Tuna or freshwater eels are a very significant, widely-valued, heavily-exploited, culturally iconic mahinga kai resource.
Common name: Freshwater eel
Scientific names: Anguilla dieffenbachii (longfin eel) and Anguilla australis (shortfin eel)
Māori names: includes tuna*
About this species
Three species of freshwater eel are found in Aotearoa: the longfin eel, shortfin eel, and most recently the Australian spotted longfin eel (A. reinhardtii). In order to complete their life cycle, freshwater eels must move between freshwater and the sea (known as a diadromy), spending extended periods in marine, estuarine, and freshwater habitats. The eel has a unique larval stage, known as a leptocephalus, which is only found in the sea. Breeding occurs in the marine environment, following an extended adult growth stage in freshwater, and a very long migration from their freshwater habitat.
Longfin eels are found throughout Aotearoa and are unique to this country. They are legendary climbers and have penetrated well inland in most river systems, even those with natural barriers. The longfin eel is the largest freshwater eel and, although it is one of our most common freshwater fish, there is concern now at the scarcity of very large specimens.
Shortfin eels are found throughout Aotearoa and on Chatham and Stewart Island. However, they are not unique to this country and also occur throughout the South Pacific (e.g., Australia, New Caledonia, Norfolk, and Lord Howe Island). Generally, shortfin eels are found at lower elevations and not as far inland as longfin eels, but they are still able to climb large obstacles, such as waterfalls, when they are young. Both species form the basis of important customary fisheries, and a commercial eel fishery has existed for over 20 years in New Zealand.
Longfin eels tend to predominate in forested streams, whereas shortfins are often more prevalent in lowland pasture streams, lowland lakes, and wetlands.
Shortfin eels are our most tolerant native fish species. They survive environmental hazards like high water temperatures or low dissolved oxygen concentrations. That means they can generally live in habitats where other species cannot survive.
*Tuna is a generic Māori word for freshwater eels; however, but there are a multitude of names that relate variously to tribal origins, appearance, coloration, season of the year, eel size, eel behavior, locality, and capture method.
Tuna are arguably one of the most important mahinga kai resources for Māori. They were abundant, easily caught, and highly nutritious. Tuna were especially important in southern New Zealand, where it was too cold to grow some crop foods. Some tuna were considered sacred, and on occasion large eels were fed and noted to be treated as gods.
Many methods were used to take tuna - a common method for capturing adult tuna was using baited hīnaki, while juvenile tuna were taken on their upstream migration in bunches of fern. Tuna were often stored live in large pots for later consumption or hung to dry.
Impacts on tuna
A number of factors have combined to impact tuna populations across Aotearoa, including forest clearance and wetland drainage, overfishing, flood protection works, and migration barriers such as hydro-dams.
When river and lake levels increase, eels forage on the terrestrial invertebrates that become accessible along the water's edge. Fluctuating water levels affect the available habitat where eels may forage. Eels tend to be scarce in winter time, when they seek cover. Both longfin and shortfin eels tolerate temperatures up to 35ºC, although they prefer temperatures around 22-25ºC.
Although suspended sediments don’t seem to directly affect eels, suspended sediments can negatively affect tuna populations because they impact the invertebrates that tuna feed on. Large eels feed on kōura, so any factors affecting kōura populations can also effect tuna. Accumulation rates of heavy metals such as mercury and organic pesticides in tuna can be much higher in pastoral streams than urban or native forest and shrub streams.
Industrial contamination of water can produce a chemical barrier to migrating tuna. However, the most common barriers to migration are hydro-dams, incorrectly installed culverts, and weirs which prevent the upstream movement of elvers (juvenile eels) to adult habitat and the downstream migration of adults to the sea, where they reproduce. Large hydro-electric dams can be surmounted if appropriate facilities are provided for juvenile eel passage (as is the case at many, but not all, sites across New Zealand); however, very few hydro-dams in Aotearoa currently have facilities to safely accommodate downstream migrating adults.