A combined acoustic and hydrographic survey was conducted west of Shetland in January 1995. The temperature salinity structure at the shelf edge north of Scotland was characterised by a narrow (30 kill) core of warm, saline water embedded within a broader distribution of Atlantic Water; this would generally mark the area of the shelf-edge current. Current measurements recorded during the period of this survey demonstration that uncharacteristically, the core did not mark the area of maximum transport along the shelf break but lay inshore of it. Hence larger scale processes associated with the north-west European shelf edge are important in determining the intermediate scale physical environment encountered by mackerel during their southerly migration to the spawning areas. Acoustic survey data revealed that a large number of the mackerel schools were located in, or close to, this warm saline core at the shelf edge, the reminder being found further inshore. Mackerel school structure varied dramatically between the large affected by this core and other parts of the survey area. In general, mackerel form large distinct schools in mid-water, and these were seen during the survey in the shelf waters. In the area of the warm core, schools were found deeper, were more diffuse and tended to form elongated thin layers. We present the hypothesis that the change in schooling behaviour reflects whether or not the schools are actively migrating, and that those schools observed in the warm water core were stationary, and those in cooler water were actively migrating.