ARS | Publication request: Aggression and mortality among Nile tilapia (Oreochromis niloticus) maintained in the laboratory at different densities

Submitted to: Popular Publication
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 7, 2008
Publication Date: January 30, 2008
Citation: Evans, J.J., Pasnik, D.J., Horley, P.J., Kraeer, K., Klesius, P.H. 2008. Aggression and mortality among Nile tilapia (Oreochromis niloticus) maintained in the laboratory at different densities. Research Journal of Animal Sciences. 2(2):57-64.

Interpretive Summary: Nile tilapia (Oreochromis niloticus) housed in aquaculture facilities or laboratory aquaria often experience overcrowding. Fish in overcrowded environments may be increasingly subject to acute or chronic stressors, which can cause decreased weight gain, decreased disease resistance, and increased mortalities. Nile tilapia commonly establish social hierarchies in fixed spaces and territories, and hierarchies among fish may be established based on size, sex, residency, previous hierarchical rank, social experience, and reproductive stage. Social status includes dominant and subordinate fish, and the dominant fish may have better access to food, territory, and mates. Subordinate fish often have increased blood cortisol and glucose levels, metabolic rates, decreased growth, immunosuppression, and increased mortality. Aggressive behavior is required to establish and maintain hierarchies among fish, and the degree of aggressive behavior may be influenced by social factors such as fish density and physical environmental factors such as aquarium size. Nile tilapia are commonly used for fish nutritional, physiological, reproductive, infectious diseases, and vaccine research world-wide. Concrete knowledge of culture conditions for Nile tilapia is still obscure in several aspects, especially the effects of high fish holding densities and behavior in experimental research. Because Nile tilapia density may affect stress levels, metabolism, and survival among tilapia in experimental studies and thus may affect experimental results, this study was designed to assess the relationship between Nile tilapia density, behavior patterns, percent cumulative survival and, blood glucose levels. Tilapia were held in a commonly-used commercial tank systems to assess the relationship between fish density and behavior patterns, percent cumulative survival and blood glucose levels. Fish (15.34 ± 0.34 g) were placed in tanks with 7 liters (7L group) of water at three different densities, 5 (7L-5), 10 (7L-10), or 15 (7L-15) fish per tank, or with 19 liters (19L group) water at three different densities, 5 (19L-5), 10 (19L-10), or 15 (19L-15) fish per tank. General patterns of behavior involving dominant and subordinate fish were observed, though no significant differences in behavior were found within or between groups. An inverse correlation was noted between the numbers of fish with normal coloration and mortalities. Mortalities in all of the 7 L tanks began one day after fish were placed in the tanks and often continued for 21 to 28 days or until only one fish remained in the tank, and mortalities in the 19L tanks began three days after fish were placed in the tanks. Percent cumulative survival was highest among the 19L-10 and 15 tanks in the 19L groups and the 7L-5 and 10 tanks in the 7L groups over the 30-day study period. Percent cumulative survival in the 7L-10 or 15 tanks was significantly lower than in 19L-10 and 15 tanks. The 19L-15 and 7L-5 tanks had approximately the same weight per volume density, but percent cumulative survival between the two groups was significantly different. Blood glucose levels of fish in the 7L tanks increased significantly over 48 hours and then decreased to baseline levels again 24 hours later. The findings in this study indicate that while Nile tilapia density can influence percent cumulative survival and blood glucose levels in a tank, the physical size and dimensions of the tank, and water volume may also affect these values. Based on this research, Nile tilapia could be held at densities of 11.0 to 21.9 kg/m3 in 7L tanks but preferentially maintained at densities of 8.1 to 12.1 kg/m3 in 19L tanks.

Technical Abstract: Because Nile tilapia (Oreochromis niloticus) density may affect stress levels, metabolism, and survival among tilapia in experimental studies and thus may affect experimental results, tilapia were held in a commonly-used commercial tank system to assess the relationship between fish density and behavior patterns, percent cumulative survival and blood glucose levels. Fish (15.34 ± 0.34 g) were placed in tanks with 7 liters (7L group) of water at three different densities, 5 (7L-5; 11.0 kg/m3), 10 (7L-10; 21.9 kg/m3), or 15 (7L-15; 32.9 kg/m3) fish per tank, or with 19 liters (19L group) water at three different densities, 5 (19L-5; 4.0 kg/m3), 10 (19L-10; 8.1 kg/m3), or 15 (19L-15; 12.1 kg/m3) fish per tank. General patterns of behavior involving dominant and subordinate fish were observed, though no significant differences in behavior were found within or between groups. An inverse correlation was noted between the numbers of fish with normal coloration and mortalities (r2 = -0.9035; P < 0.0135). Mortalities in all of the 7 L tanks began one day after fish were placed in the tanks and often continued for 21 to 28 days or until only one fish remained in the tank, and mortalities in the 19L tanks began three days after fish were placed in the tanks. Percent cumulative survival was highest among the 19L-10 and 15 tanks in the 19L groups and the 7L-5 and 10 tanks in the 7L groups over the 30-day study period. Percent cumulative survival in the 7L-10 or 15 tanks was significantly lower than in 19L-10 and 15 tanks (P < 0.0144 and P < 0.0001, respectively). The 19L-15 and 7L-5 tanks had approximately the same weight per volume density, but percent cumulative survival between the two groups was significantly different (P < 0.0173). Blood glucose levels of fish in the 7L tanks increased significantly over 48 hours and then decreased to baseline levels again 24 hours later. The findings in this study indicate that while Nile tilapia density can influence percent cumulative survival and blood glucose levels in a tank, the physical size and dimensions of the tank, and water volume may also affect these values. Based on this research, Nile tilapia (15 g) could be held at densities of 11.0 to 21.9 kg/m3 in 7L tanks but preferentially maintained at densities of 8.1 to 12.1 kg/m3 in 19L tanks.