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.
References
Aguiar, A., & Giaquinto, P. C. (2018). Low cholesterol is not always good: low cholesterol levels are associated with decreased serotonin and increased aggression in fish. Biology open, 7 (12), bio030981.
Barlow, G. (2002). The cichlid fishes: nature’s grand experiment in evolution (1st ed.): Basic Books: New York, NY, USA.
Boscolo, C. N. P., Morais, R. N., & Goncalves-de-Freitas, E. (2011). Same-sized fish groups increase aggressive interaction of sex-reversed males Nile tilapia GIFT strain. Applied Animal Behaviour Science, 135 (1-2), 154-159.
Carvalho, T. B., Mendonça, F. Z., Costa-Ferreira, R. S., & Gonçalves-de-Freitas, E. (2013). The effect of increased light intensity on the aggressive behavior of the Nile tilapia, Oreochromis niloticus (Teleostei: Cichlidae). Zoologia (Curitiba), 30 , 125-129.
Conte, F. (2004). Stress and the welfare of cultured fish. Applied Animal Behaviour Science, 86 (3-4), 205-223.
Enquist, M., & Jakobsson, S. (1986). Decision making and assessment in the fighting behaviour of Nannacara anomala (Cichlidae, Pisces).Ethology, 72 (2), 143-153.
Falter, U. (1983). Les comportements agonistiques de Sarotherodon niloticus (Pisces, Cichlidae) et la signification évolutive de l’incubation buccale. Bulletins de l’Académie Royale de Belgique, 69 (1), 566-594.
Fontenot, M. B., Kaplan, J. R., Shively, C. A., Manuck, S. B., & Mann, J. J. (1996). Cholesterol, serotonin, and behavior in young monkeys.Annals of the New York Academy of Sciences, 794 (1), 352-354.
Fricke, H. W. (1979). Mating system, resource defence and sex change in the anemonefish Amphiprion akallopisos. Zeitschrift für Tierpsychologie, 50 (3), 313-326.
Ghanawi, J., Shalaby, S. M., & Saoud I. P. (2010). Effect of Size-Sorting on Growth Performance of Juvenile Spinefoot Rabbitfish Siganus rivulatus. Journal of the World Aquaculture Society41(4), 565-573.
Giaquinto, P. C., & Volpato, G. L. (1997). Chemical communication, aggression, and conspecific recognition in the fish Nile tilapia.Physiology & Behavior, 62 (6), 1333-1338.
Gonçalves-de-Freitas, E., Bolognesi, M. C., Gauy, A. C. d. S., Brandão, M. L., Giaquinto, P. C., & Fernandes-Castilho, M. (2019). Social behavior and welfare in Nile tilapia. Fishes, 4 (2), 23.
Gonçalves-de-Freitas, E., Teresa, F. B., Gomes, F. S., & Giaquinto, P. C. (2008). Effect of water renewal on dominance hierarchy of juvenile Nile tilapia. Applied Animal Behaviour Science 112 (1-2), 187-195.
Hodapp, A., & Frey, D. (1982). Optimal foraging by firemouth cichlids, Cichlasoma meeki, in a social context. Animal Behaviour, 30 (4), 983-989.
Huntingford, F. A., & Turner, A. (2013). Animal conflict : Springer Science & Business Media.
Kaplan, J. R., Manuck, S. B., & Shively, C. A. (1991). The effects of fat and cholesterol on social behavior in monkeys. Psychosomatic Medicine .
Kaplan, J. R., Shively, C. A., Fontenot, M. B., Morgan, T. M., Howell, S. M., Manuck, S. B., . . . Mann, J. J. (1994). Demonstration of an association among dietary cholesterol, central serotonergic activity, and social behavior in monkeys. Psychosomatic Medicine, 56 (6), 479-484.
Leaver, M. J., Villeneuve, L. A., Obach, A., Jensen, L., Bron, J. E., Tocher, D. R., & Taggart, J. B. (2008). Functional genomics reveals increases in cholesterol biosynthetic genes and highly unsaturated fatty acid biosynthesis after dietary substitution of fish oil with vegetable oils in Atlantic salmon (Salmo salar). Bmc Genomics, 9 (1), 1-15.
Magnuson, J. J. (1962). An analysis of aggressive behavior, growth, and competition for food and space in medaka (Oryzias latipes (Pisces, Cyprinodontidae)). Canadian Journal of Zoology, 40 (2), 313-363.
Metcalfe, N. (1986). Intraspecific variation in competitive ability and food intake in salmonids: consequences for energy budgets and growth rates. Journal of Fish Biology, 28 (5), 525-531.
Myant, N. (1973). Cholesterol metabolism. Journal of Clinical Pathology. Supplement (Association of Clinical Pathologists), 5 , 1.
Parzefall, J. (1969). Zur vergleichenden Ethologie verschiedener Mollienesia-Arten einschließlich einer Höhlenform von M. sphenops.Behaviour , 1-37.
Saoud, I. P., Davis, D. A., Roy, L. A., & Phelps, R. P. (2005). Evaluating the benefits of size-sorting tilapia fry before stocking.Journal of Applied Aquaculture, 17 (4), 73-85.
Slavík, O., Pešta, M., & Horký, P. (2011). Effect of grading on energy consumption in European catfish Silurus glanis. Aquaculture, 313 (1-4), 73-78.
Ward, A. J., Webster, M. M., & Hart, P. J. (2006). Intraspecific food competition in fishes. Fish and Fisheries, 7 (4), 231-261.
Tables
Table 1. Ingredients of the five diets offered to Oreochromis niloticus for five weeks