\received
DD MMMM YYYY
\acceptedDD MMMM YYYYBehavioural trials
Following the final 12-week rearing period, all trout were anaesthetised
with a solution of benzocaine, measured between the tip of the snout and
the fork of the tail (i.e. ‘fork length’: 71 ± 8 mm, mean
± s.d.), and marked with a pattern of variously coloured visible implant
elastomer tags (Northwest Marine Technology Inc., Anacortes, WA, USA).
Sixty groups of three trout each were established. Within each triad,
trout were size-matched, in which the largest was larger by <
5% of the fork length of the smallest, to minimise possible effects of
size on dominance (Huntingford et al., 1990; Johnsson and Åkerman,
1998 ). Additionally, individuals were unfamiliar with one another,
having never shared a rearing tank. Each triad consisted of a dyad from
different replicates of a treatment group, plus an ‘observer’ from
another replicate. Across the 60 triads, all combinations of replicates
and treatment groups were represented with the same approximate
frequency.
Behavioural tests were held as dyadic trials in successive stages
between groups of three. Triad members were kept alone and separate from
one another before and between behavioural trials in identical 30 L
glass aquaria, adorned with an air stone and one plastic plant set in a
corner, for 24 hours and fasted. For the initial behavioural trials
(naïve trials ), a dyad was placed simultaneously into a replica
of the fasting aquaria. This trial tank was open at the top and
illuminated by a 26 W, 1750 lm ceiling lamp. The sides were visually
blocked, except the front, to allow observation by the researcher, and
one side, to allow observation by the adjacent tank inhabitant. In an
identical aquarium adjacent, the third trout of the size-matched triad
was given the opportunity to observe the first two. This aquarium was
shaded to prevent the dyad from observing the observer, and it was
visually blocked on all sides except that facing the trial tank (Fig.
2).