve. It is assumed that costs are proportional to effort ITC is a linear function) and that revenue is proportional to yield (TR is identical to the yield curve).

Recruitment overfishing is a reduction in overall stock caused b y the failure of sufficient numbers of new young fish to recruit, due to the intensity of fishing of adults before spawning. The composition of catches forms the principal source of information on stock levels and there is a time lag of one to three years (depending on the duration of the larval stage of the species) between the appearance of evidence of growth overfishing and of recruitment overfishing. This stems fran the fact that most fish larvae have different locational and feeding habits frcm adults of the same species, and are also too stall to fall prey to nets. By the time there is evidence that recruitment overfishing is occurring, up to three years of damage may have been done

Even vhen recruitment falls it may not mean that recruitment overfishing is taking place. Fisheries management must also take into account the extreme variability in survival rates of fish larvae fran year to year, owing to the effect of climatic conditions on phytoplankton production. The rate of larval survival in a particular year is traditionally ascertained by 'cohort analysis', which entails examining catches of adult fish two or three years later and classifying the fish of which they are composed, according to year of hatching. This method has shown that recruitment to the North Sea sole stock varies frcm year to year b y a factor as large as sixty, while the 1962 Year Class of North Sea haddock was twenty-five times as large as any recorded predecessor ^ . Figure 2.2 illustrates over a period of time the fluctuations of year-class strengths in the Irish Sea for four species of white fish. Heavy fishing increases the instability of recruitment which is more

Q

variable at lew stock levels . This variability in recruitment makes the problem of maintaining catch levels at OSY almost

Recruitment overfishing is a reduction in overall stock caused by the failure of sufficient numbers of new young fish to recruit, due to the intensity of fishing of adults before spawning. The composition of catches forms the principal source of information on stock levels and there is a time lag of one to three years (depending on the duration of the larval stage of the species) between the appearance of evidence of growth overfishing and of recruitment overfishing. This steins frcm the fact that most fish larvae have different locational and feeding habits frcm adults of the same species, and are also too snail to fall prey to nets. By the time there is evidence that recruitment overfishing is occurring, up to three years of damage may have been done

Even when recruitment falls it may not mean that recruitment overfishing is taking place. Fisheries management must also take into account the extreme variability in survival rates of fish larvae frcm year to year, owing to the effect of climatic conditions on phytoplankton production. The rate of larval survival in a particular year is traditionally ascertained by 1 cohort analysis' , which entails examining catches of adult fish two or three years later and classifying the fish of which they are composed, according to year of hatching. This method has shown that recruitment to the North Sea sole stock varies frcm year to year by a factor as large as sixty, while the 1962 Year Class of North Sea haddock was twenty-five times as large as any recorded predecessor ^ . Figure 2.2 illustrates over a period of time the fluctuations of year-class strengths in the Irish Sea for four species of white fish. Heavy fishing increases the instability of recruitment which is more

Q

variable at low stock levels . This variability in recruitment makes the problen of maintaining catch levels at OSY almost

R e l a t i v e y e a r - c l a s s s t r e n g t h R e l a t i v e y e a r - c l a s s s t r e n g t h

Figure 2.2: Variation of year-class strength In the Iri^h Sea o f : (A) cod, 1967-77 and vfoitinq, 1971-77,

(B) plaice, 1963-74 arai sole. 1968-76. 9

Relative year-class strength is shown b y means of an exponential

scale. The difference between two consecutive integers jjrplies the

doubling or halving o f the numbers of fish in a cohort. -1 thus

represents half the mean stock, while +1 represents double the mean stock.

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