Statistical Analysis
Data was analyzed using SAS (V.9.2, SAS Institute Inc., Cary, North
Carolina, USA). All data were reported as mean values ± standard
deviation of the mean and compared using one-way ANOVA. Student Newman-
Keuls (SNK) mean separation test was used to assess significant
differences among means. Significance level was set at p <
0.05. Original research data are not shared but can be made available
upon request.
Results
Results of the present experiment
showed that plasma cholesterol in fish offered any of the
cholesterol-enriched diets was greater than plasma cholesterol in
control fish. All fish in the various treatments had cholesterol levels
greater than detection limits (Table 3). Furthermore, there were no
significant differences in aggression among any of the treatments (Table
4). In all treatments, observers were able to identify the dominant Nile
tilapia characterized by a bright coloration, instigation of attacks and
display of the dorsal fin (Giaquinto & Volpato, 1997). Alternatively,
subordinate fish had a dark coloration or dark stripes that got darker
after confrontation with dominant fish. Similarly, in all treatments
dominant tilapia were observed to be swimming freely whilst subordinates
often grouped in a corner of the tank. Displays of dominance were
observed across treatments, whereby the dominant fish were gulping and
jotting their heads frequently. There were no significant differences in
mortality among treatments.
Discussion
Supplementing tilapia diets with cholesterol results in an increase in
plasma cholesterol levels but does not decrease aggression among the
fish. In the Aguiar and Giaquinto (2018) study, aggression of fish with
200 mg/dL plasma cholesterol was less than that of fish with 400 mg/dL.
However, in the present study fish with 187 mg/dL cholesterol were just
as aggressive as those with plasma cholesterol greater than 300 mg/dL.
Previous studies investigating the relationship between cholesterol and
aggression often attributed increased aggression to a decrease in
dietary cholesterol (Fontenot et al., 1996; Kaplan et al., 1991; Kaplan
et al., 1994). However, Kaplan et al. (1994) did mention that observed
results could actually be caused by increased dietary cholesterol
suppressing antagonistic behavior. It is interesting to note that in
batch 3, and especially in treatments 4 (1.5% cholesterol) and 5 (2%
Cholesterol), the dominance hierarchy became vague. In fact, observers
noticed the emergence of another fish that had similar characteristics
to the dominant fish, was initiating attacks on other fish and even
confronting the dominant. However, we couldn’t find a clear explanation
to such behavior.
It is important to note that the observed fish were placed in a
recirculating system and water was renewed constantly to remove feces
and excess feed. Accordingly, observed results could be caused by water
renewal diluting the chemicals needed for conspecific recognition and
thus destabilizing the hierarchy (Gonçalves-de-Freitas, Teresa, Gomes,
& Giaquinto, 2008). Another possible explanation for present results
could be that cholesterol effects on aggressiveness obeyed the rules of
saturation kinetics and when concentrations became greater than a
specific threshold, effects were no longer observed. No matter the
mechanism involved, adding cholesterol to tilapia diets did not decrease
antagonistic behavior and thus confers no benefit to farmers.
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Tables
Table 1. Ingredients of the five diets offered to Oreochromis
niloticus for five weeks