Freshwater eels have a remarkable life cycle, which begins and ends in the ocean. Spawning has never been observed.
Adult eels: spawning at sea
Adult eels probably spawn at some depth in warm seas. New Zealand’s shortfin eels produce 1.5–3 million eggs, and the longfins 1–20 million eggs. Males fertilise the eggs. After spawning, the adults die.
Larvae: drifting to land
Fertilised eggs hatch at the surface and become leaf-shaped larvae, floating on ocean currents towards the coast. They have teeth, but it is not clear for what purpose – they may store calcium for bone development. Their skin may absorb nutrients, as researchers have not found food in the larvae.
Glass eels: migrating into estuaries
Once the larvae reach land, an extraordinary transformation takes place: they become slender, transparent eels, known as glass eels. They arrive at New Zealand’s coast from July to December, with numbers peaking in spring (August–October) – the time of whitebait migration. Glass eels migrate into river mouths or estuaries in astounding numbers.
Up and over
Hydroelectric dams are an obstacle to elvers (young eels) swimming upriver. Some dams have special passes, allowing them to get round the massive concrete walls. But they don’t always need this help. Elvers can climb the 43-metre Arapuni Dam on the Waikato River, and the 75-metre Patea River dam in Taranaki.
Elvers: swimming upriver
Glass eels soon turn grey-brown, and in this form they are known as elvers. They migrate upriver, often in swarms and usually at night. Young elvers can climb waterfalls, but lose this skill as they grow.
Elvers become adults, with bigger heads and fatter bodies. After many years in fresh water, eels migrate back down the waterways to the sea. It is thought that males fertilise the eggs once the females spawn out at sea.
Migration to the sea
When they reach breeding size, eels change from ‘yellow-bellies’ to ‘silver-bellies’. The yellow-grey underside becomes grey-white, the head shape changes and the head, back and pectoral fins darken.
Shortfin males migrate in February–March, and longfin males in April. The females soon follow, and both males and females die after spawning. Studies show the species also migrate at different ages:
- Shortfin males at an average of 14 years (38–58 centimetres), females at 22 years (50–100 centimetres).
- Longfin males an average of 23 years (48–74 centimetres), females at 34 years (75–180 centimetres).
It is not known how long the journey takes. One female longfin eel that was tagged took 161 days to swim from Canterbury’s Lake Ellesmere (Te Waihora) to a point 160 kilometres north-east of New Caledonia.
Barriers across waterways have hampered their route. One estimate suggests that hydroelectric dams have blocked the longfin eel’s access to the sea in 35% of its habitat.
Fish out of water
As a young man, author David Graham worked on a Dannevirke dairy farm where eels slithered through wet grass from one waterway to another. ‘It was quite a common occurrence to leave home in the dark to bring the cows home to be milked and arrive back with several of these Eels, which were easily stunned with a blow on the head, and used for a meal.’ 1
Life cycle: a long-standing mystery
For centuries, larval eels were thought to be a separate species: they occur in the ocean and look different from adult, freshwater eels. Then in 1896 the Italian zoologist Giovanni Grassi reported that Leptocephalus brevirostris, known as a saltwater fish, was in fact the larva of the European freshwater eel. But just where at sea they bred was a mystery.
Searching for breeding grounds
In a 1923 paper, Danish biologist Johannes Schmidt stated that American and European eels spawned in the Sargasso Sea, in the Atlantic. In 1926, after sailing his research vessel Dana II to Australia and New Zealand, he concluded that New Zealand eels probably bred somewhere east of New Caledonia. But the exact locations are still not fully known.
At migration, longfin eels are more ready to reproduce than shortfin eels. Scientists thought this meant their spawning grounds were closer to shore. However, a study showed that the longfin eel had the longest larval stage of any Pacific freshwater eel reported. This suggests that the larvae actually hatch further away from New Zealand, possibly near Tonga. They were also the biggest specimens when they reached coastal waters – so they may have been at sea longer, and travelled further.