SEA, PRODUCTIVITY OF
The sea may be looked upon as a vast farm from which the human race may draw foodstuffs such as fish, crayfish, shellfish, and seaweeds. All the elements of a farm are to be found: the soil bacteria, the pastures, the animals which live on the pastures. The main differences from a farm lie, first, in man's inability to “improve” it by any such means as adding fertilisers or breeding stock, and, secondly, in the relative complexity of its structure. The fertilisers, that is, the essential minerals such as nitrates and phosphates, come from the land in rivers, from the sea bed by the action of waves, currents, and up-wellings, or from the living organisms themselves when they die and are broken down by bacteria. The plants include the seaweeds around the shore, but far more important are the tiny single-celled plant plankton or phytoplankton. Most of these plants live near the surface, in the first 100 ft or so, because, like other plants, they require light. They also possess the green pigment, chlorophyll, which enables them to use the light energy from the sun to build the complex organic compounds of which their bodies are made. The building materials are inorganic compounds of carbon, nitrogen, phosphorus, and many other elements in smaller quantities. Various attempts have been made to determine the primary production of the sea, that is, the amount of plant material which is formed in a given time. The Danish Galathea expedition which visited New Zealand in 1952 added radioactive carbon to small samples of water all over the world and, by means of a geiger counter, found how much of this made its way into plant tissue. From this they estimated that 40,000 million tons of organic matter were manufactured in the oceans of the world during a year. Many scientists do not accept this figure, but all agree that the amount is very large.
On a farm the crop is immediately edible by man, or else is eaten by the cattle or sheep which in turn are eaten, so that there are only one or two life stages between inorganic material and human food. But it is rare for humans to eat the sea plants or even plant-eating animals. There are, however, a few exceptions such as certain seaweeds (which, for example, form an important ingredient in ice cream), the toheroa, and yellow-eyed mullet, (or herring), both of which live mainly on phytoplankton. In general, however, the phytoplankton are eaten directly by small zooplankton. Neither is really an attractive item of food, nor very readily harvested in large quantities. As many, however, are edible, it is possible that perhaps one day we will find a use for them. For instance, the tiny crab larvae which sometimes occur in enormous numbers in the Hauraki Gulf are quite palatable when fried in batter. Usually, however, several stages in the food chain must be passed before a fish suitable for the table emerges. The diatom is eaten by a copepod, the copepod by a pilchard, and the pilchard by a kahawai or a hapuku. Again, the chain might be dinoflagellate-pipi-snapper-shark, though most people would prefer to stop at the snapper stage. Sometimes the process may take a step backward in the size scale, as when a large fish dies and is eaten by crabs, or is partly broken down by bacteria to become food for starfish or heart urchins, in turn to be again devoured by snapper. All of this is very inefficient since animals cannot synthesise organic material but only “burn” it, and every new step in the food cycle represents wastage of the original plant material.