© 1997 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Cross-shelf processes north of Scotland in relation to the southerly migration of Western mackerel
SOAEFD Marine Laboratory PO Box 101, Victoria Road, Aberdeen, AB11 9DB, Scotland, UK
Rijksinsituut voor Visserijonderzoek (RIVO) Haringkade 1, PO Box 68, 1970 AB, IJmuiden, The Netherlands
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 km) 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 demonstrated 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 remainder being found further inshore. Mackerel school structure varied dramatically between the areas 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 waters were actively migrating.
Keywords: mackerel, Atlantic water, shelf-edge current migration
Received 7 February 1995; accepted 7 October 1996.
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