USES AND USAGE OF MARINE PLANTS

The fisheries statistics published by F.A.O. show that the total world harvest of aquatic organisms increased steadily from about 20 million tons in 1948 to about 70 million tons in 1970, but remained more or less constant from 1970 to 1977. Since three-quarters of this total harvest was marine fish, these figures suggest that the sea fisheries of the world have almost reached the limits imposed by the primary productivity of the seas (see p. 89), and there is now general agreement among fisheries scientists — if not yet among fishermen — that the total marine harvest cannot be increased substantially by hunting the familiar species offish with ever greater intensity. Most of these commercially important fish are secondary or tertiary carnivores, however, and there would be greater scope for expansion of marine fisheries if organisms lower down the trophic pyramid could be harvested and utilized. The logical extension of this idea, of course, is to harvest marine plants rather than marine animals, but the oceanic phytoplankton that are responsible for 75% of the total marine primary productivity (see Table 4.3) are too small and too thinly distributed to be harvestable. It is, therefore, the marine macrophytes, and mainly the seaweeds, which are most likely to be of value, and this chapter is concerned with the current and potential uses for such plants, and the best ways of exploiting and managing this resource. TRADITIONAL USES: HUMAN FOOD AND AGRICULTURE The collection and sale of seaweeds for human consumption are now very localized in the western world, but three species of red algae are still utilized to a limited extent.