Schools and clusters: Interannual variability in the aggregative behavior of North Sea herring from acoustic surveys

Abstract

Commercially exploited pelagic fish species have a strong tendency to aggregate together. Individual fish aggregate into schools of varying sizes and schools will also tend to aggregate into clusters of many schools. If the stock abundance changes, it is expected that the pattern of these aggregations will change. The numbers, sizes, densities of the schools, and clusters may be expected to change with changing stock level. Such changes will in turn influence the conduct of both fishing and survey activities. A knowledge of fish aggregation patterns and the processes involved should improve the reliability of stock monitoring techniques and provide an understanding of the impact of exploitation on dynamically changing populations. Using image processing techniques a database of schools has been developed from data recorded during acoustic surveys for herring in the NW North Sea between 1993–1998. Positional, morphometric, environmental, and energetic descriptors have been computed for each school. The paper will present the results of the analysis of this database, giving particular attention to variation in schooling and clustering patterns, and the spatial distribution of these aggregations, in relation to the changing herring stock size in the period 1993–1998.