SHAGS AND TROUT

Forest and Bird, Issue 53, 1 August 1939, Page 11

SHAGS AND TROUT

IT IS FUNDAMENTALS THAT COUNT

AN article which appeared in “Forest and Bird” of February, 1938, described how the Peruvian guano supplies were furnished mainly by countless thousands of a species of shag, which is related to the Stewart Island shag. Myriads of these shags feed upon vast shoals of small fish, as also do great numbers of larger fish. Off the Peruvian coast, it was stated, there are more birds and more fish than anywhere else in the world, except perhaps in the vicinity of the sub-antarctic islands. This is the whole story so far as the superficial observer would see and if New Zealand had such a natural asset and wished to increase the extraordinary numbers of fish, the call would be to kill the shags with the inevitable result that the fish would initially increase and later become slab-like and poor in condition. Disease would then control the numbers of fish as the shags did, only in a much severer and more drastic manner. The superficial observer would not take into account what is the foundation of the presence of such great quantities of birds and fish which are to be found along the coast of Peru. But the biologist would point out that the whole lifecycle is based upon the infinitesimally small diatoms which are continuously ejected in their billions by the cold Humbolt ocean current. Upon these the small fish feed and in their turn form food for larger creatures. Increase the numbers of these diatoms, if such was possible, and both the birds and all fish will automatically increase in ratio. Totally destroy these diatoms, and the whole structure of birds and fish will tumble like a pack of cards and disappear. That kind of disturbance of Nature’s balance is even now taking place in New Zealand rivers and streams owing to forest destruction on the highlands. Thus the habitat of trout and other fish is yearly becoming more and more untenable for them because of the destruction of fish food and cover, following the erratic behaviour of the rivers, with their excessive floods which roll masses of destructive stones along the river-beds. Thus all living creatures in the rivers have the ever-recurring risk of being crushed or choked to death with silt. Millions and millions of fish-food creatures

must be killed as well as many trout. After floods come excessive low water-level periods when large areas of the river-bed are dry altogether, or contain merely pools of overheated shallow water. Then more fish-food and trout succumb. River pollution must be added to all the happenings caused by forest destruction on the uplands. The superficial observer, however, is content to view the lessening of the quantities of trout through the big end of the telescope, and blames shags without even naming the species some other native bird. That is the superficial way of looking at the matter and the easiest way to arrive at erroneous conclusions. Start at the diatom end and work upwards not downwards, and the true story will unfold itself. The crying need is undoubtedly the taming of our rivers and streams in order to produce the conditions necessary for an ample supply of fish-food.