Lake Wānaka
Geography
Lake Wānaka was formed by huge glaciers bulldozing their way through the mountains during ice ages thousands of years ago. The lake has a maximum depth of 311 metres and at 45 kilometres long and nearly 12 kilometres across at its widest, it has an area of approximately 193 square kilometres, making it New Zealand’s fourth largest lake, after Taupo, Te Anau and Wakatipu.
Lake Wānaka has four major islands: Mou Waho, Te Peka Karara (Stevensons), Mou Tapu and Ruby Island. They are mostly pest free and are important refugia for birds, lizards and mountain stone wētā.
Major rivers flowing into the lake are the Matukituki and Makarora. The only outlet is the source of the Clutha River/Mata-Au, the highest volume river and second longest in the country at 338 kilometres.
Limnology
Lake Wānaka has summer surface temperatures of between 17 and 20 degrees Celsius, depending on the weather patterns. In summer there is a marked thermocline at around 7 to 10 metres, below which it is several degrees colder.
Winter surface temperatures are around 7 to 9 degrees C and this continues all the way to the bottom of the lake, where there is very little change year round.
Lake levels can fluctuate by about two metres at any time of year, depending on rainfall in the large catchment. Following periods of high rainfall, the lake may rise by more than a metre in just one day.
Wānaka is an oligotrophic lake, meaning it has low primary production (phytoplankton) as a result of low nutrient content, especially low phosphorus levels. With so few plankton, the waters are usually very clear. Farming and urban development over the last 150 years have led to some changes with more nutrients and sediment entering the lake.
Ecology:
Aquatic Algae & Plants
Phytoplankton occur at low numbers in oligotrophic Lake Wānaka but in recent years there have been blooms of “lake snow”, an invasive algal phytoplankton Lindavia, thought to originate in North America and possibly introduced to Wānaka on fly-fishing equipment.
Didymo is another invasive algae thought to have been introduced this way but it is confined to rivers feeding into the lake.
Most of the lake is too deep for bottom-growing plants to live. But in the well-lit shallows around the approximately 120 kilometres of lake edge are 23 known species of aquatic plants – mostly growing on the softer muddy bottom:
- Potamogeton water lilies grow down to 6m.
- Charophyte algae live as deep as 16m.
- Aquatic moss as deep as 50m.
In addition there are three species of introduced, invasive plants that dominate much of the lake shallows:
- Ranunculus is an aquatic buttercup up to 2m tall and growing as deep as 3m.
- Elodea up to 3m tall and down to 6.5m deep.
- Lagarosiphon or oxygen weed, up to 6m tall and living down to 6.5m deep.
These three invasive species crowd out native plants but they also provide good habitat for the invertebrates and fish which grebes feed on. Grebes have probably benefited from the spread of Lagarosiphon, which they often pick from the lake bed and add to their floating nests. Currently there are expensive annual efforts by government and local agencies to control Lagrosiphon with the aim of eradication.
Ecology:
Aquatic Animals
The largest fish in the lake is the tuna or longfin eel, Anguilla dieffenbachii. They can grow up to two metres in length, weigh more than 20 kilograms and live a hundred years plus. Longfin eel eggs hatch at sea in the Pacific Ocean near Tonga, then float as larvae for thousands of kilometres in oceanic currents to New Zealand. They then travel upstream in rivers as elvers (juveniles) just a few centimetres long.
Tuna are blocked from reaching Lake Wānaka by downstream hydroelectric dams so each year, some elvers are caught by handnet below the Roxburgh dam and transferred to Lakes Hāwea, Wānaka and Whakatipu to restore the lake populations. A good place to see adult eels is below the main town wharf in Wānaka but be careful, they can bite! Nationwide, longfin eels are a threatened endemic species.
Another native freshwater fish species in the lake is kōaro, Galaxias brevipinnis, which grows up to 200 mm. They are one of the commonest white bait species and are found throughout the country.
The most frequently seen native fish is the toitoi or common bully, Gobiomorphus cotidianus, which grows to 100mm and lives in the shallows of the lake. It is found throughout the country and is also a major part of the grebes’ diet.
Lake Wanaka has three species of introduced Salmonid fish:
- Brown trout, Salmo trutta
- Rainbow trout, Onchorhynchus mykiss
- Quinnat salmon, Onchorhynchus tshawytscha
Although introduced, these fish are now the major predators here, eating native fish and invertebrates such as worms, snails and larvae.
Both kōaro and common bully are important to the lake as hosts for an early life stage of kākahi or freshwater mussel, Echydridella menziesii. The larvae of kākahi live for a while in the gills and on the fins of kōaro and toitoi. As these fish decline due to predation by trout and salmon, so have kākahi. These unusual shellfish are filter feeding bivalves that sieve plankton and silt from the water, helping to keep the lakes healthy and clean.
Ecology:
Water birds
Many species of birds share the Southern crested grebe’s habitat around Lake Wānaka. Some are harmless; others like karoro/black backed gulls are predators of grebe eggs and chicks. Pāpango/Scaup also disrupt nests and remove grebe eggs.
The smallest duck species found on Lake Wānaka is the endemic NZ scaup / pāpango (endemic means only found in NZ). These dark-coloured diving ducks are very social, often gathering in large groups during the autumn and winter. The males can sometimes be heard making a strange, high-pitched whistle call.
Our largest duck is the beautiful, endemic paradise shelduck / pūtangitangi. The female has a pure white head and chestnut-coloured body. The male is dark all over, except for its tri-coloured wings. They are always found in pairs and make loud honking and shrieking calls to each other when disturbed or flying.
A self-introduced native bird is the striking Australian coot with its white frontal shield and beak, strange lobed feet and slate grey plumage. Although classed as ‘naturally uncommon’, coots can be seen on many of the shallow bays around Lake Wānaka. A pair recently bred successfully on a grebe nesting platform near town.
Two gull species can regularly be seen at Lake Wānaka. The smaller black-billed gull / tarāpuka is an endemic freshwater gull, declining in numbers across NZ. Large flocks of them can be seen on the lake shore near town. They breed on the gravel flats of the Matukituki and Makarora braided rivers which flow into the lake.
The southern black-backed gull / karoro is much larger in size and a common predator of eggs and chicks of grebes and other bird species, including the black-billed gull. Karoro are native but not endemic to NZ, also found in similar latitudes around the southern hemisphere. Karoro numbers have increased greatly because of farming and other human activities and they are a threat to several species of birds.
Two species of native cormorants can be found around the lake. The little shag / kawaupaka is often seen resting on rocks or in trees overhanging the water. They have a range of colour variations from completely black, through various pied (black and white) plumages.
The much larger black shag / māpunga is often seen flying above the rivers leading into and out of Lake Wānaka. Both cormorant species are native and classified as ‘At Risk’.
A range of wading birds can be seen seasonally on the shore and in the shallow waters of the lake. The striking black and white South Island pied oystercatcher / tōrea arrives from the coast in late winter and early spring to breed on braided rivers flowing into the lake. Tōrea are endemic and declining across the South Island.
Another endemic visitor during the spring months is the black-fronted tern / tarapirohe, a graceful flyer who can be seen plunge-fishing. These ‘Nationally Endangered’ freshwater terns breed on braided rivers at the head of the lake.
The delicate looking pied stilt / poaka is occasionally seen foraging around the lake’s edges, its very long red legs distinguishing it from all other wading birds.
The pale grey white-faced heron / matuku moana may also be seen year-round in shallow bays, stalking prey or flying gracefully across the water.
Introduced waterfowl seen on the lake include the common mallard / rakiraki. These common ducks were introduced from the UK and US as a game bird and are now found in all parts of NZ. A few domestic ducks are often seen on the lakefront near town, coloured white or a mix of black and white.
Canada geese / kuihi were introduced from the US and Canada as a game bird. Large groups of these geese can be seen feeding in the lake near Wānaka at the end of summer and in early autumn.
Invasive mammals
Introduced mammal species such as rats, stoats, ferrets, hedgehogs and feral cats threaten all of our birdlife, especially species which breed on braided riverbeds. We are grateful for the efforts of hundreds of volunteers from local community groups groups such as Forest & Bird, Southern Lakes Sanctuary and Wanaka Backyard Trapping, who tirelessly trap mammalian pests to make Wanaka and our catchment a safe haven for birdlife.