The long term influence of individual biological rhythms on group feed
demand behaviour was investigated in European sea bass (Dicentrarchus labrax) held under
controlled environmental conditions with an on-demand feeding system. The
experiment was realized over 219 days with 190 fish distributed on 4 tanks.
Sea bass had a mean body mass comprised between 139 g to 183 g. The number
of feed demand acts by each individual was calculated daily, and the
population could thus be partitioned into three categories (high-, low- and
zero-triggering fish). The duration of the period that an individual held
high-triggering status could vary, but was 63 ± 16 days on average. The
transition period between two highest-triggering fish in one tank was on
average 4 ± 4 days. The group feeding rhythm followed the same pattern
of feed demand rhythm as the highest-triggering individual fish. When the
highest-triggering fish was nocturnal, the totality of feed demand in the
group was realized during the night with one peak at 22:00, corresponding to
dusk under experimental conditions. When the highest-triggering fish was
diurnal, the majority of feed demand in the group was realized during the
light period with one peak at 06:00, corresponding to dawn, and/or another
at 12:00. This study therefore highlights that sea bass group feeding
behaviour is not the sum of individual feed demand behaviours, but is
directed by the rhythm and behaviour of a few high-triggering fish. The
regular changes of high-triggering fish in the group proved that it was not
the identity of these particular fish that was most important for the group,
but their role as a feed demand leader.