The Menai Strait should be better protected because it is a marine biodiversity "hotspot," claims a report by environmental group WWF.
The waterway separating Gwynedd and Anglesey is the only "hotspot" in Wales and one of just seven in the UK.
WWF Cymru wants the Welsh Assembly Government to develop legislation to safeguard the site.
A spokesperson said the Welsh Assembly Government was currently looking at ways to best safeguard marine life.
Species in the Menai Strait include the grey seal and the honeycomb worm, which makes reefs that act as nursery areas for fish and harbour other species like crabs.
Iwan Ball, marine policy officer for WWF Cymru, said: "This report identifies the Menai Strait as one of the most valuable areas in the UK for marine biodiversity and the need to better protect it.
"Our seas are becoming busier than ever before due to a variety of human activities such as shipping, dredging and wind farms, which threaten the marine environment."
Caernarfon MP Hywel Williams said local communities would suffer if the area was not properly protected.
"Local communities depend on these magnificent resources, for fishing, sustainable tourism and for the maintenance and continuation of our way of life," he added.
A Welsh Assembly Government spokesperson said: "The protection of Wales’ marine wildlife and habitats is clearly important.
