By Sveinn K. Valdimarsson, Neil B. Metcalfe
Animal Behaviour, Volume 61, Issue 6, June 2001, Pages 1143-1149, ISSN 0003-3472, DOI: 10.1006/anbe.2001.1710.

Territory size in animals is traditionally taken to be the result of a trade-off between the spatial/temporal distribution of resources and the level of competition. However, it may also be influenced by physical constraints that themselves show temporal variability. Thus at low light levels it becomes more difficult for visually oriented animals both to see intruders (and therefore to defend a territory) and to locate the resource (e.g. food). This could result in a direct relationship between light intensity and territory size, with animals aggregating in preferred foraging areas at lower light intensities. We tested this hypothesis by recording the dispersion and aggressive behaviour (behaviours normally associated with territoriality) of replicate groups of juvenile Atlantic salmon, Salmo salar, exposed to four night-time light intensities (0.00, 0.01, 0.50 and 1.00 lx, the latter equivalent to dawn/dusk) on successive nights. There were clear effects of light intensity: fish tolerated other individuals nearer to them and showed little aggression under conditions equivalent to starlight (0.00-0.01 lx), but both aggression rate and the distance between fish increased markedly with light level. The fish were also most aggressive early in the night (possibly because of readjustments in territory mosaics). These results may indicate that territory size, and hence the territorial mosaic, in salmon is dynamic, changing greatly between low and high light intensities, with fish defending much smaller territories on dark nights (rather than being nonterritorial, as previously suggested). This has implications for population regulatory mechanisms in winter, when the fish are predominantly nocturnal.

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