Figure 2. Aquarium set for behavioural trials of a trout triad: the naïve trial (upper left tank) shows a trout dyad establishing a dominant/subordinate relationship while a third trout observes unseen (upper right tank). The observer trial (lower tank) shows the former observer and one of the previous dyad establishing a new dominant/subordinate relationship. Visual blockages on all sides, except that facing the researcher and between the naïve trail tank and the observer trout, are not shown here.
The dyad was observed by a researcher for five minutes every half hour until a dominant/subordinate relationship was seen to be clearly established, or until five hours had elapsed (vide Appendix). Dominance and subordination were assessed by assigning points at each observation using a point-scoring system that evaluated levels of concealment, activity, and aggression (Table 2), based on a previous study of brown trout behaviour (Sloman et al., 2000). A stable dominant/subordinate relationship was suspected if one fish had a positive score that was at least two points higher than the other fish’s. At the next observation following suspicion of a stable relationship, food (two single bloodworms dropped simultaneously) was introduced at the centre of the forward-facing glass panel. If the suspected dominant fish took the food first and retained a two-point or greater score advantage, a dominant/subordinate relationship wasdeclared . Otherwise, the relationship was considered unsustained. At the observation following declaration, the food drop was repeated, and if the same results were seen, the relationship was considered confirmed as stable, and the time of the declaration recorded. Without confirmation, a declaration of a stable relationship was voided. All trials were observed by the same researcher. After the naïve trials, trout were fed, then returned to their individual fasting tanks for 24 hours.
Table 2. Scoring system used at repeated observations to detect the establishment of a dominant/subordinate relationship between pairs of brown trout in behavioural trials.