Experiments were conducted to investigate the intraspecific aggressive interactions between juveniles of white‐seabream (Diplodus sargus cadenati de la Paz, Bauchot and Daget 1974) in relation to dominance hierarchy in small groups. The agonistic interactions between juvenile white‐seabream in small groups originated a peck‐dominance hierarchy. The dominant (α‐fish) generally interacted aggressively and most frequently with the subordinate that was next in line (β‐fish) within the dominance hierarchy. This β‐fish interacted aggressively more frequently with the γ‐fish, and so on. Results demonstrated that fish do not attack subordinates at random. The level of subordination in small groups of juvenile white‐seabream was related to body size. Generally, the more aggressive fish manifest a higher growth rate. Aggr. Behav. 24: 197–204, 1998. © 1998 Wiley‐Liss, Inc.

To study the effect of the degree of unsaturation of dietary fatty acids on the production of free radicals and on the vascular smooth muscle tone in rings of the aorta, Sprague-Dawley rats were fed a semipurified diet containing 5% lipids from either corn oil (CO) or menhaden oil (MO) for 8 wk. The MO diet did not change the basal or NADPH-dependent superoxide anion (O2-*) release. There were no significant differences in phenylephrine-induced contractions between the two groups in intact rings. However, these contractions increased in endothelium-intact aortic rings from the MO group after addition of the nitric oxide (*NO) synthase inhibitor NG-nitro-L-arginine and in endothelium-denuded rings, both indicating a greater endothelial basal *NO production in rats fed with the MO diet. Endothelium-dependent relaxations in response to acetylcholine were more prominent in rings from the MO group. These differences were not due to an increased smooth muscle response to.NO, because relaxations were the same using an exogenous *NO donor. Our results indicate that dietary MO did not modify O2-* release by the vessel wall or relaxation due to the cyclooxygenase pathway, but it potentiated endothelial production of *NO.

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.

The number of aggressive interactions displayed by juveniles of Diplodus sargus cadenati increases progressively in proportion to the number of intruders, with an upper threshold of more than 16 to 18 intruder fish. Beyond this density, resident aggressiveness decreases. The number of aggressive interactions that the resident fish devotes to expelling each intruder shows a clear and significant tendency to decrease as competitor density increases. The resident fish is incapable of rapidly evaluating the outweighing of its capacity for defense. Moreover, the strategy of aggressive defense of the resource against the density of competitors does not seem to be only profitability criteria dependent, but rather there is also a significant influence of the ability to fight shown by the resident. Aggr. Behav.

The behavioural patterns of juvenile white‐seabream suggest that asymmetry in residence is an important factor governing the outcome of contest between individuals of this species. Asymmetries due to resident status had strong effects on agonistic behaviour, with asymmetries in body sizes producing weaker effects. Resident fish won all the combats against intruders of lower or similar length. However, when the intruder was larger than the residents (higher than 5% in length and 20% in weight), the percentage of combats won by the residents decreased to 85.7%. There was a significant correlation between fight intensity and size asymmetry in favour of the resident fish. The resident fish was more aggressive and persistent in attacks, and contests were more intense when the size of the intruder was greater. During agonistic interactions, the frontal attacks and lateral displays were more frequent when the intruders were similar or larger than residents. Attacks to the flanks and chases were more frequent in pairs where the intruders were smaller. Aggr. Behav. 25:297–303, 1999. © 1999 Wiley‐Liss, Inc.