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