Since the biological processes are so complex and (in a chemical sense) inefficient, it is very difficult to estimate with any certainty how much food can be drawn, for example, from the waters around New Zealand. We do know, however, that the most productive waters are the shallow ones, that is, the waters less than 100 fathoms forming the Continental Shelf. This is because the plants need both light (from the surface) and minerals (from the shore or the sea bottom), and conditions are ideal only when both are close together. Some waters are more productive because an up-welling from the deep brings nutrients to the surface. There is one such region extending up the west coast of the North Island of New Zealand. Unfortunately, this coast is stormy with few good harbours so that it is difficult for scientists to study or for fishermen to work. But these waters do produce vast quantities or “blooms” of diatoms and the “primary” production on some days is thousands of times greater than that in average ocean waters. Whether these blooms provide food for any useful species of fish is not known, though we do know that without them there would be no toheroas on west coast beaches.

Today there is a gathering interest in mid-ocean fisheries. While the deep waters of the Pacific do not contain much concentrated fish food, large and powerful swimmers like the tunas are able to range over vast areas for their meals, so that the production of many square miles of ocean may thus be concentrated in one compact school of fish. In these regions fish have harvested the plankton far more efficiently than would be possible by man, and it is always a good idea to let nature do the work for us.

by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.