Underwater fish "meadows" are being lost at the same rate as the Amazon rain forests, researchers have warned.
Seagrass is a key habitat for feeding and sheltering young fish, including plaice, haddock and pollock.
But every hour an area the size of two football pitches is destroyed.
Scientists from Swansea University believe the habitats need to be protected otherwise fishing stocks could be affected.
"The rate of loss is equal to that occurring in tropical rainforests and on coral reefs yet it receives a fraction of the attention," said Dr Richard Unsworth, lead researcher.
"If you're a small fish, like a juvenile cod, then you need food and shelter. Seagrass meadows provide both."
The biggest threat is from poor water quality and damage caused by boat anchors and moorings.
The Swansea research, for the Natural Environment Research Council (Nerc), is part of a global conservation effort to save seagrass.
The team, using baited underwater camera systems and netting, took a year to measure the size and number of fish in seagrass meadows in the seas around Britain, and compared the results with nearby sand habitats.
The study included Porthdinllaen and Pen-y-Chain on the Llyn peninsula in Gwynedd.
In one seagrass site off the Gwynedd coast, divers found 42 fish species, 10 of which are important commercially.
"If there's lots of food available for them to eat and reduced predation, like there is in seagrass meadows, they don't spend all their time hunting for food so they're more likely to survive and put on weight faster," said Dr Unsworth.
"When you start to lose these habitats you'll see smaller juveniles and smaller fish stocks."
The research is part of a wider project assessing the benefits of seagrass meadows across the Atlantic, which is funded by the Welsh government and the EU.
"We want to work with partners around the country to look at trying to get this up the conservation agenda," said Dr Unsworth.