In this somewhat eccentric article, the naturalist John Arthur Thomson reviews “Ocean Research and the Great Fisheries”, a “luminous” book by GCL Howell, and in doing so explores the infinite and interlocked web of the lifecycles of fish and other sea creatures. The “fundamental fact regarding the harvest of the sea”, he writes, is that “fishes are merely links in an endless nutritive chain”. What breaks this chain is humans’ insistence of catching “food-fish” recklessly and without careful consideration of sustainability. During the First World War there was a rest in fishing, but afterwards the number of plaice fished increased. The reason for the scarcity of “good-sized” fish remained a topic of debate for scientists in 1922, as did the extent of humans’ negative impact on sealife. “It may be that a deleterious influence, such, perhaps, as oil on the waters, may operate disastrously… This question requires further investigation.”
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The peculiarity of the harvest of the sea is that man is ever gathering it in, and yet giving almost nothing back. If he did this on land there would soon come about an exhaustion of the soil; hence the use of fertilisers. But on the sea the exhaustion is rarely more than temporary and local. It can be soon compensated for by giving the area a rest. How is it that man can take so much out of the sea and put almost nothing in? As is well known, the answer is to be found in the circulation of matter.
Microscopic marine organisms possessed of chlorophyll utilise the energy of the sunlight to build up carbon-compounds from carbon dioxide and water; they form the floating sea-meadows on which small animals depend, or they give rise to organic dust which is borne outwards and downwards on the sloping floor of the sea. From one incarnation to another the nutritive material is raised; and the fishes are fed. This is the fundamental fact regarding the harvest of the sea: fishes are merely links in an endless nutritive chain.
“What have I to do with barnacles?” the skipper may say; but the minute roving larva of the shore-barnacle is an important item in the dietary of young turbot when about one-tenth of an inch long. We cannot say that the welter of water-fleas and other small fry is there in the sea in order that there may be fishes; but it is a fact that the broad foundation of prolific minutiae makes the great superstructure of fishes and fisheries possible. In the light of Mr GCL Howell’s luminous book on Ocean Research and the Great Fisheries (Clarendon Press, 220 pages, 20 plates, 26 figures, 8 maps) we wish to dwell on this fact.
The patient investigations of workers like Dr Marie Lebour on the contents of the food-canal of larval fishes, often no longer than a four-lettered word of this type, have shown the fundamental importance of the minute animals of the sea. The very young stages of plaice and herring and some other fishes eat diatoms and other unicellular organisms, but these are of much more moment indirectly, inasmuch as they afford sustenance to the small animals on which young fishes mainly depend.
By far the greater part of the food of very young fishes consists of larval Copepodsminute Crustaceans which swarm in incredible numbers in the open sea. They feed on diatoms and the like, eating almost one-tenth of their own weight in a day, and fishes feed on them. The food-canal of a single herring may be crammed with a compact pink bolus of 60,000 specimens of one species of Copepod!
Next in importance are the bivalved Cladocera, belonging to another order of minute Crustaceans popularly called water-fleas. Then there are the free-swimming larvae of rock-barnacles, which are widespread in open waters and are devoured in prodigious numbers; and along with them we may notice the Iarvae of other marine animals, such as Gasteropods and Bivalves. Miss Lebour has proved up to the hilt the cumulative importance of microscopic living creatures in the sustenance of one of the greatest of human industries. She has shown that young fishes, like herring and brill, begin to eat very early, while their yolk-sac is still unexhausted; that they feed very specifically or daintily, preferring one particular kind of Crustacean and sticking to it, thus affecting their struggle for existence; and that the presence or absence of the right kind of food at the right time means life or death to the fishes. It is obvious that the distribution of food-fishes, a matter of great practical importance to man, must be bound up with the distribution of the “small game of the sea”; and it may be that a deleterious influence, such, perhaps, as oil on the waters, may operate disastrously, not directly on the fishes themselves, but on the delicate creatures on which they feed. This question requires further investigation.
In his interesting book Mr Howell lays emphasis on another outstanding fact regarding the harvest of the sea. “We know that some cod and haddock and herrings live to fifteen or eighteen years; that some of them breed for at least fifteen years in succession; but, above all, that the stock of fishes does not (like the stock of civilised men) receive an approximately even accretion each year. There are almost incredible differences between the herring and cod populations of one year and the succeeding year; and these variations are due, apparently, to an infant mortality which in a normal year is, by human standards, cataclysmic.”
It may be explained that the estimate of the age of a fish is based on “scale-reading”, for the abundant summer growth is registered on the scale as a broad transparent zone, and the restricted winter growth as a narrow opaque ring; and it is thus possible, many experts believe, to tell the age of the fish by counting the rings on the scale, just as one tells the age of a tree by counting the rings on its cut stem. Thus it becomes possible to say that “in 1919 people were eating one lot of Norwegian herrings which were fifteen years old, and another lot which were seven. There were very few of intermediate ages. The years 1904 and 1912 were ‘good’ (or perhaps abnormal) spawning years; the intervening years were ‘bad’ (or perhaps normal).” We see then what interest attaches to the life-histories of food-fishes and to the causes influencing the infantile mortality. Following Mr Howell, let us take a single concrete example, namely, the plaice.
Britain’s catch of this very valuable food-fish amounted in 1918 to 87,872 tons, with a value of £1,095,894. British fishermen landed more than all the rest of their fishing neighbours taken together; six East Coast ports dealt with 90 per cent of the catch; and quite 70 per cent of the English catch came from the North Sea. So the problem of the plaice comes very near home.
A large female plaice may contain half a million eggs, which are liberated, in the early months of the year, chiefly in waters between 20 and 80 fathoms in depth. How many of the eggs are actually fertilised is an unanswered question. They drift about in the open sea, transparent spheres about a twelfth of an inch in diameter. In a fortnight to a month, according as the temperature is comparatively high or low (about 50°F to about 41°F), they hatch out, giving rise to larvae about two-sevenths of an inch in length. For the first few days the larval plaice absorbs the legacy of food in its yolk-sac, and drifts about in the water as helpless as the egg. About the fourth day the mouth begins to be used and the young creature makes experiments with diatoms and microscopic larvae of molluscs and the like. This post-larval period, which lasts for about a week – until the yolk is fully absorbed – is a very critical time. “If the larvae have been unable to obtain floating food to eke out the contents of the yolk from about the fourth day of their lives, they are likely to be too weak to survive the critical period when they must learn to forage successfully or die.”
During the next stage the young plaice swims like a young cod or mackerel; it feeds on the young of various small Crustaceans; when it is three weeks old it is three-eighths of an inch long. Flat fish, resting and swimming on one side – the left in plaice, sole, flounder and halibut, the right in turbot, bill and megrim – have evolved from the “round fish” type, and in their individual development they recapitulate the racial evolution. For they pass through a symmetrical stage with an eye and pigment on each side as usual, before they become asymmetrical, with both eyes on one side which has also all the pigment. About the 30th day, when the young plaice takes to eating small Copepods as well as larvae, the left eye begins to move round to the up-turned, illumined right side, a remarkable process completed about the 45th day. The “flat fish” fry, about three-fifths of an inch long, sink on to the floor of the sea and change to a diet of small worms, shrimp larvae and other Crustaceans which live on the bottom. The most riskful chapters of the life-history are now over.
Towards the end of April the plaice fry are found gradually moving inshore from the spawning areas to the “nurseries ” – shallow waters with a fine sandy bottom, where they feed and grow in huge congregations. At the end of their first summer they average three-and-a-half inches in length; a year later, five-and-three-quarter inches; in the third autumn they are eight-and-a-half inches long and weigh about three ounces. The males are usually mature at the end of the fourth summer, the females a year later, the size depending mainly on the richness of the feeding grounds. Marking experiments have shown that plaice roam freely from one feeding-ground to another; there is a general deployment northwards and offshore in summer, and concentration (generally southward) to the various spawning grounds in winter.
For many years before the war the fishermen spoke of the “fishing out” of the plaice. Good-sized fish seemed becoming scarcer in the available harvest-fields of the North Sea. But there was and is difference of opinion, especially among scientific experts, as to the precise cause of the growing scarcity. According to one school the reduction of supply was due to the destruction of large numbers of immature fish, fewer being left to grow up. The remedies proposed were various methods of restricting this wastefulness. But according to another school, the reduction of supply was due to the fact that there were too many big fish on the feeding-grounds. It was said that the big fellows left the small ones too little room and too little food. Thus they ate up the parent cockles which should have produced numbers of young cockles on which small plaice feed. It was recommended that millions of young plaice should be transplanted to areas where there would be less crowding and more food. But the first school responded that there is no appreciable competition between big plaice and little, and that the shortage was due to the large catch of immature fish. As the old fishermen said: “If you wish to live and thrive, save a little dab alive.”
Experiments are in progress to determine what is true in the opposing views; meanwhile, the practical fact is that the rest enforced by the war has been followed by increased catches of plaice and by an increased number of big ones. As an illustration of the harvest of the sea we have taken the story of the plaice; for the not less illuminating stories of other food-fishes, such as turbot, sole, halibut, cod, haddock, whiting, herring and mackerel, we must refer to Mr Howell’s admirable and timely book. It should have a big sale at the British Association meeting at Hull, where there is to be much discussion of the harvest of the sea.
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