Thursday, January 12, 2012

ZenTiger Long finned eels

Longfin eels breed only once, at the end of their life. When they are ready to breed, they leave New Zealand and swim 5000 kilometres up into the tropical Pacific to spawn, probably in deep ocean trenches somewhere near Tonga.

When they reach their destination, the females lay millions of eggs that are fertilised by the male. The larvae are called leptocephalus and look nothing like an eel - they are transparent, flat, and leaf-shaped. The larvae reach New Zealand by drifting on ocean currents [which can take 1 to 2 years].

Before entering fresh water, the leptocephalus change into a more familiar eel shape, although they remain transparent for up to a week after leaving the sea. These tiny "glass" eels enter fresh water between July and November each year, often in very large numbers.

Eels take many years to grow and it could be decades before an individual is ready to undertake the long migration back to the tropics to breed. The average age at which a longfin eel migrates is 23 years for a male and 34 for a female. The adults never return as they die after spawning.

Some may live to 80 years or so before trying to breed.

Longfin eels are fished commercially, and it's no wonder that over fishing is leading to their extinction. When they can only breed once, somewhere off Tonga, once every 30 years or so, it's not surprising to see that recovery of eel numbers would be difficult to achieve.

Reference: About the NZ Longfin Eel

7 comment(s):

robertguyton said...

When the lakes the adults are maturing in are dammed for electricity generation, the mature adults,magnificent animals with eyes that have grown bigger in order tht they can see in the darker depths of the ocean and stomachs that have disappeared as they won't be feeding on the way, intent on a trip to the Tongan trench, have to pass through the turbines to get there. They get minced.

ZenTiger said...

There are a couple of places with Eel saving programmes helping with that problem, and some National Parks have protected rivers for eels, which is good.

Andrei said...


They are not an endangered species nor are they particularly rare.

Thing about eels is that the vast majority of them never get to breed at all and the ones that do produce 10-20 million offspring who go about their business in this hostile world and for the most part come to grief one way or another before they breed.

The math is - provided 2 out of the 20 million spawned in each breeding survive to breed the population remains constant.

Nature is cruel

Adolf Fiinkensein said...

Is this the same eel I know as 'silver belly'?

Unlike the shorter finned version 'kufaru' also known as yellow belly?

David Winter said...


Both species have yellow and white bellies - the change along comes along with the shape of the head and a bunch of physiological differences when eels get ready to go out to sea.


You might want to take ecology 101. Life history theory tells us organisms that produce many gametes usually do so because the probability of any of them being fertilized then surviving is very low. You can certainly have recruitment failure in species that produce millions of eggs - see white abalone as an example.

Andrei said...

David - Billions of little glass eels enter our waterways every year and most of them get eaten, even by their own kind.

Two species occupying the same ecological niche more or less - given time one will predominate and the other will go extinct - that's nature for you - cold hearted bitch mother nature is.

David Winter said...

I don't what that has to do with overfishing but, it's usually true that two species can't occupy the same niche in the same range, so when we find two related species living in the same place it's a pretty good bet they don't occupy the same niche. That's certainly true of these species.

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