Changes in the spatial distribution of autumn spawning herring (Clupea harengus L.) derived from annual acoustic surveys during the period 1984-1996

A time series of abundance data for North Sea autumn spawning herring during the period 1984-1996 was reconstructed from the ICES coordinated acoustic survey reports. The long-term trends in spatial distribution of three age categories (2, 3, and 4+ group) were visually interpreted from contour maps and analysed using Generalized Additive Models. In general the two analyses revealed the same trends and showed a distinct northward shift of distribution with increasing age. The most northerly extent of distribution occurred during the late 1980s and in 1990. This coincided with a period of low recruitment of 0 group fish and relatively high spawning stock biomass. In contrast there was a distinct southerly shift in distribution from 1991 to 1993 when recruitment was seen to rise and biomass started to fall. This paper hypothesizes that marked year to year shifts in the distribution of the herring stock may contribute to the processes governing recruitment. (C) 1998 International Council for the Exploration of the Sea.