Synthetic fibre ropes are increasingly popular for mooring lines of marine structures. The adoption of polyester ropes for deep water offshore platforms has resulted in a large material database, but the development of floating wind turbines has widened the scope to include more dynamic loadings and more compliant mooring lines are being developed. These are needed both for the turbines, particularly in shallow water, but also for the weather buoys which are used to obtain data on site conditions before wind park installation. This study has investigated the residual properties of weather buoy mooring lines based on hybrid fibres, polyester with polyolefin fibres. A set of lines was recovered from two shallow water sites after up to 2.5 years at sea. Significant loss of strength was measured after service, up to 50%. The reasons for this, mainly damage to external polyester fibres caused by mussel attachment, are discussed.