Pearls & Parasites

by Sir A. E. Shipley

It is interesting to note the first results of the more exact system introduced in 1903. Considering only the fish caught in the North Sea and landed on the East Coast, we note a marked decline in the total catch of steam-trawlers during the years 1904, 1905, and 1906, and an increase in the catch of sailing trawlers. The former declined from 4¾ million cwt. in 1903 to 3¾ million cwt. in 1905; the latter increased from 277,000 cwt. in 1903 to 296,000 cwt. in 1905. It is shown, however, that these changes were accompanied by a considerable fall in the amount of fishing by steam-trawlers and a rise in the case of the sailing trawlers, so that inferences concerning impoverishment or the reverse would be premature. Nevertheless a fall in the abundance of haddock may be inferred from the fact that not only the total catch of this species, but also the average catch of the boats fell off continuously from 8·4 cwt. per diem in 1903 to 6·1 cwt. per diem in 1905. The fall is also seen to be mainly due to a scarcity of ‘small’ haddocks in 1904 and 1905 as compared with 1903. With the conclusions to which such data as these are likely to lead we are not now concerned; but these examples are sufficient to show that the official statistics are no longer a confused mass of useless figures, but a rational and fairly accurate system capable of analysis.

We have now to examine those experimental branches of investigation which are equally necessary for the effective solution of fishery problems. The chief possible causes of an impoverishment of the sea are three in number. First, as in the central United States the accumulated richness of a virgin soil produced at first huge crops, so, when fishing began in the North Sea an accumulated wealth, both in the number and in the greater size of the individual fish, was drawn upon. This ‘accumulated stock’ has been fished out.

Secondly, a given area of the sea, like a given area of land can support but a limited quantity of produce. There is a definite amount of food for fish in a definite volume of sea; a limit is therefore set to the number of fish in that volume of water. Professor Hensen and Professor Brandt, of Kiel, have shown that a square metre of the Baltic produces an average of 150 grammes of dry organic material in the shape of diatoms, copepods, and other floating organisms. A similar area of land produces 180 grammes of ultimate food-substance. The productivity of the sea is judged on this basis to be about 20 per cent. less than that of the land. The actual amount is of less importance than the consequences it entails. If the methods of fishing are more destructive of one species than another, comparatively worthless species may become dominant in areas where they were formerly scarce, and thus consume the food which should be reserved for their betters. It is commonly reported that the dab has tended to usurp the position formerly taken by the plaice, not only in the Scottish firths, but on the Dogger Bank, in the Devonshire bays, and in other localities. Dr. Garstang, of the Marine Biological Association, tells us that small plaice transplanted to the Dogger Bank in 1904 grew three times as much in weight as did their fellows on the coastal banks; but in the following year they grew only twice as much, owing to the presence of vast quantities of small haddocks, which ate the plaice’s food and were nevertheless too small and worthless themselves to be landed by the fishermen. Yet formerly the Dogger teemed with large plaice and haddock. It was stated to the Royal Commission in 1863 that the fishermen avoided the Bank as causing gluts of fish and depreciation of price; and witnesses from Yarmouth and Hull assured the Commission that between two and three tons of fish, chiefly haddock and plaice, were frequently taken by smacks in a three hours’ haul. As small plaice are confined to the coastal banks, and large plaice are now scarce, it follows that the great food-reserves on the Dogger Bank, which seem providentially designed for the fattening of plaice, are wasted on worthless dabs and baby haddocks. Thus may one cause of impoverishment lead on to another. Perhaps the right remedy in a case like this is to promote the wholesale transplantation of young plaice, as in the case of oysters, mussels, etc. The experiments already made by the Marine Biological Association point strongly in this direction.

Thirdly, the excessive destruction of young fish is another, and perhaps the greatest, cause of the impoverishment of the sea. The destruction is enormous. In the winter of 1882-1883 it was estimated that in the Firth of Forth, the Firth of Tay, and the Moray Firth, 143,000,000 of young herrings and a much greater quantity of sprats were captured. These were mostly sold as manure. Yet the herring does not decrease; it is the flat-fish, the plaice and the sole, that suffer most. In 1896, 368 tons of small fish were seized by the Fishmongers Company at Billingsgate; in 1897, 143 tons; and in 1898, 96 tons. These were sold as manure or destroyed. Mr. Holt estimates that, while over 7,000,000 mature plaice were landed in the port of Grimsby during the year April, 1893, to March, 1894, over 9,000,000 plaice not sexually mature were brought to port; or, taking the trade distinction between ‘small’ and ‘large’ fish, over 6,500,000 plaice under 13 inches in length were landed, as against 9,700,000 over 13 inches. So many as 10,407 young plaice have been taken from a single drag of a shrimp trawl. These are but a few instances out of many, showing the great destruction which is going on among the young of our more valuable food-fishes.

The questions they suggest are still a matter of discussion. Whether even this destruction has an appreciable effect on the adult population is debatable. It does not seem to have affected the herring; and we must not forget the prodigious number of offspring given to fish. The taking of immature fish is not in itself uneconomic, unless by that means we so far reduce the total number that the adult stock begins to dwindle. Sardines are more valuable than their adult form, the pilchard; whitebait, mainly composed of young sprats, with from 1 to 20 per cent. of young herrings, fetch more in the market than the parent form; and so long as the adults exist in sufficient number to keep up the stock of fry, sardine and whitebait fishing is perfectly legitimate.

But, assuming impoverishment from one or other or all of the causes enumerated, we should ask what steps can be taken to check it, especially as regards the more valuable flat-fish. It is at this stage that scientific knowledge becomes particularly important. At least nine out of every ten Acts of restrictive legislation have been shown by experience to be futile, or to have produced results absolutely different from those anticipated. It is equally plain that the failure of these attempts to interfere with the natural course of events has been largely due to inadequate knowledge of the complicated factors which affect the growth, multiplication, and distribution of fish, and of the influence which particular modes of fishing exert upon the sources of supply.

Let us examine the first-mentioned cause of impoverishment, the destruction of the ‘accumulated stock.’ This formula has been eagerly adopted by some who hesitate to admit the existence of any form of over-fishing. It implies that a state of equilibrium is possible between the forces of destruction and the forces of repair; that on virgin territory older individuals tend to accumulate beyond what is necessary for the maintenance of the ‘current stock’; and that their removal entails no real injury to the supply. In scientific terms this means that the average age of mature individuals of a natural stock may be reduced by man to a lower point which represents the economic optimum. The Patagonian cannibals seem to have been early converts to the soundness of this theory. The difference between the Patagonian who eats his mother-in-law and the fisherman who destroys the overgrown plaice is that the former’s actions are deliberate and limited, while the removal of the accumulated stock is not so much an object of the fisherman as an unpremeditated consequence of the intensity with which fishing operations tend to be conducted. Does the fisherman abate his operations when the economic optimum has been reached? Clearly not. He fishes till it ceases to pay; and no other motive affects him. It is plainly a question for scientific inquiry whether, in a given case, the fishery has been prosecuted to excess, and has reduced the average age too far, or not.

On this question the International North Sea Investigations have already thrown valuable light, for the study of the intensity of fishing by means of definite experiments with marked fish has formed an important part of the programme; and the investigation of the age of plaice, cod, and other species has been vigorously prosecuted. According to the latest report of the Council of the Marine Biological Association, more than 7,000 marked plaice have been set free by their staff, and 24 per cent. altogether have been recaptured. Of the medium-sized fish which, furnish the best test of the intensity of fishing, 30 per cent. in twelve months have been captured in the southern part of the North Sea, where sailing trawlers predominate, and 40 per cent. on the Dogger Bank and adjacent grounds, where the fishing is done by steam-trawlers. It seems, however, that some of the fish lose their labels before being caught again. A still closer idea of the severity of the fishing may perhaps be got from another experiment with weighted bottles, which were specially devised by Mr. G. P. Bidder to act as indicators of bottom currents, and were thrown overboard from the Huxley in the winter of 1904-1905, in the southward parts of the North Sea. Out of 600 bottles more than 54 per cent. were returned by trawl fishermen within twelve months. If anything like half the adolescent stock of plaice is taken by our trawlers every year on the deep-sea fishing-grounds, the establishment of the fact must profoundly affect our views as to the causes of depletion and the remedies to be applied; for the fishing in these instances seems not to have been on the so-called ‘small-fish’ grounds or nurseries, but in areas which have always been recognized as legitimate fields of work.

The possibility of determining the age of fish is quite a recent discovery, and is based on the observation that the scales, vertebræ, and especially the ‘otoliths’ or ear-stones of fish, show alternate dark and light rings of growth, corresponding with the summer and winter seasons of the year, exactly like the rings in the wood of trees. Many difficult problems are likely to be cleared up by a knowledge of the age of fish on different fishing-grounds; and, to judge from the scale on which this investigation is being pursued, it will not be long before we may expect something in the nature of an age-census. The Council of the Marine Biological Association have reported no less than 12,000 age-determinations of plaice by their North Sea staff up to June last; and the German and Dutch investigators are working on similar lines.

To conclude our argument, we should now examine the question whether it is possible to determine to what extent and in what manner the destruction of immature fish, which is admittedly enormous, is injurious to the permanent supply. We have already referred to Mr. Holt’s statistics, which showed that 40 per cent. of the plaice landed in Grimsby in the year 1893-1894 were below 13 inches in length. In 1904, 30 per cent. of the plaice landed from the North Sea on the whole East Coast were below 11 inches in length. German statistics show that from 1895 to 1904 there was no sensible increase in the total weight of plaice landed in that country, but the proportion of ‘small’ fish (below 14 inches in length) steadily increased from 68 per cent. in 1895 to 87 per cent. in 1904. There can thus be little doubt that the supply is being maintained only by drawing more and more upon the fish of smaller size and of less value.