AGGRESSIVE BEHAVIOR Volume 30, pages 313–318 (2004) Effect of the Light Intensity Upon the Agonistic Behaviour of Juvenile of White- Seabream (Diplodus sargus cadenati de La Paz, Bauchot and Daget, 1974) Jose ´ J. Castro* and Catalina Caballero Departamento de Biologı ´a, University of Las Palmas de Gran Canaria, Edf. de Ciencias Ba ´ sicas, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Ambient light levels were manipulated to vary potential predation risk, and aggressive behaviour was examined. Juveniles of white-seabream (Diplodus sargus cadenati) interacted aggressively more frequently at intermediate light intensity (500 lux), being agonistic interactions at higher (3000 lux) or lower (5 lux) illumination less frequent. Aggr. Behav. 30:313–318, 2004. r 2004 Wiley-Liss, Inc. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Keywords: light intensity; aggressive behaviour; white-seabream; Diplodus sargus cadenati INTRODUCTION Despite the generally poor quality of underwater images [Guthrie and Muntz, 1993], fish depend a great deal on vision as a source of sensory information. Many fundamental activities (feeding, resting, breeding, etc.) and behavioural pattern (way of feeding, strategies of avoiding predators, matting, etc.) of fish that live in shallow and well illuminated waters are regulated by levels of light, and specially by the day-night cycle [Helfman, 1993; Lythgoe, 1979]. The fish eye is able to distinguish a variety of different properties of visible objects (brightness, texture, shape, contour, shade, colour, etc.) that facilitate exploration [Guthrie, 1986; Guthrie and Muntz, 1993; Nikolsky, 1963; Russell, 1993]. In this way, fish are able to identify and recognise the members of their group, the mate, a potential prey, a predator, etc. [Coates, 1980; Hairston et al., 1983; Karplus et al., 1982; Pitcher et al., 1979;]. Also, in some fish species, small visual signs (especially the colour) are indicatives of hierarchical range or degree of territoriality [Lorenz, 1962; Tinbergen, 1957]. In example, dominant Cyrtocara moorei show a vivid blue colour that distinguish them from the other member of the group, *Correspondence to: Jose´ J. Castro, Departamento de Biologı´a, University of Las Palmas de GranCanaria, Edf. de Ciencias Ba´sicas, Campus de Tafira, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain. E-mail: jcastro@pesca.gi.ulpgc.es Received 3 August 2002; amended version accepted 17 March 2003 Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/ab.20023 r 2004 Wiley-Liss, Inc.