FFK Blog Fehmeeda Farid Khan Blog for Unsolved Miseries
Somewhere in the remote Pacific Ocean, there’s a place only eels know. It’s a graveyard and a nursery; the beginning and end point for one of nature’s most poetic and mysterious life cycles.
It’s a truism that we know more about outer space than Earth’s oceans. This is made evident by an enduring mystery whose resolution has proved as slippery as an eel: despite advances in ocean research technology, marine biologists have yet to discover the secret spawning grounds of the New Zealand longfin eel.
Longfin eels (Anguilla dieffenbachii), called ōrea or tuna in Māori, are a common sight in the rivers, lakes, and streams of New Zealand, where they are the only endemic eel species. They are the largest and longest-lived species of freshwater eel; females can grow up to 2 meters long and live over 100 years (though the average lifespan is 20-60 years). Like all freshwater eels, they evolved from a marine ancestor, which gave rise to their unusual life cycle.
Though they spend most of their lives in freshwater, they’re born in the ocean—and, when the time comes, they return to it. Longfin eels mate only once, at the end of their long lives, in the same deep ocean trenches where they were spawned. It’s a bittersweet homecoming. We’ve never witnessed their mating process, but once the eggs are fertilized, the adult eels die.
This final journey takes them from the inland waterways of New Zealand to an area at least 2500 km away, somewhere between New Caledonia and Tonga. The precise location has not yet been pinpointed, though some likely candidates are the Tonga Trench and the eastern Fiji Basin.
There have been attempts to map the eels’ movements with tracking tags, which have helped narrow down the possibilities. However, few of the tagged specimens survived the perilous migration, the remainder dislodged their tags along the way, and the devices were hindered by the unexpected depths the eels would dive to avoid predators. Future expeditions using updated tracking technology may finally reveal where these elusive creatures go.
However, their breeding grounds’ location isn’t the only secret longfin eels are keeping. Upon hatching, their larvae, called leptocephali, drift on ocean currents back to the estuaries of New Zealand, where they undergo metamorphosis into freshwater-adapted juveniles. Despite this, no longfin larvae or eggs have ever been observed or collected. We can assume, however, that the larvae resemble those of other eel species: transparent, leaf-shaped, and among the ocean’s tiniest animals.
Also not fully understood is what prompts the mature eels to prepare for spawning. Evidence points to a combination of physiological and environmental factors. Before they migrate, they stop eating—the entire trip, which takes 5-6 months, is made without food. As they travel from inland waters toward the sea, they go through a series of physical changes, such as developing larger eyes and sleeker bodies better suited to ocean life. Once in brackish water, their gills undergo a true sea change: freshwater respiration back to saltwater, in a reversal of the metamorphosis they went through as juveniles. They’re a species for whom the beginning and the end are elegantly mirrored.
Besides curiosity, there are practical reasons to learn as much as we can about these strange fish. Although they are endangered, commercial fishing continues even as their numbers decline. The threats of habitat loss, pollution, construction of hydroelectric dams, and other human factors could be mitigated with a better understanding of their life cycles. They’re an important food source, particularly in Māori culture, and their presence is necessary for the health of New Zealand’s ecosystem. Learning more about their breeding habits could help determine if aquaculture is feasible, and if their declining population can be replenished.
