Sulphur Bay

Forest and Bird, Volume 15, Issue 2, 1 May 1984, Page 19

Sulphur Bay

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Regional Representative, OSNZ — Volcanic Plateau and

Zoology Department, University of Canterbury

John Innes

Graeme Taylor

A THERMALLY HEATED WILDLIFE AREA

Much of the water in Sulphur Bay, Lake Rotorua, is devoid of aquatic plants and animals. Why then, do the majority of New Zealand’s little black shags nest here? What brings black-billed gulls 1,000km north of their normal South Island braided river nesting habitat, to raise chicks ona steamy silica spit a few hundred metres from downtown Rotorua? Why do dabchick, pied stilt and welcome swallow gather here in large numbers each winter?

Inhospitable bay? Looking at the Bay itself does not immediately answer the questions. It is the most southern bay of Lake Rotorua and measures 1,300m by about 750m. Adjacent ground includes the well known Government Gardens complex; the large Travelodge hotel, and extensive natural silica flats with an intact manuka frontage as old as Rotorua city itself and now uncommon on the lake edge. The Puarenga Stream, which drains the Whakarewarewa Thermal Reserve, flows in from the south. However, the Puarenga does not supply the thermal odours and suspensions which gave the Bay its name. These discharge directly into the Bay from an active thermal field under and adjacent to the lake water. The water is warm; it is murky, sulphurous, low in oxygen and shallow. You can walk across most of the Bay, which alarms tourists at the Travelodge! The water is quite acid — as lowas pH 3.5 in places — and we often see gulls with feet like blackbirds because the webs of their feet have not withstood years in the acid water. The bottom is covered with fine mud near the thermal discharges and fine quartz sand further away from them which contain the only abundant insect group — midge larvae. There are no lakeweeds in the sulphurous part of the Bay. Sanctuary and refuge Sulphur Bay was declared a game reserve in 1904, for ‘‘native and imported game, primarily ducks’’. The breeding area for red-billed and black-billed gulls at Sulphur Point, and its surroundings have been a Wildlife Sanctuary since 1964 and special permission is needed from the Wildlife Service to go there. The water of the Bay has been a Wildlife Refuge since 1967. We have records of 62 species of bird from the area of which 45 are native. Some visitors are rare vagrants; others are present all year round in constant numbers while more still are periodic users, either

coming in the spring and summer to breed or in late Ssummer/autumn to moult or wait out the winter. The summer breeders Nationally significant colonies of red-billed and black-billed gulls and little black shag occur in Sulphur Bay. Also, the banded dotterels which nest on the silica flats behind the Travelodge Hotel are special for the Rotorua region. Red-billed gulls: Red-billed gulls are common coastal breeders throughout New Zealand but there are few inland breeding colonies. They have been known to nest in Sulphur Bay since at least 1939-40, and their numbers on the Sanctuary colony have steadily increased — from 27 nests in 1945 to 150 pairs in 1956/57, to 400 pairs in 1961/62 to about 1,000 nests in the 1982/83 and 1983/84 seasons. First eggs are laid in early October and the usual clutch is two. The red-billed gulls prefer to nest on the rocky edges of the colony, while the top flat section is used by black-billed gulls. Red-billed chicks fledge in about five weeks. The foods we’ve observed them regurgitate include dragonfly larvae, beetles, bullies and smelt. The adults rarely feed in the sulphurous water, but a common sight is several hundred red-bills wheeling and diving for smelt in a line along the margin between the sulphurous water of the bay and the cleaner water of the open lake. Smelt spawn mostly in the spring and early summer in Lake Rotorua. Dr Peter Mylchrest, Fisheries scientist in Rotorua, has suggested to us that smelt may be rendered groggy by the sulphurous water in the Bay, which could explain why the gulls constantly work the mix-zone. Chicks fledge steadily from about midDecember to mid-February. Except for the odd chick that falls into a steaming hole, the nesting birds don’t seem to be affected by the thermal nature of their colony. From the time they leave the Sanctuary the newly flying gulls can be seen wherever the adults feed and roost. This includes the local yo ~.

rubbish tip — 10km away — and nearby parks, especially the Rotorua Lakefront where tourists, children and birdwatchers can all get very close to them. One day in late February or early March, the entire colony will vacate as you enter. The intense site attachment shown during breeding is finished, and another season is over. Some gulls always remain during winter but banding has shown that some fly to Maketu, a Bay of Plenty beach. This year we have put orange bands on the legs of over 300 red-billed gull young to improve our knowledge of their movements. Biack-billed gulls: If the red-billed colony can be called ""unusual’’, then the black-billed gulls are extraordinary. O’Donnell and Moore (1983) grouped this endemic species with the wrybill plover and black-fronted tern as having ‘‘. . . specific adaptations for breeding and feeding on riverbeds’’. Although it sometimes breeds in association with red-billed gulls in the South Island, it is typically a South Island braided river breeder. Other North Island colonies — some recently discovered — also exist, but the Rotorua breeding population is largest. Black-bills have bred here since the early 1930s and perhaps longer, but their numbers on the Sanctuary are declining. In 1951/52, they outnumbered red-bills three to one (Black, 1954). In 1961/62, they just held their majority, having 460 nests to 400 of red-bills (Reid and Reid, 1965). In 1982/83, only 310 pairs built nests, compared to about 1,000 pairs of red-bills; the number had dropped further to 230 pairs during the 1983/84 season. This species commences nesting three weeks later than the red-billed gull and deserts three weeks earlier. Their rapid synchronised breeding is one of several behaviours adapting them to the floodprone South Island riverbeds. In the South Island, the gulls lay ‘‘almost as soon as a nesting site is secured’, and "‘the adults and young move away from the nests within a few days of hatching of the eggs .. .’’ (Beer, 1966). There is already evidence that the Rotorua black-bills are learning new ways, more in keeping with a stable colony site that isn’t washed away with each flood. At Rotorua, many gulls are on the colony two months before first eggs are laid, and the chicks don’t leave the site until they are almost ready to fly. However the nesting period is the same length — about 10 weeks. Few black-bills are seen around town where their red-billed cousins are scrounging food, and we have never seen them at the rubbish tip. They feed on pasture around Rotorua or on lakes in the region. Winter movements of the black-billed gulls which breed at Rotorua are a mystery. It is likely that many go to the Firth of Thames, although this has yet to be proven. Some remain in the Bay of Plenty. Birds banded on the Sanctuary have been seen on Lakes Rotoehu and Taupo in winter. Fifty Sulphur Bay black-bills were banded with orange leg bands this year to find out the movements of the birds. Little black shags: Six years ago, no little black shags nested in Sulphur Bay. In 1982/83 there were 980 nests crammed onto the small Island off

Motutara Point — most of the New Zealand population. Until recently little was known about the species in New Zealand. A few little blacks arrive from their wintering grounds in September, but most appear in early October. First eggs appeared in midOctober in 1982/83 but the next year not until 4 December. Incubation takes 26-28 days and the usual clutch is four eggs. Chicks are ugly but lovable! At fifteen days the chicks can be banded and are fully fledged within seven weeks. Juveniles are distinctively brown-plumaged. As with the gulls, these shags don’t feed in the sulphurous water. Hundreds of them fly to the northern bays of Lake Rotorua daily to dive for smelt. Their feeding is characteristic — hunting in groups, they herd smelt ahead of them by diving repeatedly in front of each other to reach the small fish. Little black shags banded in Rotorua in the 1982/83 season have been seen or captured in winter at Bay of Plenty beaches; at Oputere on the east coast of the Coromandel Peninsula; near Panmure and on the Waitemata Harbour in

Auckland, and much further north, near Kerikeri. In the 1983/84 season only 300 shags nested on the small island. Red leg bands have been put on 160 young to identify them more clearly as being Rotorua-born. Meanwhile the few trees — old manuka — on the island are long dead. But it doesn’t matter, since 90% of the nests are on the ground. Banded dotterel: Banded dotterel are rare in the Rotorua Lakes district. Ten pairs still nest atop Mt Tarawera, in scattered localities and despite the airstrip there. The only other known breeding site is on the thermal silica flats of Sulphur Bay between the Travelodge Hotel and the Puarenga stream mouth. The dotterels breed there from August to February, then depart until June or July. In 1982/83 up to 12 were present and four pairs nested. Seven chicks hatched with probably four fledging. The biggest threats to the continued survival of these banded dotterels are motorbikes and ‘‘development’’. Many people see the flats as unproductive

wasteland, which needs to be developed for tourism or commerce. Every summer evening and weekend motorbikes scream up and down the sand and mud. The dotterels fly unnoticed ahead of the bikes, their chip-chip call warning chicks to keep their heads down. Unfortunately their breeding area is outside the Wildlife Refuge. The overwinterers The New Zealand dabchick population in the Bay shows a strong seasonal trend. Flocks form in autumn and disperse during the spring. Few remain in high summer. In 1982 the winter flock peaked at 95, while in 1983 about 70 birds used the Bay. The estimated population of this endemic bird in New Zealand (and therefore in the world) is 1,150-1,400 birds (B. Heather, 1978, OSNZ News No 8). Sulphur Bay therefore supports at least 5% of the total population in winter, and ranks as a wetland of international importance under IUCN Criteria (any area which supports more than 1% of the world population). Dabchick feed along the zone between the Puarenga Stream and Ngapuna Stream currents, and in the evening — when the lake is more peaceful — move out towards Hinemoa Point. Pied stilts feed, roost and breed around the bay shoreline, mainly at the Puarenga Stream mouth and on the Travelodge flats. Alarge flock (170 in February 1983) forms after the breeding season in late summer. This declines in autumn, possibly as birds move down to the Bay of Plenty coast. Numbers build up again in May as duckshooters take to our wetlands, and most of this flock disperses in July to farm ponds and lakes in the region to breed. Welcome swallow are virtually absent from October to February but appear in large numbers in May. In June 1983, over 700 were estimated to be hawking up and down the thermal Puarenga Stream at its mouth. They remain until August. It seems probable that in summer they breed in farm buildings, near ponds and on bridges within 10-15km of Rotorua. They use the bay over the winter period probably because its warmth supplements their body heat and it contains a ready winter food supply of midges.

Regulars, moulters and shelterers Other regular users of the bay include black swan, paradise duck, mallard, scaup, grey teal, black-backed gull and Caspian tern. Numbers of waterfowl, especially paradise duck, shoveller, grey teal, mallard and grey duck are highest after breeding, in January and on into autumn. Presumably the bay provides peace for moulting, but few breed there. Other birds present at different times are little shag, large black shag (up to 60 in December/January, after breeding) and white-faced heron. What of the future? The questions posed in the introduction to this article still need an answer. For the banded dotterel, gulls, pied stilts and other species which breed and feed here, Sulphur Bay provides a large, sheltered, flat surface close to a lake. This habitat type is regionally rare. The black-billed and redbilled gulls and little black shags which have large breeding colonies here mostly feed outside the Bay, and certainly feed outside the thermal section of it. For all species, freedom from disturbance is a key factor. The bay isn’t popular with tourists or anglers, and few visitors walk on the thermal flats. However tourist interests are keen to make use of the area but for the wrong reasons. A hovercraft company is now considering starting an ‘‘airport to accommodation"’ service, actually running the noisy machine out of the water within 30m of where red-billed and black-billed gulls and banded dotterel nested this year. A jet helicopter also takes sightseers over the bay from vacant land within 200m of the Puarenga Flats. Roosting gulls take to the air as the machine covers the bay. Red-

billed and black-backed gulls are good adapters, but we do wonder at the decline of black-billed gulls on the Sulphur Point Sanctuary. This year a colony of black-bills at the nearby tranquil Lake Rerewhakaaitu fledged over 200 chicks, about four times the chick production at the Sanctuary. Perhaps they’re telling us something? We think there is excellent potential for developing the Sulphur Bay refuge for better viewing by the public, but on foot, not from a hovercraft or helicopter! Unfortunately the message does not yet seem to have reached many tourist operators that bird watching is a very popular activity worldwide. Natural history and activity tourism are the major growth area of tourism in New Zealand today. However it is essential that we safeguard the attractions the visitors come to see. References Beer, C. G. 1966: Adaptations to nesting habitat in the reproductive behaviour of the black-billed gull (Larus bulleri). Ibis 108 pp394-410. Black, M. S. 1954: Some notes on the blackbilled gull (Larus bulleri) at Lake Rotorua, with special reference to the breeding cycle. Notornis Vol 6 (1) pp167-170. O’Donnell, C. F. J. and Moore, S. G. M. 1983: The wildlife and conservation of braided river systems in Canterbury. Fauna Survey Unit Report No 33, Department of Internal Affairs, Wellington. Reid, Denise and Brian, 1965: The Sulphur Point (Lake Rotorua) Gull Colony. Notornis Vol 12 (3) pp 138-157. a Editor’s Note: Any readers who sight any of the banded birds described in this article may wish to contact: John Innes, Loop Road, Lake Okareka, Rotorua with their sightings.