Dietary Cholesterol and Aggression in Nile tilapia Oreochromis
niloticus
Razan ZeinEddine1, Magida Tabbara2and I. Patrick Saoud1*
1 Department of Biology, American University of Beirut
2 School of Fisheries and Allied Aquacultures, Auburn
University
Abstract
Tilapia farmers would benefit tremendously if they could decrease
aggression among fish. Conspecific aggression affects growth, feed
conversion and general wellbeing of fish. Previous studies established
an inverse relationship between blood cholesterol levels and aggression
in fish, whereby a decrease in cholesterol led to an increase in
aggression. The present study assessed the effect of an increase of
dietary cholesterol on blood cholesterol and possible decrease in
aggression of Nile tilapia Oreochromis niloticus . Nile tilapia
were stocked in an outdoor recirculation system then offered one of five
diets: 0% cholesterol, 0.5% cholesterol, 1% cholesterol, 1.5%
cholesterol and 2% cholesterol. Five fish of each treatment were moved
to a glass tank and monitored for signs of aggression for 10 minutes,
twice a day. This experimental procedure was repeated five times using a
new set of fish every time. Results show an increase in cholesterol
levels in the blood but that there are no significant differences in
aggression among treatments. Accordingly, the present study suggests
that an increase in dietary cholesterol increases blood cholesterol in
fish but does not have a significant effect on antagonistic patterns in
Nile tilapia.
Keywords: Nile tilapia, aggression, dietary cholesterol, antagonistic
behavior.
Introduction
Nile tilapia is a territorial fish whose social interactions are
hierarchical. The position/ rank of the fish in the hierarchy is
established through aggressive displays and can grant fish better access
to food (Hodapp & Frey, 1982; Magnuson, 1962; Metcalfe, 1986; Ward,
Webster, & Hart, 2006), mates (Fricke, 1979; Parzefall, 1969) and
territory (Barlow, 2002; Huntingford & Turner, 2013). As such, when
fish of various sizes occupy the same niche, the dominant Nile tilapia
grows more than subordinates because of differences in feed acquisition,
digestion rate, and energy expenditure (Gonçalves-de-Freitas et al.,
2019), with economic costs to the farmer.
In aquaculture, fish are often size sorted before stocking to ensure
optimum growth (Saoud, Davis, Roy, & Phelps, 2005; Ghanawi, Shalabi &
Saoud 2010) and equal access to resources. The problem is that fish
grading allows unfamiliar fish of similar fighting ability to be sorted
together (Slavík, Pešta, & Horký, 2011). Fish of similar sizes tend to
have prolonged and escalated fights (Boscolo, Morais, &
Goncalves-de-Freitas, 2011; Enquist & Jakobsson, 1986) that result in
harm to the individuals involved. Moreover, aggressive interactions
among fish increase the metabolic cost and energy expenditure of fish
and trigger social stress thus affecting fish health (Carvalho,
Mendonça, Costa-Ferreira, & Gonçalves-de-Freitas, 2013;
Gonçalves-de-Freitas et al., 2019; Metcalfe, 1986). Because stress is a
major cause of morbidity and mortality in aquatic systems (Conte, 2004),
farmers try to manage stress to improve fish welfare.
Cholesterol is a zoosterol that has important structural and biochemical
roles in the body. For instance, cholesterol is an important constituent
of cell membrane and plasma lipoproteins as well as a precursor of
various steroid hormones (Myant, 1973). Teleost fish, like other
vertebrates, are capable of synthesizing cholesterol de novo(Leaver et al., 2008) and thus don’t require it in their diets. In 2018,
Aguiar and Giaquinto (2018) treated Nile tilapia with a
cholesterol-lowering statin and noticed that low plasma cholesterol was
associated with an increase in aggression in the fish. Results of the
study corroborated the findings of previous studies performed on
primates (Fontenot, Kaplan, Shively, Manuck, & Mann, 1996; Kaplan,
Manuck, & Shively, 1991; Kaplan et al., 1994) reporting an inverse
relationship between blood cholesterol levels and aggression. In the
present study we propose a twist on Aguiar and Giaquinto (2018) where we
incorporate cholesterol in the diets of Nile tilapia, and test whether
dietary cholesterol mitigates antagonistic interactions in the fish. We
investigated whether an increase in dietary cholesterol is associated
with an increase in blood cholesterol in Nile tilapia, and whether that
increase is inversely correlated with aggression.
Materials and Methods