Cross-scale observations on distribution and behavioural dynamics of oceanfeeding Norwegian spring-spawning herring (Clupea harengus L.)

Interpretation of the behavioural dynamics and distribution of herring requ ires explicit consideration of spatial and temporal scales since no single mechanism can explain the pattern on an scales. High resolution sonar and e chosounders were used as our observation tools. The cluster ratio, mean Nea rest Neighbour Distance (mean ISD): mean of the average Inter-school Distan ce (mean ISD) provides a descriptive index linking scales of distribution p attern within and between clusters of schools. It can be used to compare sc hool clustering patterns for surveys made at different places and seasons. Two mesoscale patterns of school clustering were observed, as indicated by the distribution of nearest neighbour distances; one at 0.05-0.3 km reveale d by sonar and another at 0.8-2.5 km revealed by echosounder. In general, s chools were tightly aggregated, whilst school clusters were patchily distri buted. Heightened feeding motivation explains the predominance of relativel y small schools and their tendency to be found closer together and more clu stered than large schools. Differences in distribution and density of food patches and predators may account to some degree for the observations (1) s chools were distributed according to their size; both small and large schoo ls had nearest neighbours of similar size more often than expected, (2) spl itting and joining of schools were the most frequently recorded behavioural events. A considerable amount of variation in distribution and dynamics ca n be accounted for by changes in behaviour associated with diurnal vertical migrations. Some schools apparently did not perform diurnal vertical migra tions. We suggest that when profitable, some herring may "dip in to the fri dge" (feed in deep, cold water) during the day rather than pursue food on p otentially high energy migrations. (C) 1999 International Council for the E xploration of the Sea.