Aspects of the Distribution and Growth of Bluemouth (Helicolenus dactylopterus, Delaroche 1809) since its Invasion of the Northern North Sea in 1991

Recent trends in the abundance, distribution and growth of bluemouth in Scottish waters are described. The observations are all based on an ecologically important time-series database of trawl surveys conducted annually between 1925 and 2004 by Fisheries Research Services, Aberdeen. We examine the period 1985-2004 in detail. Bluemouth abundance has increased over the last decade both along the continental shelf west of Scotland and in the North Sea. In 1991 bluemouth appeared in the northern North Sea as a distinct cohort, with total lengths ranging between 6 and 8 cm. 1991 was a year of exceptional Atlantic inflow into the North Sea, and it is likely that the bluemouth used this water mass as a highway, entering from the north and west as small individuals. The fish from 1991 grew steadily and formed a distinct length mode that we track here until its disappearance in 2002. In the first few years following this influx, bluemouth were recorded all around the northern North Sea, but since 1997 numbers have declined and its distribution has shrunk into one of two International Council for the Exploration of the Sea (ICES) statistical rectangles (39F1 and 40F2) which are positioned in the central part of the North Sea. Since 1991 only one subsequent and relatively minor immigration of bluemouth into the North Sea has taken place, in 1998. The composition of the North Sea bluemouth population as a single cohort allows its spatial distribution, expansion and growth rate to be charted, discussed and described. Key words: bluemouth, growth rates, long-term spatio-temporal changes, North SeaJRC.G.3-Agricultur