Glannau Porthaethwy is a designated Site of Special Scientific Interest

Site of Special Scientific Interest

A Site of Special Scientific Interest is a conservation designation denoting a protected area in the United Kingdom. SSSIs are the basic building block of site-based nature conservation legislation and most other legal nature/geological conservation designations in Great Britain are based upon...

 (SSSI) along the northern shore of the Menai Strait

Menai Strait

The Menai Strait is a narrow stretch of shallow tidal water about long, which separates the island of Anglesey from the mainland of Wales.The strait is bridged in two places - the main A5 road is carried over the strait by Thomas Telford's elegant iron suspension bridge, the first of its kind,...

 in North Wales

North Wales

North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...

. It is a linear designation that extends some 4 km (2.5 mi) along the shore from Britannia Bridge

Britannia Bridge

Britannia Bridge is a bridge across the Menai Strait between the island of Anglesey and the mainland of Wales. It was originally designed and built by Robert Stephenson as a tubular bridge of wrought iron rectangular box-section spans for carrying rail traffic...

 in the west to Craig y Don in the east. It also includes the island shorelines of Ynys Gorad Goch, Ynys Welltog, Ynys Benlas, Ynys Tysilio, Half Tide Rock, Ynys Faelog, Ynys Tobig, Ynys Gaint

Ynys Gaint

Ynys Gaint is a small island in the Menai Strait connected to the town of Menai Bridge on Anglesey by a causeway and also a concrete bridge erected by Sir Guy Fison in the 1930s....

, Ynys Castell

Ynys Castell

Ynys Castell is a small island in the Menai Strait which separates Anglesey and mainland Wales. It is an extruding piece of Precambrian schist lying to one side of the Afon Cadnant estuary. It lies between Ynys y Bîg and Ynys Gaint. There is a causeway running to the island that is covered at high...

 and Ynys y Big

Ynys y Bîg

Ynys y Bîg is a small private island in the Menai Strait attached to the island of Anglesey by a short causeway. The causeway runs from the grounds of a private house, itself also called Ynys y Bîg, preventing any public access....

.

It was designated in 2003, as it is the most extensive sheltered rock shore in the area between Bardsey Island

Bardsey Island

Bardsey Island , the legendary "Island of 20,000 saints", lies off the Llŷn Peninsula in the Welsh county of Gwynedd. The Welsh name means "The Island in the Currents", although its English name refers to the "Island of the Bards", or possibly the island of the Viking chieftan, "Barda". It is ...

 and Great Orme

Great Orme

The Great Orme is a prominent limestone headland on the north coast of Wales situated in Llandudno. It is referred to as Cyngreawdr Fynydd in a poem by the 12th century poet Gwalchmai ap Meilyr...

’s Head and because it supports the greatest diversity of marine plant and animal communities.