A north to south running deep-water channel (St. Georges Channel) with a maximum depth of 150m separates the relatively shallow western and eastern regions of the Irish Sea (Figure 1b). The main flow of water through the relatively narrow western Irish Sea flows south to north from the North Channel, whilst an anti-clockwise gyre dominates circulation patterns in the eastern Irish Sea (Figure 1c). There are two main seasonal fronts in the Irish Sea: the Western Irish Sea Front, which separates mixed waters to its south-east from the stratified waters to its north-west and the Celtic Sea Front that separates the cooler, mixed waters of St. Georges Channel from the warmer surface waters of the Celtic Sea. Seasonal fronts may also occur in the eastern Irish Sea and Cardigan Bay (Figure 1b). The temperature in the Irish Sea ranges from 6ºC in the winter to 16ºC in the summer. Sand is the dominant sediment type in the western and eastern Irish Sea, whilst gravel is the prominent type in the deeper waters of the mid Irish Sea and St. Georges Channel. There are also two offshore mud grounds; one in the northeast and a more extensive one in the northwest (Figure 1d). Parker-Humphreys (2004) provides a more detailed description of the physical characteristics of the Irish Sea.