Is the level of aggression and dispersion in territorial fish dependent onlight intensity?

Valdimarsson, SK Metcalfe, NB

Sk. Valdimarsson et Nb. Metcalfe, Is the level of aggression and dispersion in territorial fish dependent onlight intensity?, ANIM BEHAV, 61, 2001, pp. 1143-1149

59

INGLESE

Article

Animal Sciences","Neurosciences & Behavoir

ANIMAL BEHAVIOUR

0003-3472 → ACNP

61

2001

6

1143 - 1149

ISI

0003-3472(200106)61:<1143:ITLOAA>2.0.ZU;2-F

Territory size in animals is traditionally taken to be the result of a trad e-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 beco mes more difficult for visually oriented animals both to see intruders (and therefore to defend a territory) and to locate the resource (e.g. food). T his could result in a direct relationship between light intensity and terri tory size, with animals aggregating in preferred foraging areas at lower li ght intensities. We tested this hypothesis by recording the dispersion and aggressive behaviour (behaviours normally associated with territoriality) o f replicate groups of juvenile Atlantic salmon, Salmo salar exposed to four night-time light intensities (0.00, 0.01, 0.50 and 1.00 Ix, the latter equ ivalent to dawn/dusk) on successive nights. There were clear effects of lig ht intensity: fish tolerated other individuals nearer to them and showed li ttle aggression under conditions equivalent to starlight (0.00-0.01 Ix), bu t both aggression rate and the distance between fish increased markedly wit h light level. The fish were also most aggressive early in the night (possi bly because of readjustments in territory mosaics). These results may indic ate that territory size, and hence the territorial mosaic, in salmon is dyn amic, changing greatly between low and high light intensities, with fish de fending much smaller territories on dark nights (rather than being nonterri torial, as previously suggested). This has implications for population regu latory mechanisms in winter, when the fish are predominantly nocturnal. (C) 2001 The Association for the Study of Animal Behaviour.