Aggression and Mortality among Nile Tilapia (Oreochromis niloticus) Maintained in the Laboratory at Different Densities

Because Nile tilapia (Oreochromis niloticus) density may affect stress levels, metabolism and survival among tilapia in laboratory experimental studies and thus may affect experimental results, tilapia were held in a commonly-used commercial tank system for 30 days 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 L (7L group) of water at 3 different densities, 5 (7L-5; 11.0 kg m 3), 10 (7L-10; 21.9 kg m 3), or 15 (7L-15; 32.9 kg m 3) fish per tank, or with 19 L (19L group) water at 3 different densities, 5 (19L-5; 4.0 kg m 3), 10 (19L-10; 8.1 kg m 3), or 15 (19L-15; 12.1 kg m 3) fish per tank. General patterns of behavior involving dominant and subordinate fish were observed: Biting was correlated with poor fin and body condition, ramming and mouth-fighting, while ramming was correlated with mouth-fighting. Mortalities were negatively associated with light coloration. Mortalities in all of the 7 L tanks began one day after fish were placed in the tanks and often continued for 21-28 days or until only one fish remained in the tank and mortalities in the 19L tanks began 3 days after fish were placed in the tanks. Percent cumulative survival was highest among the 19L-10 and 15 (100-93%) tanks in the 19L groups and the 7L-5 and 10 (53%) 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 2 groups was significantly different (p<0.0173). Blood glucose levels of fish in the 7 L tanks increased significantly over 48 h and then decreased to baseline levels again 24 h 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 our research with two commercial aquaria and tank systems commonly used in laboratories, 15 g Nile tilapia could be held at densities of 11.0-21.9 kg m 3 in 7 L tanks but preferentially maintained at densities of 8.1-12.1 kg m 3 in 19 L tanks to reduce mortality from aggression and ensure proper fish welfare.