Over 60 tonnes of heating oil escaped from a storage tank at Friction Dynamex’s former car parts factory at Griffiths Crossing near Caernarfon and ran into surface water drains.
The oil flowed into the Strait through an outfall, causing an eight-mile slick stretching virtually the length of the Strait.
The area is designated as a special area of conservation under the EU habitats Directive.
However, the Environment Agency noticed little impact on the shoreline and says the slick dispersed without the use of detergents.
The site’s owner, Ferotec Realty, paid for contractors to clean the drainage system.
Appearing before Caernarfon magistrates on 26 April, Ferotec Realty pleaded guilty to allowing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991.