“So, if the middle of the ocean is a desert, what do the tuna eat?”
I was talking to a group of young people for the UK’s Potential Plus program for gifted kids. It was a great question
If you happen to be herding schools of tuna through the South Pacific gyre, and a large portion of it happens to be a desert-like expanse of ocean, where will your tuna find food? Tuna are voracious eaters, and they are always on the move.
Well, it so happens that the seas are only like deserts – with just a scattering of plants and animals – in the upper 200 meters of water. This is because the waters which are warmed by the sun create a barrier called thermoclines that prevent the nutrients in the colder depths from rising up to the top. This leaves the upper 200 meters of water with little resources and so little life.
Just below 200 meters, down to 1000 meters, is the ‘twilight zone’ of the sea because little light makes it this far down. An unexpectedly large number of creatures live in this twilight area, called the mesopelagic zone: marine species like lanternfish, bristlemouths, jellyfish and squid. In fact, the current estimate is that 95% of the ocean’s fish reside in the mesopelagic zone of the open oceans. That’s right, 95%!
Humans don’t harvest much from this zone. Even trawling fishing vessels don’t snare these creatures – apparently they are very adept at avoiding the nets.
This is a very recent discovery and means, ahem, that the open ocean is not quite the desert it seems. Tuna have been shown to dive up to 1000 meters to get food, so these vast feeding grounds are open and available to the Pelagic tuna herders.
In the story pf Pelagia, robotic ‘guide tuna’ bots lead their schools of fish to feeding grounds like this: feasting in the midst of ocean deserts.
The wonder of the seas never ceases to surprise me!