The high-tech mission involved sending an unmanned glider to an area around 50 miles (80km) south west of Milford Haven, called the Celtic Deep.
The glider travelled 372 miles (600km) in 30 days, undertaking nearly 3,000 dives to the seabed.
Data from the robot shows a previously hidden plankton concentration around 30m (98ft) below the surface.
Scientists said this marine "larder" is likely to be a foraging area for a range of creatures.
A busy shipping route and fishing ground, the Celtic Deep is also a haven for wildlife, attracting dolphins, porpoise and the world's second largest animal - the fin whale.
The information gathered is part of a bigger project run in partnership by WWF and the National Oceanography Centre (NOC) to improve understanding of our seas.
The data will be used to support calls for further sites, such as the Celtic Deep, to be added to a network of marine protected areas around Wales.
A public consultation on the issue, on behalf of the Welsh Government, has been held this year - although the Celtic Deep is not one of the proposed sites for extra protection.
"This incredible technology is giving us a completely new level of understanding of our seas," said Dr Lyndsey Dodds, WWF-UK head of marine policy.
"The latest evidence pinpoints important food areas for creatures such as porpoises - something that would be invisible to satellites.
"Data like this is really important because it gives us a clearer picture of life beneath the surface. That is vital for Welsh and UK governments to help them improve the way they manage our waters and help both people and nature."
This weekend sees the start of the next phase of the project, with researchers launching another robot from Newlyn in Cornwall.
The innovative surface vehicle named "Thomas" will gather data and images from another "hotspot" off the Isles of Scilly, and will again be paired up with a submarine glider.
It is the second attempt to launch Thomas after he ran into technical problems in Milford Haven in Pembrokeshire last summer.
Powered by the sun, wind and waves, the robots are operated remotely by scientists in Southampton and Portsmouth via satellite link.
NOC's chief scientist of marine autonomous and robotic systems, Prof Russell Wynn, said: "We have already successfully demonstrated the ability to capture valuable data from beneath the surface, and now we hope to capture images and sounds of marine life at the sea surface."
A "hidden larder" for sharks and whales has been discovered off the west coast of Wales by a pioneering marine robot.