Monitoring aggression
Fish used in the present experiment were acclimated in five 1
m3 outdoor holding tanks connected to a biological
filter in a recirculation system. Water was aerated using submerged air
diffusers connected to a regenerative blower. Nile tilapia were size
sorted by hand, and 34 fish (46.0 ± 14.97 g; mean ± SD) were stocked in
each tank. Each tank was assigned one of the five dietary treatments and
fish were offered feed at 3% body mass (BM) divided into two daily
portions, at 8:00 and 17:00 hours, for two weeks. At the end of the two
weeks, a subset of the fish in each tank were relocated from the holding
tanks to 52-L glass tanks connected to a biological filter, and a sand
filter in a recirculation system, to monitor aggression. Five fish of
similar sizes were randomly collected from each of the five holding
tanks, group-weighed and stocked in the glass tanks. Fish were left to
acclimate in the glass tanks for 24 hours, and offered their respective
feeds at 3% BM, twice daily, at 8:00 and 17:00 hours. Fish were then
monitored for signs of aggression using an adaptation of the Falter
(1983) ethogram. Aggressive behavior such as nipping, mouth fighting/
attack, lateral fighting/ attack, chasing and flight, and cornering were
quantified twice daily for 10 minutes (after morning feeding and before
second feeding) on days 2, 3 and 5 after stocking. All observations were
recorded by two people and their observations compared. Number of
animals per tank hindered the quantification of aggressive behavior,
therefore frequency of aggressive behavior was not considered but rather
the presence or absence of aggressive behavior. Data collected was used
to calculate an aggression index whereby the presence of a criterion
elicits a score of “1”, and the absence a score of “0”. As such,
each tank was given a score (on a scale from 0 to 5) and average scores
of three monitoring days were recorded. Observers also monitored
position of fish in the tank and coloration of fish. The monitoring
procedure was replicated five times using new fish from the holding
tanks (Table 2). Fish stocked in previous experimental weeks continued
to be observed throughout the duration of the experiment but only data
collected on days 2, 3 and 5 were used to calculate the aggression
index.