Warning On Eutrophication

Press, Volume CVI, Issue 31223, 22 November 1966, Page 3

Warning On Eutrophication

(N.Z. Press Association) NELSON, Nov. 21.

Serious stock and fish deaths in New Zealand lakes and dams could develop very rapidly from enrichment of their waters by fertilisers, said a world authority on the food and growth of herbivorous fish. Dr. G. A. Prowse, in Nelson today.

Dr. Prowse, director of the Tropical Fish Culture Institute, at Berendam, Malaysia, said toxic blue-green algae, a microscopic organism, had already reached a critical stage on Lake Tutira, Hawke's Bay, and would soon reach a criti-

cai stage on Lake Karapiro, Waikato.

Water weeds and algae were becoming a serious problem in inland waters all over the world as fertiliser was deposited in increasing quantities in slow-flowing lakes and rivers.

The weed might be regarded as a nuisance to fishermen unable to launch their boats, but its danger was much more significant than that The day-to-night fluctuations in oxygen levels could become so great that at some stages there was virtually no oxygen present and fish died in large numbers.

The algae danger could be even more severe because it struck suddenly, said Dr. Prowse. The toxic algae was the final stage of a series of nontoxic algae that built up

i steadily in enriched waters, and suddenly culminated in an accumulation of bluegreen algae “blooms” forming a powdery effect on the surface. Deaths of both fish and farm stock had definitely been traced to this source. Besides sounding a warning, Dr. Prowse offered a proven remedy. Through their studies in Malaysia, he and his staff have found that two varieties of carp—the grass and silver carp—can control these growths. Carp will graze the nontoxic algae and thus prevent the later toxic stages developping. These had proved much more successful than chemical controls which killed the weed and algae in situ and could create an even greater problem with continuing fertiliser build-up in impounded i waters.

'“I don't recommend the introduction of these carp lightly,” said Dr. Prowse, “but I am absolutely certain that they will not spawn in New Zealand waters.

“Their only spawning grounds known are in two rivers in China, one small river in Japan, and one lake in Taiwan. They’ve been trying for years unsuccessfully to breed them elsewhere, so they’re not likely to become a menace here.

“They do not compete for food with trout or other fish and they live amicably with them. They’re also good eating.

“If New Zealand doesn’t control the weed in her lakes soon, she’ll finish up with no fish. From questioning about New Zealand waters I find the authorities know very little about your rivers.”