Aquatic Pests



Didymo (Didymosphenia geminata), commonly known as Rock Snot, is a diatom that produces nuisance growths in freshwater rivers and streams that have low water temperatures and low nutrient levels. Diatoms are a major group of algae which are unicellular and in the case of didymo exist in colonies covering rock in rivers. Fossil evidence suggests that diatoms have been around since before the early Jurassic Period (150 – 200 million years ago).
Didymo is not a significant human health risk, but can affect sources of food, such as insects, for fish, as well as making fishing an unpleasant experience. Didymo is widespread in the Northern Hemisphere, being found in Russia, Europe, Asia and North America. It was first discovered in New Zealand in 2004 in the Waiau River in Southland. This was the first discovery of didymo in the Southern Hemisphere. In subsequent years it has been found to have invaded many other South Island Rivers. The whole of the South Island has been declared a controlled area requiring all boating, fishing gear and vehicles to be cleaned before being moved to another waterway.
Alarmingly, in 2007 dead didymo cells were detected in water samples taken from the Whanganui, Tongariro and Whakapapa Rivers. Fortunately, it has not spread to become a problem in the North Island. Study by NIWA scientists has linked the growth of didymo to the levels of dissolved reactive phosphorus in the waterways and rivers. Where the phosphorus levels are low the didymo cell division rates increase causing large blooms. The North Island rivers have more intensive catchment development resulting in higher phosphorus levels which inhibits the growth of didymo.
However, we cannot become complacent and must follow the methods recommended by MAF to stop the spread of didymo:
Check: Before leaving the river or lake look for all clumps of algae that may be caught in equipment and vehicles. Leave it on site, if you later find clumps of algae, don’t wash them down the drain, but treat them using the methods below and when dry place in the rubbish.
Clean: Soak and scrub all items for at least 1 minute in either hot water (60°C), a 2% solution of household bleach, a 5% solution of salt, dishwashing detergent or antiseptic hand cleaner.
Dry: If cleaning is not practical, after the items are completely dry, wait another 48 hours before contact or use in another waterway.

Lindavia Intermedia (Lake Snow)

Lindavia Intermedia commonly known as Lake Snow, is another diatom which has found its way into our waterways. Genetic testing by Landcare Research indicates that lindavia intermedia found in our lakes has most likely come from North America, its genetic make up matches that of specimens taken from Lake Youngs in the USA and Cultus Lake in Canada. So, it seems likely that visitors from North America have introduced this new algal pest to our lakes.
Lindavia Intermedia was first discovered in Lake Waikaremoana in 2008 and has now found its way into a number of our lakes and was detected in Lake Taupo in 2018. Being a microscopic organism, it is hard to detect and does not pose a threat to humans or animals. It is also unlikely to affect the lakes ecological health. It does however, cause “Lake Snow”, a sticky mucus like substance that hangs in the water and can be a nuisance to water users, in that it sticks to fishing gear, boat hulls and will clog filters in boats and those in water supplies. So far, no evidence of Lake Snow has been found in Lake Taupo. It is not known what causes Lindavia Intermedia to produce Lake Snow, but it is thought that it has been present in Lake Taupo for more than a decade, and in that time has not produced Lake Snow.
Being a microscopic organism, Lindavia Intermedia cannot be easily detected on boats or fishing gear, this emphasises the importance of cleaning and disinfecting all gear before moving waterways.


Hornwort (Ceratophyllum demersum) is a highly invasive submerged freshwater weed first recorded in waters near Napier in 1961. It has since spread to many of the North Island lakes including Lake Taupo where it was first detected in 1980. Hornwort is well established in Waihi Bay, Stump Bay and Acacia Bay with considerable potential to spread to other areas such as Motuoapa Bay and Two Mile Bay. Hornwort has been largely eradicated from the South Island with no plants detected since 2008.
Hornwort is a non-rooted plant, anchoring itself to the bottom sediment with buried stems and the lower leaves. It reproduces from broken off fragments that are easily moved to new sites by the action of boats, boat trailers and fishing gear. Even wind and wave action can relocate it to new sites where just a small fragment can generate enough new plants to fill a lake.
When it comes to aquatic weeds, hornwort holds a number of records, it is ranked New Zealand’s worst submerged weed, the tallest growing submerged weed ( can grow up to 10M tall), the deepest growing submerged weed (can grow up to depths of 15M), and the only non-rooted submerged weed. It was first introduced to New Zealand through the aquarium industry as an aquarium plant, but its sale has been prohibited since 1979.
Because of its ability to spread rapidly, hornwort beds outcompete and smother all other aquatic plants, including native plants and oxygen weeds such as Egeria (Egeria densa) and Lagarosiphon (Lagarosiphon major).
As with other aquatic weed pests, to prevent their spread, we must follow the routine of – Check, Clean and Dry.

Egeria Densa

Egeria Densa is a freshwater pond weed native to South America that was introduced to New Zealand waterways via the aquarium trade. First discovered in Lake Rotorua in 1977, Egeria densa is well established in North Island lakes and has found its way into some South Island waterways. Egeria densa is the only oxygen weed with visible white flowers, only male plants are found in New Zealand and so it spreads, similar to hornwort, by the dispersal of plant fragments which quickly establish themselves as new plants. The dense canopy that Egeria densa forms is light blocking and that combined with the nutrients that it takes from the water results in reduced levels of phytoplankton, it can, however, act as a shelter for zooplankton.
Egeria densa, having escaped from cultivation, has become an unwanted invasive species in many parts of the world, including Europe, North America, South Africa, Asia and Australia to name a few. It has a major impact on hydroelectric dams as well as impeding the flow of water in irrigation and drainage waterways.
So, like hornwort, Egeria densa is easily spread between waterways by the action of boats, boat trailers and fishing gear making it important to check and clean gear before moving to another area.

Lagarosiphon Major (Curly Oxygen Weed)

Lagarosiphon Major, commonly known as curly oxygen weed was introduced from South Africa as an ornamental aquarium and pond plant in the 1950’s. It has since spread and is common in lakes and waterways in both the North and South Islands. Only female plants are found in New Zealand producing tiny pink flowers, but it is spread by fragmentation with plant fragments quickly establishing themselves to form new colonies. Lagarosiphon Major can form vast underwater meadows shading out native species and preventing them from establishing. Large clumps can dislodge from the meadows and become a nuisance to lake users and eventually drift into shallow water where they will rot killing flora and fauna.
As with many aquatic weeds it is spread by fishing and boating activities emphasising the need to clean and dry gear after leaving the water.

Elodea Canadensis (Canadian Pond Weed)

Elodea Canadensis or Canadian Pondweed was the first invasive weed species introduced into New Zealand in 1868. It is thought to have been introduced to oxygenate waterways to support future species of introduced fish. It is now distributed widely throughout New Zealand. It is very similar to Egeria Densa and Lagarosiphon Major and can be distinguished from Egeria in that it has leaves arranged in whorls of three compared with Egeria which has whorls of at least four. Lagarosiphon’s leaves curl downwards and are arranged in spirals about the stem.
Like Egeria Densa and Lagarosiphon Major, Elodea Canadensis is distributed by fragmentation. Fragments can float away and then root to form a new plant eventually forming a thick mat which out competes other plant species for light and space. Elodea Canadensis fragments have a large survival rate which enables them the be dispersed over large distances. It is therefore, a major threat to waterways with recreational activities believed to be main dispersal method assisting in the spread of this invasive weed.

Nymphaea Mexicana (Mexican Water Lily)

Nymphaea Mexicana or Mexican Water Lily is an aquatic plant native to the Southern United States and Mexico. It was first discovered in Lake Ohakuri in the Waikato in 1982, and at this stage has been confined to the Waikato and Auckland regions. It was most likely introduced as an ornamental pond plant but is best known as a noxious weed outside its native countries.
Nymphaea Mexicana is a bottom aquatic plant similar to other water lilies, it has vertical underwater rhizomes and round heart-shaped floating leaves (up to 25cm diameter) on long stalks. The leaves are mainly purple underneath with a reasonably large cleft giving them a heart shape. Large star shaped flowers (Up to 15cm across) are produced, they are pale yellow and produce seeds 2 – 3mm long. A distinguishing feature of Nymphaea Mexicana are brown blotches that often occur on the upper side of the leaves that are not found on the common water lily.
Rhizomes, tubers, and seeds are the main means of distribution with fragments picked up on boats and trailers assisting distribution. Once established, Nymphaea Mexicana forms dense mats of floating leaves which can cover a large area clogging waterways. This shades out other aquatic plants and is an obstruction to water users.

Gymnocoronis Spilanthoides (Senegal Tea)

Gymnocoronis Spilanthoides is a Perennial aquatic herb which grows up to 1m tall along water margins. First discovered growing in Papakura, Auckland in 1990, it is targeted for eradication but has since spread throughout the North and South Islands. Gymnocoronis Spilanthoides produces long (up to 1.5M) floating stems which take root at nodes, the leaves are dark green, slightly waxy with a serrated edge. Clover like flowerheads with many thin white petals are produced between November to April, followed by yellow-brown seeds around 5mm in diameter which are long lived and become well dispersed. In colder climates the plant will die back to root stock in winter but resprouts in spring.
Seed, stem, and root fragments are distributed by water and wave action along with being transported by recreational activities such as boating and fishing. This plant damages the environment by forming dense mats which prevent the seedlings of native species from establishing, it clogs waterways causing flooding and rotting vegetation damages water quality.

Ameiurus Nebulosus (Bullhead Catfish)

The Bullhead Catfish is a North American species first introduced to Auckland in 1877 and stock then sent to Wellington and Hokitika in1885. As a result of this, catfish are now widespread throughout the central North Island with a few scattered pockets in the South Island. They are relatively large (up to 40cm), dark brown to olive green in colour with paler underbellies. A distinctive feature is the eight barbels around the mouth looking much like whiskers, hence the name catfish. Their skin is smooth with no visible scales and the leading edge of the dorsal and pectoral fins have sharp spines which can inflict injury.
Catfish are extremely robust and can survive for long periods out of water, hence they are easily distributed via boat trailers and fyke nets used for eeling. Catfish spawn in shallow depressions in the substrate in shallow water. The male guards the eggs during development and then the young fish for around a week after hatching.
Catfish are carnivorous and use their sensitive barbels to probe the substrate locating insects, crustaceans, molluscs, and small fish. In Lake Taupo freshwater crayfish are a major prey for catfish.

Cyprinus Carpio (Koi Carp or Common Carp)

Koi Carp are an invasive fish species originating from Asia, they are thought to have arrived in New Zealand in the 1960’s having been accidentally introduced as part of a goldfish consignment. They were first noticed in the Waikato region in 1983 by which time they had already established a breading population. The release of koi carp into the wild from private ponds and flooding have seen them spread to other regions, mainly Auckland and the Waikato. Koi carp are often mistaken for goldfish but can be identified by two barbels at each corner of the mouth. Their colour varies greatly in the wild, they are usually an olive green to bronze colour while the ornamental variety are brightly coloured with red, white, and orange markings. Once released into the wild they soon revert to their wild colouring of olive green to bronze.
Koi carp feed by sucking up bottom sediments and blowing out what is not wanted. In doing so they stir up sediment and leave the water discoloured, and they also dislodge aquatic plants causing a loss of habitat and food for native fish plus waterfowl and invertebrates.
A mature female koi carp can lay half a million eggs at a time resulting in a population density such that their average weight has halved since they were first discovered in New Zealand waterways.
A couple of innovative ways of controlling the carp population have developed in the Waikato. For the last 25 years an annual bow hunt has been held with hunters gathering from all over the county. This has resulted in an estimated 25,000 carp being removed from our waterways. The second method is a project called “Carpuccino”, where carp are trapped, fed into a digester and then with the aid of geothermal bacteria, turned into organic fertiliser. Because the carp are living in waterways in the dairy intensive Waikato region, they are rich in nutrients such as zinc, potassium, and nitrates. These nutrients are bad for our waterways, but carp thrive in the adverse conditions they cause. So is a unique way of not only removing carp from our waterways, but in returning nutrients to the land for the benefit of the dairy herds.

Gambusia Affinis (Mosquito Fish)

Gambusia Affinis were introduced into New Zealand from the Gulf of Mexico and released into an Auckland Botanical Gardens pond in 1930. Further transfers into Northland, Taranaki and Wellington are documented. Since then they have become widely distributed throughout the top half of the North Island. Originally introduced to control mosquito larvae they have become an unwanted and aggressive species frequently attacking native fish by nipping at their eyes and fins and preying on their eggs.
Gambusia are small with a greenish silvery sheen, the female grows to 6cm and the male 3.5cm. They mature at 6 weeks and are short lived but breed rapidly enabling populations to grow rapidly. A single female produces several broods a year with around 50 offspring per brood. The female gives birth to live young which means a single pregnant female can establish a new population. Gambusia can tolerate high salinity and so readily colonises brackish estuaries and mangrove swamps at stream and river mouths into harbours.
The irony is, Gambusia were introduced to control mosquito larvae, a job which native species such as whitebait, bullies, eels, and aquatic invertebrates do quite well.
As with other aquatic pests they are often spread by fishermen using boats and fyke nets to other waterways.