Fish and rearing environment
Approximately 1000 late-stage ‘eyed’ brown trout (Salmo trutta ,
Linnæus 1758) eggs were obtained in early February 2021 from a stocking
hatchery (AE Fishery, Moffat, UK). The eggs were the offspring of a
mixture of riverine and lacustrine parents removed to captivity from the
wild three to four generations ago. They were transported to the
Scottish Centre for Ecology and the Natural Environment (SCENE) on Loch
Lomond, UK and approximately evenly distributed between 12 cylindrical
30 L flow-through tanks held at ca. 4°C. Hatching was complete
within two weeks of arrival at the centre.
At first feeding (ca. 10–14 days post hatch), to ensure that
differences between individuals in aggression, swimming abilities, etc.
did not affect group-level results in later behaviour trials,
alevins-turned-fry were randomly split into two diet groups and moved to
bare flow-through tanks as above, six replicates per diet group, but
without temperature control. Ambient water temperature naturally warmed
throughout the growth season from ca. 4–21°C. Two diet types of
near-identical nutritional value were specially prepared by
Garant-Tiernahrung GmbH (Pöchlarn, Austria): one diet was high in n-3
LC‑PUFA (EPA = 3.69%, DHA = 4.95%); the other diet was deficient in
n-3 LC‑PUFA (EPA = 0.9%, DHA = 0.89%) (Appendix, Table A1). Fry were
fed to satiation twice daily for ca. four months.
For the final twelve weeks of rearing, trout from each diet treatment
were further randomly divided between ‘simple’ and ‘complex’ habitats.
Simple habitats consisted of bare flow-through cylindrical 90 L tanks
with water supplied directly from Loch Lomond at ambient temperatures;
30–40 individuals per tank approximated conventional hatchery
densities, which have been shown to be socially simpler than natural
densities by inhibiting the development of competitive behaviours
(Brockmark and Johnsson, 2010). Complex habitats used the same tank type
but were heavily ornamented, with ornaments changing position every
second day (i.e. physical complexity); 6–7 fish per tank and
weekly part-exchanges of fish between pairs of replicates fostered
social complexity by allowing the continuous formation and reformation
of dominance hierarchies (Sloman et al., 2000). All trout remained on
their original diets and were, for the final twelve weeks, fed once
daily to satiation, giving four distinct treatment groups, from which
selections were made for further analysis: simple habitat, high n-3
LC-PUFA diet (four replicates); simple habitat, low n-3 LC-PUFA diet
(four replicates); complex habitat, high n-3 LC-PUFA diet (eight
replicates organised as four pairs part-exchanging inhabitants); and
complex habitat, low n-3 LC-PUFA diet (eight replicates organised as
four pairs part-exchanging inhabitants) (Table 1; Fig. 1).
Table 1. Treatment groups and replicates at three stages of
experimental rearing period of captive brown trout