Man gets food from the sea essentially by hunting and gathering. Yet the farming of fish and shellfish has been pursued for some 2,000 years, and its potentialities are far from being exhausted.

 A major concern of modern man is the possibility that the earth will not be able to produce enough food to nourish its expanding population. A particularly controversial issue is the question of how much food can ultimately be obtained from the sea. It is argued on the one hand that, on the basis of area, the oceans receive more than twice as much solar energy-the prime source of all biological productivity as the land. This suggests that the oceans' potential productivity should greatly exceed the lands. On the other hand, most of the sea is biologically a desert. Its fertile areas are found where runoff from the land or the upwelling of nutrient rich deep water fertilizes the surface water and stimulates the growth of marine plants, the photosynthetic organisms on which all other marine life depends. Even at today's high level of exploitation the fisheries of the world provide only a small fraction of human food needs, and there is some danger that they may supply even less in the future because of overfishing.

Does this mean that there is no hope of increasing our yield of food from the sea? I do not think so. It does mean, however, that instead of concentrating exclusively on more efficient means of fishing we must also learn io develop the potential of the oceans by farming them, just as early man learned that farming rather than hunting was the more effective method of feeding a human population. The purpose of this article is to examine briefly the contribution marine farming now makes to our food supply, and to consider some possibilities for its future role.

Ancient Marine Farming

Marine farming has a long history. The earliest type of farming was the raising of oysters. Laws concerning oyster-raising in Japan go back to well before the time of Christ. Aristotle discusses the cultivation of oyst6rs in Greece, and Pliny gives details of Roman oyster-farming in the early decades of the Christian Era. By the 18th century the natural oyster beds in France were beginning to be overexploited and were saved only by extensive developments in rearing practices.

 Carp (Cyprinus carpio) were commonly raised in European freshwater ponds in both Roman and medieval times. Records concerning the regulation of salt or brackish ponds for raising milkfish (Chanos chanos) in Java date back to the 15th century. Carp and milkfish are both herbivores that thrive on a diet of aquatic plants. Oysters, as filter-feeders, can also be loosely classified as herbivores.

The humble Oyster

Oysters are particularly appropriate for marine farming because their spawn can be collected and used for seeding new areas of cultivation. An oyster produces more than 100 million eggs at a single spawning. The egg soon develops into a free-swimming larval form, known as a veliger, which settles to the bottom after two or three weeks. Veligers attach themselves to any clean surface and develop into miniature adult oysters, called 11 spat" because oystermen once believed the adult oysters spat them out. At this point the oyster farmer enters the picture. He distributes a supply of "cultch": clean material with a smooth, bard surface, such as old oyster shell or ceramic We. The cultch receives a "set" of spat and is then used to seed new oyster beds.

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The bottom is prepared for seeding by removing as many natural enemies of the oyster as possible. In the eastern U.S. this is usually done by dragging a rope mat along the bottom to sweep the area clear of starfish, one of the major predators. In France, where more intensive labor is employed, the spat are usually planted on the exposed bottom of an estuary at low tide. The predators are removed by hand and the oyster bed is fenced to prevent their return. The oysters are moved after a few years to claires, special fattening areas where the water is rich in diatoms. This produces oysters of improved taste and color. When the oysters have reached marketable size, they are moved again to shallower water, where they must stay closed for longer periods at low tide. The French oystermen believe this treatment prepares the oysters for their trip to the market.

A significant advance in oyster-farming is the use of suspension cultures. This method, pioneered in Japan, is now spreading to the rest of the world. The spat are collected on shells that are strung in long bundles and immersed in tidal water. The strings, which do not touch the bottom, are sometimes attached to stakes but more generally are attached to rafts. The suspension method has a number of advantages over growth on the bottom. The oysters are protected from predators and from silting, and they feed on the suspended food in the entire column of water rather than being limited to what reaches the bottom. The result is faster growth, rounder shape and superior flavor.

 In small areas of Japan's Inland Sea suspension cultures of oysters annually yield 46,000 pounds of shucked meats per acre of cultivated area. This does not mean that one can multiply the total acreage of the Inland Sea by this figure to estimate the potential productivity of the area. Tidal flow allows the anchored oysters to filter much larger volumes of water than surround them at any given time. In addition, inshore waters are generally more productive than those farther from land. The figure does illustrate, however, the production of meat that is possible with our present farming practices in inshore waters.

 Luther Blount of Warren, R.I., has tested oyster suspension cultures in Rhode Island waters over the past several years, using spat set on scallop shells. Blount spaces seven scallop shells well apart on each suspension string. At the end of seven months' growth he harvested one group of suspended oysters from 3,200 square feet of float area. The oysters weighed nearly 40,000 pounds and yielded 2,500 pounds of oyster meat.

 His experience suggests that the coastal waters of the eastern U.S. might yield more than 16,000 tons of meat per square mile of float per year.

The Mighty Mussel

Although the farming of oysters in suspension cultures is a comparatively recent development, the same technique has long been used in Europe to raise mussels. The Bay of Vigo is one of the many Spanish ports where acres of mussel floats are a common sight. French and Italian mussel growers are less inclined to use rafts. Their mussel strings are usually suspended from stakes set in the estuary bottom.

 John H. Ryther and G. C. Mattbiessen of the Woods Hole Oceanographic Institution have studied the yields obtained by the mussel farmers of Vigo. The annual harvest produces an average of 240,000 pounds of mussel meat per acre. This is equivalent to 70,000 tons of meat per square mile of float, or better than four times the yield of oysters in suspension cultures in the U.S. and Japan.

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Fish are Harder

The farming of fish is more difficult than the farming of bivalves for at least two reasons. First, the fish, being motile, must be held in ponds. Second, the saltwater species that are most commonly farmed-milkfish and mullet (Mugil)breed only at sea. This means that the fry have to be caught where and when they occur naturally, and in some years the supply is not adequate. Furthermore, unwanted species and predators have to be sorted out by band, with the inevitable result that some of both are introduced into the ponds along with the desired species.

 In spite of such handicaps pond farming is remarkably productive. In the Philippine Republic, for example, the annual milkfish harvest is estimated at some 21,000 tons and the productivity of the ponds averages 78 tons per square mile. A comparable estimate for the annual productivity of free- swimming fish in coastal waters, as calculated by Ryther and Matthiessen, falls between six and 17 tons per square mile. In the Philippines, moreover, it is not customary to enrich the pond waters artificially, a process that accelerates the growth of the fishes' plant food. In Taiwan, where milkfish ponds are fertilized, the average annual yield is 520 tons per square mile, and in Indonesia, where sewage is diverted into the ponds in place of commercial fertilizer, the annual yield reaches 1,300 tons per square mile.

 Fish farming in Asia is still a long way from reaching its maximum potential. The United Nations Food and Agriculture Organization has calculated that more than 140,000 square miles of land in southern and eastern Asia could be added to the area already devoted to milkfish husbandry. Even if this additional area were no more productive than the ponds of Taiwan, its yield would be more than today's total catch from all the world's oceans. Assuming an adequate supply of milkfish fry, such an increase could be achieved without any technological advance over present methods of pond farming, Even the fry problem may be close to solution. Mullet, a largely herbivorous fish, is now extensively farmed not only in Hawaii and China but also in India and even in Israel. Recently it has proved possible to breed mullet in the laboratory, which brings closer the prospect of mullet hatcheries and a steady supply of mullet by.

HIGH PRODUCTIVITY of upwelling areas and coastal waters, in contrast to the low productivity of the open sea, is not due only to greater mineral enrichment. In upwelling areas (a) the phytoplankton at the bottom of the food chain are usually aggregates of colonial diatoms that are large enough to feed fish of exploitable size. As a result the food chain is very short, with an average of 1.5 steps. The food chain in coastal water (b) is longer, averaging 3.5 steps. In the open sea (c), where phytoplankton at the bottom of the chain are widely scattered, single-celled diatoms, five steps are

In looking for ways to increase the potential yield of fishponds throughout the world, we are faced with two problems. The first is whether or not we can overcome the sanitary and aesthetic objections to using sewage as a growth stimulant. This is a complex question, but it is worth noting that some practical progress is being made by transferring shellfish from polluted areas to unporluted ones for a period of "cleaning" before shipment to market.   

An equally important question is to what degree commercial fertilizer could increase productivity. Oysters or mussels suspended from rafts in small ponds should provide a simple test organism for such experiments, and they are particularly appropriate because of their high natural yields.

 The effect of adding commercial fertilizer to Long Island Sound water has been studied by Victor L. Loosanoff of the U.S. Fish and Wildlife Service. He wanted to produce large amount of marine plants as food for experiments in rearing oysters and clams. He found marked stimulation of plant growth, but the zooplankton-the marine animals in the water-also grew and ate the plants, thus competing with the shellfish for food. After trying various methods of inhibiting the zooplankton's growth, Loosanoff finally came to the use of pure cultures of the plants, but this would be a very expensive practice on a commercial scale.

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The growing of both marine plants and marine animals in a pond could be rewarding, and the zooplankton could be converted from a pest to an asset by adding an organism that feeds on them. Rainbow trout might fill this requirement: they are carnivorous, adapt readily to salt water and are said to grow faster and have a better flavor than when they live in fresh water. In addition they are readily available from hatcheries and have a good market value. To dispose of the inevitable organic debris sinking to the bottom of the pond one might add clams and a few lobsters, since both are in demand and their young are being reared in hatcheries and could be obtained.

Follow the Principles

It seems to me of the utmost importance that we follow the principles of ecology in our efforts to develop marine farming, by working with nature to establish balanced, stable communities rather than by supporting large single crops artificially, as we do on land, with what are now becoming recognized as disastrous side effects. Perhaps the single most exciting challenge we have in marine farming is this opportunity to make a new start in the production of food, utilizing the ecological knowledge now available.

If the results of the pond experiments are satisfactory, it is technically feasible to consider applying fertilizer to estuaries or even to the open ocean. The mechanical problem here is that the applied fertilizer sinks to the bottom in estuaries and tends to become absorbed by mud, and in the open ocean it simply sinks below the zone where the marine plant life grows. A solution for this problem would be to combine the fertilizer with some floating material that would disintegrate and liberate it slowly. The political and legal problems of controlling the harvest of the crop seem more difficult than the technical one of developing floating fertilizer.

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