Thermal Tolerance and Survival of Nile Tilapia and Blue Tilapia under Rapid and Natural Temperature Declination Rates

Blue Tilapia Oreochromis aureus and Nile Tilapia O. niloticus are indigenous to Africa and the Middle East but now are globally popular in aquaculture and for private and public stocking as forage fish for sport species and biological control for nuisance vegetation. The invasive nature of these species at a global scale and the lack of recent and applicably tested thermal tolerance methodology require research to assess the current potential for invasion risk in North Carolina and elsewhere. In this thermal tolerance experiment we focused on a moderate declination rate (1 degrees C per 3d) and a slow declination rate (1 degrees C per week) in water temperature to assess the thermal minima of Blue Tilapia, common Nile Tilapia, and a white strain of Nile Tilapia. The moderate rate was used to compare results to previous literature, and the slow rate was used to mimic natural seasonal water temperature decline in North Carolina. As lineage and breeding history may vary by supplier, fish from multiple suppliers within each species group were tested. Our results indicate that in general, these tilapias cannot survive below 8 degrees C. We did not observe differences in temperature at death or survival curves between species in the moderate rate treatment. Under the slow declination rate, we found significant differences in the mean temperatures at death of Blue Tilapia (9.5 degrees C) and white Nile Tilapia (9.3 degrees C) relative to the less-cold-tolerant common Nile Tilapia (10.8 degrees C); this result was reflected in statistically different survival curves. In addition, although there was some variation in temperature at death among different supplier groups (0.0-0.6 degrees C) within a species, all fish died at 8 degrees C or above. Our results suggest that selective breeding of white Nile Tilapia may have resulted in increased thermal tolerance during the process of developing this color variant. Furthermore, the minimum lethal temperature of 8 degrees C may explain why these tilapia species have been restricted to reservoirs with heated effluent in North Carolina.