Social and spatial structure in brook chars (Salvelinus fontinalis) under competition for food and shelter/shade

Abstract

Salmonids, outside their reproductive period, are seen to have two types of territory called "territorial mosaic" and "partial territory". The first aspect of this research aimed at identifying the type of territory established by mature brook chars in artificial streams. After this, the biological value of spacing out was studied with regard to two resources: food, and shelter/shade which gives protection. Three 5 X 1 X 1 m artificial streams were built on the edge of a natural brook which provided a continuous water supply. One hundred and fifty mature brook chars (Salvelinus fontinalis) taken from that brook were distributed into 30 colonies with 5 members apiece. Three experimental conditions were created, and 10 colonies were submitted to each of these. In the first experimental condition, the quality of shelter/shade differed in 3 sectors of the artificial streams, whereas the quantity of food remained the same for all 3. In the second experimental condition, the quality of shelter/shade was identical, while the quantity of food differed in the 3 sectors. In the last condition, conflict was created: the fish had to choose between an area which offered excellent shelter/shade but no food, one which provided ample nourishment but no shelter/shade, and one in which all those resources were present at intermediate levels. Observation of the 30 colonies revealed "partial territory" in all cases. These corresponded to more or less complete aggressive-dominance hierarchies. Almost every alpha established territory, and the number of territorial individuals progressively decreased throughout inferior ranks. Alphas had exclusive use of their territory. Lower-ranking individuals successfully defended their territory against their subordinates, but were unable to drive away higher-ranking conspecifics. Overall results also indicated that the highest-ranking brook chars in the aggressive-dominance hierarchies more frequently established their territories in sectors of the streams with good shelter/shade than in sectors with good alimentary conditions.

The role of density-dependent individual growth in the persistence of freshwater salmonid populations

“…Moreover, especially in fragmented streams, the movement of fish, both density-dependent and density-independent, and its relationship with population dynamics are worthy of investigation. It is well known that competition for feeding and spawning sites in many stream-dwelling salmonid populations involves the finescale movement of individuals related to body size (Caron and Beaugrand 1988;Grant et al 1998;Hughes 1998) and can thus influence population dynamics. Thus, a spatiallyexplicit individual-based model of population dynamics may provide other insights on the role of density-dependent growth in population dynamics.…”

Kin discrimination in salmonids

“…For example, coho salmon fry which do not successfully defend foraging territories in streams have a greatly reduced probability of returning to the natal site to spawn as adults (Crone and Bond 1976;Mason, 1976). In addition, territoriality in coho salmon and brook charr has been shown to reduce predation risk significantly (Symons, 1974;Dill et al, 1981;Caron and Beaugrand, 1988). As such, territorial defence behaviour patterns serve to increase the individual's direct fitness by increasing its probability to survive until reproduction.…”

Methods for assessing social status in Arctic charr

“…Consequently, scores on position in relation to a defensible food resource or food intake, have been used more often to assess social status have actual scores on aggression (Metcalfe et al, 1989;Grant, 1997). Both of these parameters have a high correlation with aggression and social status in stream-living salmonids (Li & Brocksen, 1977;Fausch, 1984;Metcalfe, 1986;Caron & Beaugrand, 1988;Metcalfe et al, 1989;Huntingford et al, 1990;Hughes, 1992;Johnsson et al, 1996). Therefore it would be advantageous if alternative methods of socially ranking a lake-dwelling species such as Arctic charr Salvelinus alpinus (L.) could also be correlated with measures of aggression.…”