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

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.