No abstract

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°C per 3 d) and a slow declination rate (1°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°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°C) and white Nile Tilapia (9.3°C) relative to the less‐cold‐tolerant common Nile Tilapia (10.8°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°C) within a species, all fish died at 8°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°C may explain why these tilapia species have been restricted to reservoirs with heated effluent in North Carolina.

Redbelly Tilapia Tilapia zilli and Blue Tilapia Oreochromus aureus are indigenous to Africa and the Middle East, but now occur throughout the southeastern United States. It is fairly well understood that foraging flexibility and wide salinity, oxygen, and temperature tolerances have led to general invasion success for most tilapia species. Conflicting empirical information about the influence of tilapia on native sport fish species, however, leaves natural resource managers unable to anticipate and prevent further introductions and potential associated risks to native sport fishes. This investigation focused on two North Carolina reservoirs with tilapia populations: Hyco Lake, a large reservoir in northcentral North Carolina and Lake Julian, a small impoundment in western North Carolina. These reservoirs receive heated effluent from power generation, which is assumed to provide winter refuge for tilapia at a time when temperatures would otherwise lead to mortality. We assessed relative abundances of the tilapias and native sport species (Bluegill Lepomis macrochirus, black basses Micropterus spp., and Black Crappie Pomoxis nigromaculatus) in these systems and examined their trophic interactions via diet and stable isotope analyses in both lakes. In Hyco Lake we assessed potential spatial interactions and influences on relative abundance using a priori candidate models and model selection via Akaike's information criterion (AIC). These analyses indicated little evidence of tilapias influencing the relative abundances of the sport species in Hyco Lake. In both lakes, however, we did find substantial diet overlap between the tilapias and Bluegill along with isotopic niche overlap between Blue Tilapia and Bluegill in Lake Julian and Redbelly Tilapia and Bluegill in Hyco Lake. Due to the omnivorous and flexible diet of all three species these interactions were not surprising, but the ultimate influence on Bluegill growth and size structure is unknown. Overall, our results suggest that tilapia are exploiting trophic niches that are not used by the sport fishes of interest in these two very different reservoir systems and that their direct impact on sport species is not substantial.