On the relation between schools, clusters of schools, and abundance in pelagic fish stocks

Small pelagic fish are known to aggregate into schools and clusters of scho ols. It is commonly assumed that the number of such schools and clusters, a s well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. T he study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those sc hools. The study also showed that the number and structure of the school cl usters was strongly correlated with the number of schools seen. An increase d number of schools in an area tended to be linked with denser clusters (mo re schools per kilometre) and a higher occupation of the survey area by tho se clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundanc e are due to density independence in aggregation patterns or whether such d ensity dependence is only functional at relatively low stock abundance leve ls. (C) 2001 International Council for the Exploration of the Sea.