Age census

Stock samples are examined to determine the number of fish in each age group. Where annual markings are absent, unreadable or of doubtful reliability, Petersen's method of age analysis is used. Measurements are made of the length of each fish in representative samples of stock, and length/frequency graphs are plotted. These polymodal curves can be broken down to their separate modes, each representing an age group within the sample (Figure 9.32). The age assigned to each mode of the curve can be checked against meristic markings of the majority of that group.

The Petersen method is usually applied to the analysis of cod populations because the scale markings are only reliable up to 3 years of age. The age of each length group is taken as the scale age of the majority of fish in each length group.

Studies of the age composition of herring shoals in the southern North Sea have shown that here the adult shoals contain fish from 3 to 11 years old, the maximum age normally reached by herring in this area. Off the Norwegian coast, where the fish mature later, the age range is from 4 to 15 years or older. Great variations occur from year to year in the numbers of fish entering the adult shoals (see 'Recruitment' below), and this is reflected in the relative abundance of each year class (Figure 9.33). Strong year classes, where many young survive and are recruited into the adult population, may dominate the population for many years. Knowledge of the age composition of shoals in a particular year enables a prediction to be made of the probable composition of the next year's shoals, which may be a useful guide to sensible regulation of fisheries.

Figure 9.32 Dissection of length/frequency data for cod on Petersen's principle. Polymodal curves are pecked.

(From Graham, M., Buckland Lecture (1948), based on Fisheries Investigation Ser. 2, XIII, No. 4, p. 54, Figure 18, by permission of the Controller, HMSO.)

Figure 9.32 Dissection of length/frequency data for cod on Petersen's principle. Polymodal curves are pecked.

(From Graham, M., Buckland Lecture (1948), based on Fisheries Investigation Ser. 2, XIII, No. 4, p. 54, Figure 18, by permission of the Controller, HMSO.)

Sizes and growth rates

The modes of the length/frequency curve indicate the length distribution of each age group. From these, average growth rates can be estimated.

Sex ratio

From examination of stock samples, the sex ratio and proportion of mature and immature fish can be determined.

Stock size

There are two principal ways in which estimates of total numbers in fish stocks may be attempted. One is the tagging and recapture method (Cormack, 1968). A known number of tagged fish are released into the sea, and time allowed for dispersal. Records are then kept of the numbers of tagged and untagged fish that are captured. There are several sources of error, but taking the simplest case where mortality can be ignored, where no tags are lost, where experience of capture, tagging and release has not taught the fish to avoid nets and where the tagged fish are evenly distributed throughout a stock unaffected by migrations, the total population could be obtained from the relationship:

No. of tagged fish released X Total no. of fish caught

No. of tagged fish recaptured

Years of age 3 456789 10 11

1—

19

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1923

P7Z

20

19

p 18;

1924 TTI—TT5l_

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20

19

1925

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22

I

20

1926

24;

23

22

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1

23"

22

21;

1928

nr

25

T-K

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26

25

8 '24;

1930

fir

(27;

—fed—

29

28

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26

1932

3456789 10 11

3456789 10 11

Years of age

Figure 9.33 Ages of East Anglian herring shoals, 1923-32. The numbers identify the year in which each cohort of fishes was born.

(From Hodgson, W.C. (1934). The natural history of the herring of the southern North Sea. London; Arnold. By courtesy of the Buckland Foundation.)

As an illustration, Graham gives the following figures. Over a period of years before 1914 the results of tagging experiments on North Sea plaice indicated a fishing mortality of about 70 per cent of the stock. Average landings of plaice from the area being some 50 000 tonnes, the stock weight was presumably about 70 000 tonnes. Allowing 3000 fish to the tonne gives an average stock of 210 000 000.

Tagging experiments have demonstrated that in some areas fishery for plaice is very intensive. In experiments in the North Sea, over 30 per cent of tagged plaice have been recaptured within 12 months of their release. By relating the percentage recapture of tagged plaice to the total landings of the commercial fishery, an assessment of the total plaice population of the North Sea has been made.

An alternative method is based on egg counts. We have previously referred to the method of plotting egg density contours. From these, an estimate can be made of the total number of eggs laid in a season within a spawning area. If the average number of eggs produced by spawning females is known, the total number of spawning females may be determined. If the sex ratio is known, the total number of spawning males can also be calculated. Taking account of the proportion of the population that are immature, and any fish that may spawn outside the main spawning area, a calculation may be made of total stock numbers.

This method was applied by Wollaston to estimate the plaice population of the southern North Sea. In 1914, an exceptionally good spawning year, the total egg production in the Flemish Bight spawning area was reckoned to be about 3.5 X 1012. Taking a mean fecundity of 70 000 eggs per female plaice, the number of females spawning in this area would be:

3.5 X 1012

70 000

50 000 000

This figure must be doubled to allow for the number of mature males, and should probably be about doubled again to allow for plaice spawning in other parts of the southern North Sea, making 200 000 000. As approximately half of the fishable stock are immature fish this gives a final figure for the total fishable population of 400 000 000 plaice. These figures (Graham 1956) are a revision of Wollaston's original estimate, which is now regarded as considerably too low, his figure of 200 000 for the mean fecundity of plaice probably being too high by a factor of about three.

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