Effects of water diversion on the Whanganui whio

Forest and Bird, Volume 22, Issue 4, 1 November 1991, Page 26

Effects of water diversion on the Whanganui whio

by

Forest and Bird energy campaigner

Keith Chapple,

The future of one of the key populations of blue duck (or whio as used here) hangs on the outcome of Court action being taken by Electricorp. This publicly owned corporation is challenging a Planning Tribunal decision for the partial return of water to the upper Wanganui catchment taken for the Tongariro Power Scheme.

ors HE UPPER WHANGANUI CATCH- ". MENT comprises six tributaries: Whakapapa, Okupata, Taurewa, Tawhitikuri, Mangatepopo, uppermost Whanganui. These rivers are all prime whio habitat. Tributaries of these streams are beheaded the water being piped under the hills to the Tongariro Power Scheme. The Whakapapa has a mere five percent residual flow, while the other tributaries are left dry below the intake. This is known as the Western diversion. Little is known about whio on the Tongariro Scheme’s Eastern diversion (Rangipo), which comprises more dam sites and more be-headed rivers. The Tongariro river is the only one in the power scheme to have been studied in depth. The whio population on that river plummeted from 32 to five when water diversion commenced in 1984. Numbers may have risen a little since then.

Early population density

The size of the Whanganui whio population before the power scheme began is uncertain. No population counts or studies were carried out as part of the power scheme investigation work. The only biological study of note is the ‘Woods Report 1957’ which was concerned only with eels and whether they would migrate through the tunnels and infest Lake Taupo. Whio were known to populate the Whakapapa/Whanganui confluence and were recorded in the early 1950s. The population at this time would certainly have been in the region of hundreds of pairs. The Whakapapa currently supports 10-11 pairs, but carrying capacity would be much greater with full water flow. In May 1991, the river was running naturally due to the diversion tunnel being blocked off for repairs. Two fledglings were sighted at Owhango, having migrated about 12 kilometres downstream. It was a tragedy in the making, because the next day the water was diverted back into the tunnel. The birds have since

disappeared as the low water level has left much of their habitat high and dry. The Owhango sighting demonstrates the probable elimination of 12 pair on the Whakapapa river. The power scheme has eliminated habitat for at least 16 pairs on the Whanganui and Mangatepopo tributaries. Two pairs have been eliminated from the Okupata river (five pairs presently live here). Dewatering of the Taurewa and Tawhitikuri rivers, plus farming and forestry activity have probably eliminated a further ten pairs. Thus, about 40 pairs — half the blue duck in these tributaries — have been eliminated since 1972.

Significance of the Whanganui whio population

The upper Whanganui river and tributaries affected by the power scheme currently holds about 43 to 49 whio pairs. This is thought to be the largest whio concentration in the North Island. In addition, the Manganui-a-te-Ao, ° where the most intensive whio study has been carried out, holds a further 36 to 40 pairs. The Manganui-a-te-Ao is also a tributary of the Whanganui. It is unaffected by the power scheme and fully protected by a National Water Conservation order.

Vulnerability

Conservation status could change overnight. The Whakapapa colony is the most vulnerable because it is on a lahar path (volcanic mud eruption). One major lahar could wipe out the entire colony. The Manganui-a-te-Ao is equally vulnerable; it too lies ona lahar path. The single most important conservation action for North Island whio would be to restore the water to the Mangatepopo river and the upper headwaters of the Wanganui. These tributaries are more secure habitat because they are not on potential lahar flows.