Abstract

In fish schools the density varies per location and often individuals are sorted according to familiarity and/or body size. Highdensity is considered advantageous for protection against predators and this sorting is believed to be advantageous not only toavoid predators but also for finding food. In this paper, we list a number of mechanisms and we study, with the help of anindividual-based model of schooling agents, which spatial patterns may result from them. In our model, schooling is regulated bythe following rules: avoiding those that are close by, aligning to those at intermediate distances, and moving towards othersfurther off. Regarding kinship/familiarity, we study patterns that come about when agents actively choose to be close to relatedagents (i.e., ‘active sorting’). Regarding body size, we study what happens when agents merely differ in size but behave accordingto the usual schooling rules (‘size difference model’), when agents choose to be close to those of similar size, and when smallagents avoid larger ones (‘risk avoidance’). Several spatial configurations result: during ‘active sorting’ familiar agents grouptogether anywhere in the shoal, but agents of different size group concentrically, whereby the small agents occupy the center andthe large ones the periphery (‘size difference model’ and ‘active sorting’). If small agents avoid the risk of being close to largeones, however, small agents end up at the periphery and large ones occupy the center (‘risk avoidance’). Spatial configurationsare also influenced by the composition of the group, namely the percentage of agents of each type. Furthermore, schools areusually oblong and their density is always greatest near the front. We explain the way in which these patterns emerge and indicatehow results of our model may guide the study of spatial patterns in real animals. Key words: assortment, density distribution, fish,self-organization, school form, spatial structure.