Increase in Aggression and in Strength of the Social Hierarchy among Juvenile Atlantic Salmon Deprived of Food

Abstract

Aggressive behaviour of Atlantic salmon (Salmo salar) parr, tested in groups of eight, was greater during 3 days of deprivation of food than during 3 days of feeding. Frontal and lateral displaying appeared to increase more than charging and nipping. I.rcreased agression was only partly a result of increased locomotion, and therefore was partly a direct effect of hunger.Strong social hierarchies developed, larger fish nipping smaller ones more than the reverse. The correlation between size and hierarchical status was usually sufficient to account for the strength of hierarchy observed.Upon deprivation of food the dominant of any pair of socially unequal fish on the average increased its nipping of the subordinate. The nipping rate of dominanis by subordinates did not change consistently; many low ranking fish decreased their nipping rate upon deprivation. These results would strengthen hierarchies during deprivation. Initiation of attacks by small fish, as opposed to nipping, increased consistently during deprivation.Nipping was more frequent between fish of equal status and size than between fish of unequal status or size. This could have accounted for the strength of hierarchies in groups in which the correlation between size and position was insufficient to do so.The increase in aggression upon food deprivation could function to increase the size of feeding territories when food is scarce. This and the strengthening of hierarchies would cause emigration of some fish from the area. Concentration of aggression between fish of equal size and status could permit parr of different ages to coexist.

Social interactions between 0+ brown and rainbow trout in experimental stream troughs

“…The low aggression between the size-classes indicates that these fish do not show severe competition. Greatest social conflict usually occurs between fish of similar size (Newman 1956;Symons 1968), and small differences in size can determine dominance (Chapman 1962). However, with increasing size disparity fish require different foods, hence behaviour which focuses competition on fish of similar size would be adaptive (Symons 1968).…”

Variation in individual food consumption rates of fish and its implications for the study of fish nutrition and physiology

“…The advantage of using feeding behaviour to assign dominance is that it enables the assessment of social hierarchies in larger groups of fish than is possible using the standard approach of observing aggressive interactions between individuals. The distribution of food between individuals within a group and the strength of the social hierarchy will be dependent on the availability of food (Symons, 1968;McCarthy et al 1992;Olla et al 1992). This is demonstrated in Fig.…”

“…Although the fish in both groups were of a similar size and under the same feeding regimen, both the range of meal sizes and the range of individual values for CV, were significantly different for the two groups indicating that the strength of the social hierarchy, inferred from the correlation coefficient, varied between the two groups. Previous studies have shown that the strength of the social hierarchy can vary between groups of fish given the same treatment (Symons, 1968;Winberg et al 1992Winberg et al , 1993. Under these conditions, it appears that the strength of the social hierarchy is dependent on the particular individuals present in the group and the differences in competitive ability and aggression between individuals in each group.…”

Sensory deprivation and sensory input in rearing experiments

“…For instance, SHELBOURNE (1964) reported that smaller fish were more prone to pigment deficiency, but RIL~X & THACK~V, (1963) found no effect of size. Aggressive behaviour of young salmon, Salrno salar (SYMoNS 1968) in the form of frontal and lateral displaying as well as in charging and nipping was more prevalent during lack of food. Strong hierachies based on size then developed.…”