Regional and seasonal differences in growth of larval North Sea herring (clupea harengus L.) estimated by otolith microstructure analysis

The ecology processes of the larval life of autumn-spawned North Sea herring have been studied in a multidisciplinary and internationally coordinated research programme (ACE). The programme focused on larval advection and the importance of the autumn/winter circulation in determining larval distribution. In order to evaluate the effect of potential changes in spatial distribution of larvae through time, the growth pattern of spatially separated groups of larvae has been analysed. Growth of larvae was estimated by comparing mean length of groups in four surveys and by examining the relationship between larval length and otolith ring structure within defined geographical regions and restricted periods of time. The analysis indicates a 45% decrease in growth rates through the autumn/winter period along with a substantial difference between southern and northern areas, the growth rates in the south being about 50% higher. The significance of differences is evaluated in relation to method of analysis and assumptions on ring deposition rates. The findings point to the importance of spatial distribution pattern in determining overall growth and survival of herring larvae.