Fat for One Last Mission

Wildlife biologist Guy Verreault describes the importance of the fat accumulated by eels when they live in fresh water.

A close-up head and shoulder view of Guy Verreault appears on the screen. He is facing the camera and gestures with his hands from time to time as he speaks.

[Interviewer] How do you know when an eel is mature?

[Guy Verreault] There are physiological signs that let eels know when they’re ready to make the journey.

The main indication for an animal is its percentage of fat. We don’t notice such things [about ourselves] anymore because we’ve now become a bit disconnected from our survival.

Map showing the migration route of eels, from the St. Lawrence River to the Sargasso Sea.

In the case of an animal, fat is what gives it a chance to survive. In the case of an eel that has to migrate, fat is what gives it a chance to live.

There are 4000 km between the habitat where eels spend almost their entire lives and the place where they go to reproduce. 4000 km, that requires a lot of energy. When we drive our cars, we need gas; when eels migrate, they simply need fat.

A close-up head and shoulder view of a jovial-looking Guy Verreault reappears on the screen.

So, eels need fat to travel. But they also need fat to reproduce.

A normal eel from here, measuring about 1 m long, can contain 12 to 20 million eggs. Each one of those little eggs needs a drop of lipid, or fat, so that it can float and provide food for future larvae. Therefore, when an eel contains 20% fat, that’s OK. Its fat content is 20%. It starts getting signals that it can migrate. It then starts to migrate. It has enough fuel to get to the place where it will reproduce. Once it gets there, it will still have enough resources to put a small drop of fat in each of its eggs so that they can survive and start the entire cycle all over again.

So that’s the main physiological signal that eels receive.