New Zealand is home to two native eel species that occupy nearly every type of freshwater habitat in the country, from high-altitude mountain streams to lowland farm ponds and coastal lagoons. The longfin eel (Anguilla dieffenbachii) is found nowhere else on Earth and can live for many decades, while the shortfin eel (Anguilla australis) also inhabits parts of Australia. Together, these eels sit at or near the top of freshwater food chains and hold deep cultural importance for Māori, yet their biology remains surprisingly mysterious in some respects, particularly when it comes to where and how they spawn.
Two Species, Two Strategies
Though the longfin and shortfin share rivers, lakes, and wetlands across both the North and South Islands, they carve out space differently. Research in New Zealand streams found that the size distribution of longfin eels was best explained by local physical habitat features like water velocity and the amount of cover available for fish. Shortfin eels, by contrast, were more influenced by landscape-scale variables such as distance from the sea and channel slope.1New Zealand Journal of Marine and Freshwater Research. Relative influence of local and landscape‐scale features on the density and habitat preferences of longfin and shortfin eels In practical terms, longfins tend to push farther inland and higher up into headwater tributaries, while shortfins concentrate more in lowland waterways closer to the coast.
Despite these habitat differences, the two species eat much the same food. A study in the Horokiwi Stream showed that all size classes of both species fed at night on similar prey, with no evidence of the two species separating their diets by time of day or by what they ate. The researchers concluded that habitat separation, not dietary specialization, is the main mechanism keeping competition between them in check.2New Zealand Journal of Marine and Freshwater Research. Prey selection and dietary overlap of shortfinned (Anguilla australis) and longfinned (A. dieffenbachii) eels during summer in the Horokiwi Stream, New Zealand Both species are opportunistic predators. Longfin eels in particular will take terrestrial invertebrates that fall into the water, alongside the aquatic insects and crustaceans that make up the bulk of their diet.3New Zealand Journal of Marine and Freshwater Research. Food webs in forest and pasture streams in the Waikato region, New Zealand: A study based on analyses of stable isotopes of carbon and nitrogen, and fish gut contents
Growth rates differ between populations and habitats, but longfin eels in one long-term study grew on average about 3.8 centimeters per year over a decade after release, faster than the roughly 2.4 centimeters per year estimated at the time they were first captured.4PubMed Central. Growth patterns and age validation from otolith ring deposition in New Zealand longfin eels Anguilla dieffenbachii recaptured after 10 years at large Those numbers might sound modest, but longfin eels compensate with sheer longevity. Individuals over 80 years old have been documented, and some may live past a century. By the time a large female longfin reaches maturity and begins preparing to migrate out to sea, she can exceed a meter in length and weigh several kilograms.
The Journey In and the Journey Out
Every New Zealand eel begins life somewhere in the tropical Pacific Ocean, hatched from eggs laid at a spawning ground that no researcher has ever directly observed. After hatching, the larvae drift as leaf-shaped leptocephali on ocean currents until they metamorphose into transparent glass eels and arrive at the New Zealand coast. In the Waikato River, glass eel migrations peaked around spring tides, typically within a few hours of high tide, and two to four days after the day of the spring tide itself. Water temperature and river discharge both had inverse relationships with glass eel catches, with temperature alone explaining more than 30 percent of the variation in catch timing.5PubMed. Recruitment of Anguilla spp. glass eels in the Waikato River, New Zealand. Evidence of declining migrations?
Seasonal patterns add another layer. Glass eel densities at river mouths are generally low in August, peak during September and October, and then taper off through November and December. Longfin glass eels tend to arrive before shortfins, and they probably reach the North Island before the South Island, while shortfins show the opposite trend.6New Zealand Journal of Marine and Freshwater Research. Seasonal arrival patterns of juvenile freshwater eels (Anguilla spp.) in New Zealand How the glass eels navigate from their tropical birthplace to the correct coast remains partially unresolved. Otolith analysis of shortfin glass eels has shown that New Zealand arrivals are unlikely to have been carried across the Tasman Sea from Australia by the East Australian Current, suggesting they follow a different oceanic route entirely.7Marine Ecology Progress Series. Dispersal pattern of glass eel stage of Anguilla australis revealed by otolith growth increments
Once glass eels enter freshwater, they pigment up, become elvers, and begin the long upstream journey. Some stop in estuaries or coastal lagoons; others push far inland over years. Otolith microchemistry, which tracks the ratio of strontium to calcium laid down in an eel’s ear bones over its lifetime, has revealed that individual eels use surprisingly varied habitats. Some remain in pure freshwater their entire lives, while others occupy areas with noticeably higher salinity, and some individuals shift environments more than once during their residence in a waterway.8Marine Ecology Progress Series. Evidence of different habitat use by New Zealand freshwater eels Anguilla australis and A. dieffenbachii, as revealed by otolith microchemistry
The journey out is the mirror image, and it is a one-way trip. After decades of feeding and growing, eels undergo a dramatic physical transformation into their “silver” stage: their eyes enlarge, their skin thickens and turns silvery, their gut degenerates, and their gonads begin to develop. They then migrate downstream and out to sea, swimming thousands of kilometers to a spawning ground somewhere in the western tropical Pacific. They spawn once and die. No adult New Zealand eel has ever been tracked to its spawning site, and no eggs or newly hatched larvae have been collected there. The location is inferred from where the youngest leptocephali have been caught in plankton nets.
What Eels Mean for Freshwater Food Webs
Because longfin eels are apex predators in most of New Zealand’s rivers and streams, what feeds them has outsized consequences for the entire freshwater ecosystem. A striking illustration comes from Waituna Creek in Southland, where researchers measured exceptionally high biomasses of large predatory fish, on the order of 610 to 830 kilograms per kilometer of stream. This productivity was not sustained by the degraded farmland surrounding the creek. Instead, small migratory fish called īnanga (Galaxias maculatus), swimming upstream from a near-pristine estuarine lagoon, supplied 60 to 80 percent of longfin eel biomass over two sampling years. The researchers estimated that large predatory fish in the creek consume roughly two tonnes of migrating whitebait annually.9New Zealand Journal of Marine and Freshwater Research. Sea to the mountains: quantifying freshwater eel and trout diet reliance on marine subsidies from upstream migrating fish
Follow-up work drove the point home further. When migratory prey fish were available, longfin eel growth increased by roughly 350 percent compared with periods when migration was blocked by natural variability in lagoon mouth openings. The eels were already at carrying capacity in terms of population density, so the extra food did not attract more eels; it simply made existing eels grow much faster. That finding carries a strong conservation message: restoring fish passage at river mouths and lagoon openings can dramatically boost eel production even in habitats that are otherwise degraded.10Journal of Applied Ecology. Cross‐ecosystem subsidies support growth‐based freshwater eel conservation It also challenges the traditional assumption that stream restoration should always begin at the headwaters and work down. For eels, keeping the connection to the sea open at the bottom of the catchment may matter just as much.
Cultural Significance for Māori
Eels, or tuna in te reo Māori, have been a staple food and a cultural touchstone for Māori communities for centuries. Customary harvest of tuna is not simply about catching fish; it plays a role in expressing Māori identity, maintaining relationships with particular places and ancestral connections (whakapapa), and passing traditional ecological knowledge (mātauranga) from one generation to the next.11New Zealand Journal of Marine and Freshwater Research. Understanding Population Sustainability and Safe Consumption Limits to Inform Cultural Harvest of Tuna Within Nelson Lakes National Park Specific waterways, their eel populations, and the practices surrounding their harvest carry deep meaning that goes well beyond food production.
Reconciling customary harvest with conservation has become an increasingly visible part of freshwater management in New Zealand. One approach that has shown promise combines mātauranga Māori with conventional scientific assessment to evaluate both population sustainability and food safety. Research in the Nelson Lakes National Park area demonstrated that harvest guided by tikanga (customary protocols) and informed by scientific monitoring can be sustained safely, even within protected conservation lands. The researchers described the model as transferable to other iwi-led freshwater management efforts around the country.12New Zealand Journal of Marine and Freshwater Research. Understanding Population Sustainability and Safe Consumption Limits to Inform Cultural Harvest of Tuna Within Nelson Lakes National Park
Dams, Turbines, and Getting Eels Past Them
Hydroelectric dams are one of the most direct threats to New Zealand eels, and the problem cuts both ways. On the upstream side, dams block juvenile eels from reaching habitat above the barrier. On the downstream side, mature silver eels migrating to sea can be killed or injured by turbines. Because longfin eels may spend 30 to 80 years in freshwater before migrating, a single turbine strike destroys decades of investment in a single individual.
To address the upstream barrier, fisheries managers have transferred elvers from the base of dams into reservoirs above them. Programs on the Patea, Waikato, and Rangitaiki rivers have all successfully established fishable eel populations within six years of the first transfers. In Lake Arapuni on the Waikato, eels reached marketable size in under four years.13Fisheries Management and Ecology. Enhancement and management of eel fisheries affected by hydroelectric dams in New Zealand Getting mature eels back downstream past the dams is harder. Spillway openings during autumn rain events and netting in headraces have been used, but preventing eels from being drawn into turbine intakes in the first place remains an ongoing engineering challenge.
Potential solutions include operational changes like turbine shutdowns during peak eel migration periods, ensuring that spillways are available as safe passage routes, and installing bypass channels in power station forebays to divert eels away from the intakes.14PubMed. Evaluating the impact of hydropower on downstream migrating anguillid eels: Catchment-wide and fine-scale approaches to identify cost-effective solutions None of these measures is universally adopted yet, and cost remains a factor. But the research suggests that focusing mitigation on the few specific structures where eels are most likely to enter forebays can be more effective than trying to protect every meter of canal.
Contaminants in Eel Tissue
The longevity that makes eels biologically fascinating also makes them efficient accumulators of environmental pollutants. Mercury is the most widely studied contaminant in New Zealand eels. In longfin eels from three South Island rivers, most of the mercury in muscle tissue was in the form of methylmercury (over 84 percent), and concentrations increased linearly with both body length and age. Eels from urban-dominated waterways had higher concentrations than those from rivers running through native bush or agricultural land.15PubMed. Methyl mercury bioaccumulation in long-finned eels, Anguilla dieffenbachii, from three rivers in Otago, New Zealand
Mercury is not the only concern. Eels from harvesting sites in the South Canterbury region contained measurable concentrations of several persistent organic pollutants, including the DDT breakdown product p,p’-DDE, PCBs, dieldrin, and chlordane compounds.16PubMed. Organochlorines and heavy metals in wild caught food as a potential human health risk to the indigenous Māori population of South Canterbury, New Zealand Many of these chemicals entered the environment decades ago through agricultural use and persist in sediments. Because eels are long-lived apex predators, they accumulate these compounds over years and concentrate them to levels that can exceed those in their prey. The contamination issue carries particular weight for Māori communities who harvest eels as customary food, and it underscores why food-safety assessment is now being integrated into harvest management frameworks.
Parasites and Disease
New Zealand eels carry a substantial diversity of parasites. An early survey of 839 shortfin and 459 longfin eels from 34 locations around the country identified 30 parasitic species, 26 of which were considered regular parasites of eels rather than accidental infections.17New Zealand Journal of Marine and Freshwater Research. Distribution of some parasites of freshwater eels in New Zealand Most of these are protozoans, trematodes, nematodes, and larval freshwater mussels that attach to gills. They are part of the normal ecological community associated with eels and, in most cases, do not cause obvious harm.
The parasite that has attracted the most research attention is Anguillicola novaezelandiae, a swimbladder nematode native to New Zealand. (A close relative, Anguillicola crassus, has devastated European eel populations after being introduced from Asia.) In New Zealand shortfin eels, prevalence of A. novaezelandiae was low, under 12 percent at the sites studied, and infected eels typically carried between one and five worms each.18New Zealand Journal of Marine and Freshwater Research. Anguillicolosis in the short‐finned eel Anguilla australis: Epidemiology and pathogenicity Compared with the severe swimbladder damage caused by A. crassus in European eels, the damage in New Zealand eels was minimal, with no detectable effect on body condition even in large silver eels about to migrate to their spawning grounds. This is consistent with what you would expect from a long-evolved host-parasite relationship where the host has developed tolerance.
Interestingly, the parasite’s population structure differs between the North and South Islands. Eels in the South Island carried only larval stages, while both adult and pre-adult worms were found in North Island eels. This unusual pattern of larval-dominated infections may reflect seasonal dynamics in the parasite’s life cycle, something not previously described for any species in the Anguillicola genus.19PubMed. Natural Anguillicola novaezelandiae infection–is there seasonality in New Zealand?
Unexpected Eel Habitat on Farmland
When people think of eel habitat in New Zealand, they typically picture rivers, lakes, and wetlands. But eels turn up in places that are easy to overlook. Surveys of 56 small ponds built for waterfowl hunting on private agricultural land across Southland found eels in about half of them. Shortfin eels appeared in 28 ponds with an average population of 22 individuals and a mean biomass of about 9 kilograms per pond. Longfin eels were in 26 ponds, averaging nine individuals per pond but carrying a higher average biomass of about 7.6 kilograms, reflecting the longfin’s tendency to grow larger.20New Zealand Journal of Zoology. Waterfowl hunting wetlands as habitat for two New Zealand eel species
Scaling those numbers up to the estimated 7,000-plus duck ponds in Southland alone, the researchers calculated that these small artificial wetlands collectively support roughly 36,000 shortfin eels weighing about 15,500 kilograms and around 60,000 longfin eels weighing an estimated 53,000 kilograms.21New Zealand Journal of Zoology. Waterfowl hunting wetlands as habitat for two New Zealand eel species That is a meaningful amount of longfin eel biomass sitting in habitats that do not appear in any conservation plan. The finding illustrates how adaptable eels are: give them water, cover, and a connection to the sea at some point in their lives, and they will show up. It also raises questions about how habitat loss is measured, because gains in unlikely places like farm ponds could partly offset losses elsewhere without anyone noticing.