/. mar. biol. Ass. U.K. (1995), 75,849-855 849 Printed in Great Britain DEEP-WATER SPAWNING OF ATLANTIC MACKEREL SCOMBER SCOMBRUS, WEST OF IRELAND I.G. PRIEDE, T. RAID* AND J.J. WATSON Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB9 2TN 'Estonian Marine Research Institute, 32 Lai Street, Tallinn EE-0001, Estonia Atlantic mackerel (Scomber scombrus L.) were sampled by pelagic trawl over the Porcu- pine Bank to the west of Ireland between 52°N and 54°N at peak spawning time, between 27 May and 10 June 1992. Distribution of juveniles (age classes 1 and 2) was inversely related to depth, comprising over 90% of hauls close to the coast of Ireland and 20% of the catch on the Porcupine Bank. Egg production increased westwards and was high along the continental shelf edge. Adult spawning fish were found throughout the shelf area and beyond, into waters over 3000 m deep. The results indicate that mackerel spawn over deep water some distance from the shelf edge and may exploit oceanic food resources from deep water. INTRODUCTION The Scombridae are pelagic fishes with often ocean-wide distributions, but the Atlantic mackerel (Scomber scombrus) is generally thought to be confined to the temper- ate shelf areas (Lockwood, 1988) with depths of no more than 200-250 m (Collette, 1986). Greer-Walker & Nichols (1993) observed mackerel feeding on deep-water myctophids (Benthosema glaciale) that come to the surface at night along the edge of the continental shelf, and suggested that spawning mackerel may be able to exploit this food resource beyond the shelf edge. Around the British Isles two main stocks of mackerel are recognized, a North Sea stock that spawns in the North Sea and a western stock that spawns along the European continental shelf edge from the Bay of Biscay northwards (Hamre, 1980; Lockwood et al., 1981a). Spawning occurs between February and July each year with planktonic eggs being released in the surface layers above the thermocline or to depths of over 200 m when no thermocline is present early in the season (Coombs et al., 1981). The peak of spawning is typically in May to early June but significant changes in distribution, abundance and timing of spawning have occurred since studies first began (Coombs & Mitchell, 1981) with the North Sea stock now severely depleted so that the European mackerel fishery is largely sustained by the western stock (Priede & Watson, 1993). In 1977 the spawning stock biomass of the western stock was first estimated using egg abundance from plankton surveys as an index of adult female biomass (Lockwood et al., 1981b). Since then surveys have been carried out every three years to estimate the annual egg production by the western mackerel as a basis for stock assessment, using