32 L. Stanton Hales, Jr. 



1977), has reached a point in many lakes of Florida and Texas at which 

 it is adversely affecting reproduction of native centrarchids (Radonski et 

 al. 1984, Taylor et al. 1984). The blue tilapia is the dominant species in 

 cooling lakes at several power plants in Texas; several native fishes are 

 scarce or absent in those lakes (Dolman 1990). Tilapia aurea has been 

 reported to reduce the organic content of sediments, which eliminated 

 blooms of the blue-green alga Oscillatoria chalybea in some Israeli 

 reservoirs (Leventer 1981). The blue tilapia has been observed to depress 

 populations of large phytoplankton and vulnerable zooplankton while 

 enhancing small algae and evasive zooplankton (Drenner et al. 1984, 

 1987). Presumably through competition, the blue tilapia has been 

 reported to reduce the catch of desirable fish in Lake Kinneret, Israel 

 (Gophen et al. 1983, Vineyard et al. 1988). The effects on native fishes 

 through removal of vegetation have not been documented (Taylor et al. 

 1984). The effects of blue tilapia on estuarine communities, including 

 recreationally and commercially important fishes, have not been investi- 

 gated. Given the inherent variability of the estuarine physical environ- 

 ment and an estuarine fish community that includes seasonal and 

 transient members, demonstration of the adverse effects of blue tilapia 

 on estuarine fishes is very difficult. This difficulty suggests that a 

 conservative approach should be taken and that in the absence of 

 demonstrated benefits, the reported population should be eliminated, if 

 feasible. 



Eradication of the Skidaway population may be possible given the 

 present occurrence in only one tidal creek. The recorded juveniles 

 escaped from a raceway on Skidaway Island that empties into the tidal 

 creek. The raceway is one of two built in the early 1970s at the 

 Skidaway Institute of Oceanography and has been used since its 

 construction for aquaculture experiments and to hold surplus fishes. 

 The drain in these raceways consists of an uncovered standpipe, from 

 which raceway overflow dumps into a drainage culvert and a connecting 

 tidal creek. Until the recent construction of the drainage culvert, the 

 raceway overflow emptied directly into a tidal creek. With construction 

 of the drainage culvert, runoff from several labs at Skidaway empties 

 into the culvert, where it then drains into the tidal creek. Because of the 

 form of this culvert and its tidal elevation, water from the lab does not 

 flow immediately into the creek. In winter, these features may enhance 

 overwintering survival in the creek by providing a large (approximately 

 13,000 liter) storage pool of warm water. 



Eradication of the population may be possible through the applica- 

 tion of rotenone or other ichthyocides to the small creek, but such a 

 procedure has not been attempted. Modification of the raceway drain or 

 elimination of the raceway population would prevent future introduc- 



