FOOD OF FISH

New Zealand Herald, Volume LXXIII, Issue 22518, 8 September 1936, Page 7

FOOD OF FISH

HARBOUR BED SURVEY VALUABLE INFORMATION SCHNAPPER FEEDING GROUNDS In the light of the researches of Mr. A. W. B. Powell, conchologist of the Auckland War Memorial Museum, in his comprehensive survey of the sea bottom of Waitemata Harbour, it is considered that it may be possible to indicate approximately the feeding grounds of the schnappor, month by month. The dominant animals in the sea bottom figure prominently in the food cycle of the schnapper, and the various communities have been plotted and defined on charts. Data was made available to Mr. Powell by the Fisheries Department, which indicated the seasonal diet changes of the schnapper. With the correlation o:f this information with the types of animal communities defined on the charts, an approximate idea should be possible of i the feeding grounds of the schnapper at various periods.

Eflect of Reclamation

Dealing with his opinion that the reclamation of mud fiai;s has affected the feed of the schnapper, Mr. Powell points out that a sea grass, zostora, which grows on mud flats, harbours the microscopic diatoms that are a food for many other primitive animals. These in turn make food for the schnapper. Further than that, masses of zostera decay in duo course and are washed out by tides. This decaying vegetation impregnates all the soft muds and supplies rich foods for marine worms, heart urchins and other burrowing organisms, which are preyed upon by the schnapper. It is evident that with the reclamation of 7 many mud flats big areas of zostera have been removed from the harbour. This causes a paucity in the food content of the muds and a consequent falling off in the animals dependent on such form of foods, which form the basic food of the schnapper. So possibly one of the main reasons why the fish is not so plentiful in the harbour as before is not so much because of anything directly affecting the schnapper, but the indirect one of the reduction of the food supply. It is considered a significant fact that in the outflow channels, where there is a concentration of decaying matter, were found the richest animal communities in the harbour or the gulf.

Harbour Pollution The dredging operations by Mr. Powell in making hia survey did not reveal anything detrimental to the organisms of the sea bottom caused by harbour pollution, except in the immediate vicinity of the sewer outfall. Generally speaking, the condition of the sea bottom appeared normal; indeed, the richest bottom from the point of view of organisms suitable for fish feeding was found in the vicinity of the steamer anchorage off Hobson Point, which is fairly near to the eewer outfall. If pollution could be expected in the sea, one would expect that it would be in that neighbourhood. The laboratory and dredging results form the subject of several charts and diagrams. One cross-section of the material found indicates the changes in the animal community and arouses speculation of how certain factors came into operation and probable changes. Naturally, changes may occur, from time' to. time! "but the. present material should serve as a btisis in determining both the extent and the reasons for such change's. On the physical side alone, Mr. ■ Powell's ,labours have been heavy. Hauling -in* a weighty hand dredge to the launch, sometimes in deep water, was often a difficult task. While most of the harbour was found to be four or five fathoms in depthi the main channel was six to . eight fathoms, generally speaking, although there were much deeper places, notably off Bean Rock, where the line went to 15 fathoms.